Martin280s
New Member
I found this article while I was looking for info as to why my W124 was not starting properly. I hope it is opf use to somebody.
------------------------------------------------------------------------
Troubleshooting D-Jetronic
No start and fuel pump not working;
1) Test the fuse, relay, wiring and fuel pump.
No start and fuel pump working;
2) Test the main relay and power from ECU. Ensure there is a good ground from ECU terminal #11 to ground.
3) If the ignition system has a good spark, good power supply, ground to the ECU and continuity at the trigger points (cover this later) and injectors fail to pulse, replace the ECU.
Starts when cold and dies when the key is released;
1) Trigger points in distributor lack continuity. When the engine starts, it is starting off the cold start injector and as you release the key, the cold start injector shuts off.
Stalls when engine is cold;
2) Inspect the auxiliary air valve. When the engine is cold, remove the hose on the intake side of auxiliary air valve and let air to be drawn into it. If the engine speed increases greatly, replace the auxiliary air valve.
3) Temperature sensor 2 is also critical to cold operation.
Misfire or unstable idle;
1) Ensure that the ignition system is in good shape and also the intake is in good order. These need to be confirmed first as they are more likely to cause this problem.
2) Inspect the circuits and sensors of temperature sensor 1 and 2 and continuity through both sides of trigger points.
3) Inspect the vacuum hose that goes between the pressure sensor and the intake manifold. It should have no kinks and not collapse with vacuum present.
4) If all the above check out, do an injector flow test.
Misfire while driving;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Do a fuel pressure and volume test.
3) If the fuel pressure is constant, test temperature sensor 2, pressure sensor and throttle switch circuits.
Power is suffering;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Most likely this is due to low fuel pressure. Do a fuel pressure-volume test and pay close attention for loss of pressure.
3) If correct, test the circuits of the pressure sensor and temperature sensor 2. If these meet specs, do an injector flow test.
4) If everything checks out, repeat this test.
Stalls when you decelerate;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Inspect the hoses and wiring for good contact.
3) Check your throttle stop adjustment and curb idle RPM's.
Smokes and runs rough;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Do a fuel pressure and volume test.
3) Inspect the circuits of the temperature sensor 2, trigger points, pressure sensor and injectors. If they check out fine, do an injector flow test.
Misfire under Load;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Do a fuel pressure-volume test.
3) Inspect the circuits of temperature sensor 2, pressure sensor, trigger points and injectors.
4) If the above are good, do an injector flow test.
Trailer hitching;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Do a fuel pressure-volume test.
3) Inspect the circuits of temperature sensor 2, pressure sensor, trigger points and injectors.
4) If the above are good, do an injector flow test.
5) Check the throttle switch.
Idle speed too high;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Check and adjust the throttle stop and curb idle speed.
Idle speed too low;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Check and adjust the throttle stop and curb idle speed.
Main relay;
1) There should be 12 volts at pin # 24 of the ECU.
2) If not, check for 12 volts at pin # 24 of the relay.
3) If 12 volts is not present at the relay but it clicks, replace with a new relay.
4) If there is 12 volts at pin # 24 of the relay, repair # 24 wire between relay and ECU.
5) If the relay does not click, inspect the wire from the main relay pin # 45 to ground.
6) If the ground is good and 12 volts at relay pin # 38 when the engine is cranked, replace the main relay.
Trigger points;
1) With the engine not running and ECU harness disconnected from the ECU, check the resistance between ECU harness 12 and 21, then from 12 to 22. One should show a low resistance and the other should show an open circuit.
2) Rotate the engine 360 degrees and check the resistance. They should have an opposite reading now.
3) If not, perform the same test at the trigger points themselves. This will tell you if it a component or wiring fault.
Temperature sensor 1
1) With the engine not running and the ECU harness disconnected from the ECU, connect an ohmmeter between ECU harness pin # 1 and 13. At an ambient temperature, the reading should be 200 ohms.
2) If the resistance is well above 200 ohms, check resistance at the sensor itself to determine if it is a component or wiring fault.
3) Check resistance of all the terminals to chassis ground. There should always be an open circuit.
Temperature sensor 2
1) With the engine not running and the ECU harness disconnected from the ECU, check the resistance between harness terminal 23 and ground. The resistance should be 2000 ohms at ambient temperature.
2) If the engine is near operating temperature, the resistance should be below 1000 ohms.
Throttle switch
1) Test the throttle switch with the key on, engine off.
2) Slowly open the throttle, and as it opens the injectors should alternately click. You should hear 20 evenly spaced clicks.
3) If this is not met, place a 0.016 in. (0.4 mm) feeler gauge between the throttle stop and throttle stop screw. Connect a voltmeter to terminal 17 of the throttle switch. The voltmeter should show voltage while the ignition key is in the on position. Remove the feeler gauge and the voltmeter should show no voltage now.
4) If this fails, loosen the screws and rotate the switch until it meets these requirements.
5) If they cannot be met and voltage is always present no matter where you position, replace the throttle switch.
6) If the voltmeter reads no voltage, ensure voltage is being supplied to the switch. If so, replace the throttle switch.
7) If no clicks are heard while performing test 1 and 2, check wire numbers 20, 17, 14 and 9 for continuity end for end from the ECU to throttle switch.
Pressure sensor
1) With the engine not running and the ECU harness disconnected from ECU, check the resistance from ECU harness terminal # 7 to 15. Should be 90 ohms and then check the resistance between # 8 and 10. Should be 350 ohms. If this is not met, test the sensor itself.
2) On occasion, the sensor resistance will check out, but will not hold vacuum. Using a hand held vacuum pump perform this test again (step #1).
Injector circuit
1) With the engine not running and the ECU harness disconnected from ECU, check resistance of ECU harness pins # 3, 4, 5, and 6. All should have less than 25 ohms.
2) If the resistance is greatly higher or lower than 25 ohms, test the injector itself.
3) If the injectors pass, repair the wiring harness.
Sensors
Trigger Points
The trigger points are located in the base of the distributor below the ignition point breaker plate. These points are used to produce the signal that synchronizes the injector to the crankshaft. They consist of two low voltage, low current contact points which are driven by a cam located on the distributor shaft. Each set of points initiates the opening of one or two groups of injectors.
Unlike ignition points, the trigger points can last 100, 000 miles or more. As the distributor rotates, a pulse is created by the opening and closing of the trigger points. This pulse is sent to the ECU. The ECU uses this signal to open the injectors and will use the inputs from the other sensors to determine when to close them.
Temperature Sensor I
Temperature Sensor I is an ambient air temperature sensor. As the ambient air temperature decreases, the density of the air increases. As a result, the ECU must inject more fuel on a cold day than on a warm day.
Temperature Sensor I is a temperature sensitive resistor known as negative temperature coefficient thermistor (NTC). This sensor has a resistance of between 400 and 500 ohms at 50 degrees F. At 100 degrees F., the resistance is between 150 to 200 ohms.
Temperature Sensor I actually has little effect on the operation on most D Jetronic equipped vehicles. This is because most of these cars have tens of thousands of miles on the engine and are running very rich due to this wear. The troubleshooting consequence of this is that disconnecting the air temperature sensor during the diagnostic procedure may have little effect on the way the engine runs, and in some cases, it may actually improve the way it runs.
Temperature Sensor II
Temperature Sensor II is the coolant temperature sensor on water-cooled engines. Like temperature sensor I, it is a NTC thermistor. The temperature of the engine is important because the intake manifold design of fuel-injected engine does not permit the use of an air restrictive choke. Additionally, restricting the air to enrich the engine during warm up would cause inaccurate readings from the manifold pressure sensor. The warm up choke function is therefore performed by Temperature Sensor II.
When the coolant temperature is about 50 degrees F., the resistance of Temperature Sensor II is between 3000 and 4500 ohms. As the temperature increases to more that 120 degrees F., the resistance drops to less than 1000 ohms. Thus, the sensors work is done once the engine is warmed up. In addition, it must be continuously remind the ECU that the engine has warmed up. If damaged, Temperature Sensor II will not continue to partially function( such as the resistance values shifting) . It will completely fail, creating an open short or ground.
An open circuit in Temperature Sensor II or the wiring leading to it will cause the engine to run extremely rich once warmed up. Symptoms would be dark smoke from the tailpipe (most noticeable at idle), rough idle and poor power. Keep in mind that these same symptoms could also be caused by engine compression problems and ignition.
Should the sensor become shorted or the wiring harness grounded, the effects may not be noticeable at all when the engine is warmed up. The symptoms would be more like a carburettor with the choke stuck open-rough or erratic idle, stalling or hesitation when the engine is cold and progressively running better as the engine warms up.
Throttle Switch
The throttle switch tells the ECU when the throttle is closed, when the throttle is wide open and when the throttle is moving toward the open position. The switch consist of twenty two contacts, with a set of wiping contacts that move across them as the throttle progresses from the closed position to the wide open position.
One wiping contact is used to inform the ECU that the throttle is closed. Another makes contact only when the throttle is wide open and a third makes and breaks contact twenty times as the throttle opens. The electrical pulses created by the making and breaking of the contacts signals the ECU to open the injectors more frequently, thereby enriching the mixture for acceleration. This feature behaves much like an accelerator pump on a carburettor.
Symptoms associated with a defective throttle switch include a rich(smoky) idle and hesitation. An intermittent condition at cruise-which feels like you shut off the engine and immediately turned the key back on, can also be caused by the throttle switch.
To test the throttle switch, open the throttle with the key on but the engine not running. The injectors should open exactly twenty times, evidenced by twenty evenly spaced clicks.
Pressure Sensor
The D-Jetronic pressure sensor is known as a linear variable displacement transducer (LVDT). It consists of a pair of coils, one with about 150 ohms of resistance and the other with about 85 ohms of resistance. An iron core attached to a diaphragm runs through the centre of these coils. As changes in manifold pressure moves the diaphragm, the iron core moves inside the coils, causing ripples in the current flowing through these coils. This signal is used by the ECU to monitor the relationship between barometric pressure and manifold pressure.
The most common symptom from a defective pressure sensor is a rich running condition. Of course, rich running can be caused by several other defects as well.
Of all the sensors used on D Jetronic, this one both the easiest and the most difficult to test. Usually a simple resistance test of the coils is enough to determine if the unit is good or bad. On the other hand, the only way to be sure is to replace it with a known good unit.
Electronic Control Unit (ECU)
The ECU receives input signals from the pressure sensor, Temp Sensor I, Temp Sensor II and the throttle switch to determine how long to leave the injectors open. It is only able to respond to air-fuel ratio request from one sensor at a time. As a result, whenever a sensor fails, the tendency will be for the ECU to send the injectors a full rich supply of fuel.
The electronic unit has no serviceable components. In the event of a failure, the ECU is replaced as a unit. Failures are extremely rare and usually result in a no start.
D Jetronic that have an adjustable air fuel ratio have a detent potentiometer on the side of the ECU. This potentiometer can be used to fine-tune the air-fuel ratio during a tune up.
Pin # 19 and 25 of the ECU are connected to the fuel pump relay. When the ignition switch is turned to the on position, the ECU energizes the fuel pump to ensure that the fuel system is filled for ease of starting. If the engine is not cranked, the ECU will shut off the fuel pump after one or two seconds. If the engine is started, the fuel pump runs continuously until the engine is shut off.
In Tank Filter
Located inside the fuel tank is a screen or filter designed to protect the fuel pump from rust, dirt and debris. Although seldom the cause of a drivability problem, the in tank filter should be high on the list of items to check. In many cases, these filters have been ignored, even on well-maintained vehicles.
Fuel Pump
The fuel pump is a high-speed roller vane pump. It is capable of pumping fuel at pressures and volumes much higher than the engine or injection system would ever require. This type of pump is very efficient at pushing fuel bur does not do a good job of pulling fuel. As a result, these pumps are located very close to the fuel tank or even inside the fuel tank to reduce the chance to vapour lock.
The fuel pump can react adversely and even fail as a result of using some fuel additives. Use extreme when selecting them to ensure that they do not contain methanol or other corrosive substances.
A defective fuel pump can cause low fuel pressure that would result in symptoms such as hesitation, stalling and poor power. Often a defective fuel pump bypasses the poor running stage and simply stops operating. This causes the engine to die or keeps the engine from starting.
Fuel Filter
The fuel filter is the only real protection the injection system has from internal contamination from dirty fuel. Therefore, the filter should be changed every time the spark plugs are changed.
Fuel Pressure Regulator
The fuel pressure regulator used on D Jetronic systems consist of a valve connected to a spring-loaded diaphragm. The regulator controls the fuel pressure at 28-32 psi and is adjustable so it can ensure the proper fuel pressure throughout the life of the vehicle. Incorrect fuel pressure can cause a lean running engine if the fuel pressure is too low and a rich running engine if the fuel pressure is too high.
A defective fuel pressure regulator can result in high fuel consumption, rough or erratic idle and poor power.
Injectors
The injectors are solenoid operated, normally closed valves controlled by the ECU. Grounded to the engine block or chassis, each injector is opened by a 3-volt pulse from the ECU. The length of the pulse is only a few milliseconds (2 to 5), and it takes time for the injector to close from the spring tension. Thus, the injector is open for a total of about 3 to 6 milliseconds.
Very little goes wrong with the D Jetronic injectors themselves. An occasional burned out solenoid winding or restriction from contamination is the most common problem. Another problem is leaking from the hoses that attach the injector to the fuel rail.
Typical symptoms of injector problems include rough idle and poor power.
Fuel Rail
The injectors and the fuel pressure regulator are attached to steel tubing known as the fuel rail. The inbound fuel lines from the fuel pump and filter feeds fuel to the injectors and the fuel pressure regulator through the fuel rail.
As a passive component of the system, very little can go wrong with it except for leaks and restrictions.
Cold Start Injector
Also attached to the fuel rail is a solenoid-operated valve known as the cold start injector. Since the earliest applications of the D Jetronic system were four cylinders, the cold start valve picked up the moniker "fifth injector.” It stuck in some circles, even for six and eight cylinder applications.
The cold start injector receives battery voltage whenever the starter is engaged and is grounded through a device known as a thermo-time switch. The thermo-time switch is a temperature sensitive bimetal switch designed to provide a ground for the cold start injector when the temperature of the engine is less than 95 degrees F. A second circuit in the switch is an electric heating element intended to heat the bimetal as the engine is being cranked.
Consequently, the cold start injector should operate only when the engine is being cranked, the temperature of the engine is less than 95 degrees F. and for a maximum of five to twelve seconds.
------------------------------------------------------------------------
Troubleshooting D-Jetronic
No start and fuel pump not working;
1) Test the fuse, relay, wiring and fuel pump.
No start and fuel pump working;
2) Test the main relay and power from ECU. Ensure there is a good ground from ECU terminal #11 to ground.
3) If the ignition system has a good spark, good power supply, ground to the ECU and continuity at the trigger points (cover this later) and injectors fail to pulse, replace the ECU.
Starts when cold and dies when the key is released;
1) Trigger points in distributor lack continuity. When the engine starts, it is starting off the cold start injector and as you release the key, the cold start injector shuts off.
Stalls when engine is cold;
2) Inspect the auxiliary air valve. When the engine is cold, remove the hose on the intake side of auxiliary air valve and let air to be drawn into it. If the engine speed increases greatly, replace the auxiliary air valve.
3) Temperature sensor 2 is also critical to cold operation.
Misfire or unstable idle;
1) Ensure that the ignition system is in good shape and also the intake is in good order. These need to be confirmed first as they are more likely to cause this problem.
2) Inspect the circuits and sensors of temperature sensor 1 and 2 and continuity through both sides of trigger points.
3) Inspect the vacuum hose that goes between the pressure sensor and the intake manifold. It should have no kinks and not collapse with vacuum present.
4) If all the above check out, do an injector flow test.
Misfire while driving;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Do a fuel pressure and volume test.
3) If the fuel pressure is constant, test temperature sensor 2, pressure sensor and throttle switch circuits.
Power is suffering;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Most likely this is due to low fuel pressure. Do a fuel pressure-volume test and pay close attention for loss of pressure.
3) If correct, test the circuits of the pressure sensor and temperature sensor 2. If these meet specs, do an injector flow test.
4) If everything checks out, repeat this test.
Stalls when you decelerate;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Inspect the hoses and wiring for good contact.
3) Check your throttle stop adjustment and curb idle RPM's.
Smokes and runs rough;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Do a fuel pressure and volume test.
3) Inspect the circuits of the temperature sensor 2, trigger points, pressure sensor and injectors. If they check out fine, do an injector flow test.
Misfire under Load;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Do a fuel pressure-volume test.
3) Inspect the circuits of temperature sensor 2, pressure sensor, trigger points and injectors.
4) If the above are good, do an injector flow test.
Trailer hitching;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Do a fuel pressure-volume test.
3) Inspect the circuits of temperature sensor 2, pressure sensor, trigger points and injectors.
4) If the above are good, do an injector flow test.
5) Check the throttle switch.
Idle speed too high;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Check and adjust the throttle stop and curb idle speed.
Idle speed too low;
1) Ensure that the ignition system is in good shape and also the intake is in good order.
2) Check and adjust the throttle stop and curb idle speed.
Main relay;
1) There should be 12 volts at pin # 24 of the ECU.
2) If not, check for 12 volts at pin # 24 of the relay.
3) If 12 volts is not present at the relay but it clicks, replace with a new relay.
4) If there is 12 volts at pin # 24 of the relay, repair # 24 wire between relay and ECU.
5) If the relay does not click, inspect the wire from the main relay pin # 45 to ground.
6) If the ground is good and 12 volts at relay pin # 38 when the engine is cranked, replace the main relay.
Trigger points;
1) With the engine not running and ECU harness disconnected from the ECU, check the resistance between ECU harness 12 and 21, then from 12 to 22. One should show a low resistance and the other should show an open circuit.
2) Rotate the engine 360 degrees and check the resistance. They should have an opposite reading now.
3) If not, perform the same test at the trigger points themselves. This will tell you if it a component or wiring fault.
Temperature sensor 1
1) With the engine not running and the ECU harness disconnected from the ECU, connect an ohmmeter between ECU harness pin # 1 and 13. At an ambient temperature, the reading should be 200 ohms.
2) If the resistance is well above 200 ohms, check resistance at the sensor itself to determine if it is a component or wiring fault.
3) Check resistance of all the terminals to chassis ground. There should always be an open circuit.
Temperature sensor 2
1) With the engine not running and the ECU harness disconnected from the ECU, check the resistance between harness terminal 23 and ground. The resistance should be 2000 ohms at ambient temperature.
2) If the engine is near operating temperature, the resistance should be below 1000 ohms.
Throttle switch
1) Test the throttle switch with the key on, engine off.
2) Slowly open the throttle, and as it opens the injectors should alternately click. You should hear 20 evenly spaced clicks.
3) If this is not met, place a 0.016 in. (0.4 mm) feeler gauge between the throttle stop and throttle stop screw. Connect a voltmeter to terminal 17 of the throttle switch. The voltmeter should show voltage while the ignition key is in the on position. Remove the feeler gauge and the voltmeter should show no voltage now.
4) If this fails, loosen the screws and rotate the switch until it meets these requirements.
5) If they cannot be met and voltage is always present no matter where you position, replace the throttle switch.
6) If the voltmeter reads no voltage, ensure voltage is being supplied to the switch. If so, replace the throttle switch.
7) If no clicks are heard while performing test 1 and 2, check wire numbers 20, 17, 14 and 9 for continuity end for end from the ECU to throttle switch.
Pressure sensor
1) With the engine not running and the ECU harness disconnected from ECU, check the resistance from ECU harness terminal # 7 to 15. Should be 90 ohms and then check the resistance between # 8 and 10. Should be 350 ohms. If this is not met, test the sensor itself.
2) On occasion, the sensor resistance will check out, but will not hold vacuum. Using a hand held vacuum pump perform this test again (step #1).
Injector circuit
1) With the engine not running and the ECU harness disconnected from ECU, check resistance of ECU harness pins # 3, 4, 5, and 6. All should have less than 25 ohms.
2) If the resistance is greatly higher or lower than 25 ohms, test the injector itself.
3) If the injectors pass, repair the wiring harness.
Sensors
Trigger Points
The trigger points are located in the base of the distributor below the ignition point breaker plate. These points are used to produce the signal that synchronizes the injector to the crankshaft. They consist of two low voltage, low current contact points which are driven by a cam located on the distributor shaft. Each set of points initiates the opening of one or two groups of injectors.
Unlike ignition points, the trigger points can last 100, 000 miles or more. As the distributor rotates, a pulse is created by the opening and closing of the trigger points. This pulse is sent to the ECU. The ECU uses this signal to open the injectors and will use the inputs from the other sensors to determine when to close them.
Temperature Sensor I
Temperature Sensor I is an ambient air temperature sensor. As the ambient air temperature decreases, the density of the air increases. As a result, the ECU must inject more fuel on a cold day than on a warm day.
Temperature Sensor I is a temperature sensitive resistor known as negative temperature coefficient thermistor (NTC). This sensor has a resistance of between 400 and 500 ohms at 50 degrees F. At 100 degrees F., the resistance is between 150 to 200 ohms.
Temperature Sensor I actually has little effect on the operation on most D Jetronic equipped vehicles. This is because most of these cars have tens of thousands of miles on the engine and are running very rich due to this wear. The troubleshooting consequence of this is that disconnecting the air temperature sensor during the diagnostic procedure may have little effect on the way the engine runs, and in some cases, it may actually improve the way it runs.
Temperature Sensor II
Temperature Sensor II is the coolant temperature sensor on water-cooled engines. Like temperature sensor I, it is a NTC thermistor. The temperature of the engine is important because the intake manifold design of fuel-injected engine does not permit the use of an air restrictive choke. Additionally, restricting the air to enrich the engine during warm up would cause inaccurate readings from the manifold pressure sensor. The warm up choke function is therefore performed by Temperature Sensor II.
When the coolant temperature is about 50 degrees F., the resistance of Temperature Sensor II is between 3000 and 4500 ohms. As the temperature increases to more that 120 degrees F., the resistance drops to less than 1000 ohms. Thus, the sensors work is done once the engine is warmed up. In addition, it must be continuously remind the ECU that the engine has warmed up. If damaged, Temperature Sensor II will not continue to partially function( such as the resistance values shifting) . It will completely fail, creating an open short or ground.
An open circuit in Temperature Sensor II or the wiring leading to it will cause the engine to run extremely rich once warmed up. Symptoms would be dark smoke from the tailpipe (most noticeable at idle), rough idle and poor power. Keep in mind that these same symptoms could also be caused by engine compression problems and ignition.
Should the sensor become shorted or the wiring harness grounded, the effects may not be noticeable at all when the engine is warmed up. The symptoms would be more like a carburettor with the choke stuck open-rough or erratic idle, stalling or hesitation when the engine is cold and progressively running better as the engine warms up.
Throttle Switch
The throttle switch tells the ECU when the throttle is closed, when the throttle is wide open and when the throttle is moving toward the open position. The switch consist of twenty two contacts, with a set of wiping contacts that move across them as the throttle progresses from the closed position to the wide open position.
One wiping contact is used to inform the ECU that the throttle is closed. Another makes contact only when the throttle is wide open and a third makes and breaks contact twenty times as the throttle opens. The electrical pulses created by the making and breaking of the contacts signals the ECU to open the injectors more frequently, thereby enriching the mixture for acceleration. This feature behaves much like an accelerator pump on a carburettor.
Symptoms associated with a defective throttle switch include a rich(smoky) idle and hesitation. An intermittent condition at cruise-which feels like you shut off the engine and immediately turned the key back on, can also be caused by the throttle switch.
To test the throttle switch, open the throttle with the key on but the engine not running. The injectors should open exactly twenty times, evidenced by twenty evenly spaced clicks.
Pressure Sensor
The D-Jetronic pressure sensor is known as a linear variable displacement transducer (LVDT). It consists of a pair of coils, one with about 150 ohms of resistance and the other with about 85 ohms of resistance. An iron core attached to a diaphragm runs through the centre of these coils. As changes in manifold pressure moves the diaphragm, the iron core moves inside the coils, causing ripples in the current flowing through these coils. This signal is used by the ECU to monitor the relationship between barometric pressure and manifold pressure.
The most common symptom from a defective pressure sensor is a rich running condition. Of course, rich running can be caused by several other defects as well.
Of all the sensors used on D Jetronic, this one both the easiest and the most difficult to test. Usually a simple resistance test of the coils is enough to determine if the unit is good or bad. On the other hand, the only way to be sure is to replace it with a known good unit.
Electronic Control Unit (ECU)
The ECU receives input signals from the pressure sensor, Temp Sensor I, Temp Sensor II and the throttle switch to determine how long to leave the injectors open. It is only able to respond to air-fuel ratio request from one sensor at a time. As a result, whenever a sensor fails, the tendency will be for the ECU to send the injectors a full rich supply of fuel.
The electronic unit has no serviceable components. In the event of a failure, the ECU is replaced as a unit. Failures are extremely rare and usually result in a no start.
D Jetronic that have an adjustable air fuel ratio have a detent potentiometer on the side of the ECU. This potentiometer can be used to fine-tune the air-fuel ratio during a tune up.
Pin # 19 and 25 of the ECU are connected to the fuel pump relay. When the ignition switch is turned to the on position, the ECU energizes the fuel pump to ensure that the fuel system is filled for ease of starting. If the engine is not cranked, the ECU will shut off the fuel pump after one or two seconds. If the engine is started, the fuel pump runs continuously until the engine is shut off.
In Tank Filter
Located inside the fuel tank is a screen or filter designed to protect the fuel pump from rust, dirt and debris. Although seldom the cause of a drivability problem, the in tank filter should be high on the list of items to check. In many cases, these filters have been ignored, even on well-maintained vehicles.
Fuel Pump
The fuel pump is a high-speed roller vane pump. It is capable of pumping fuel at pressures and volumes much higher than the engine or injection system would ever require. This type of pump is very efficient at pushing fuel bur does not do a good job of pulling fuel. As a result, these pumps are located very close to the fuel tank or even inside the fuel tank to reduce the chance to vapour lock.
The fuel pump can react adversely and even fail as a result of using some fuel additives. Use extreme when selecting them to ensure that they do not contain methanol or other corrosive substances.
A defective fuel pump can cause low fuel pressure that would result in symptoms such as hesitation, stalling and poor power. Often a defective fuel pump bypasses the poor running stage and simply stops operating. This causes the engine to die or keeps the engine from starting.
Fuel Filter
The fuel filter is the only real protection the injection system has from internal contamination from dirty fuel. Therefore, the filter should be changed every time the spark plugs are changed.
Fuel Pressure Regulator
The fuel pressure regulator used on D Jetronic systems consist of a valve connected to a spring-loaded diaphragm. The regulator controls the fuel pressure at 28-32 psi and is adjustable so it can ensure the proper fuel pressure throughout the life of the vehicle. Incorrect fuel pressure can cause a lean running engine if the fuel pressure is too low and a rich running engine if the fuel pressure is too high.
A defective fuel pressure regulator can result in high fuel consumption, rough or erratic idle and poor power.
Injectors
The injectors are solenoid operated, normally closed valves controlled by the ECU. Grounded to the engine block or chassis, each injector is opened by a 3-volt pulse from the ECU. The length of the pulse is only a few milliseconds (2 to 5), and it takes time for the injector to close from the spring tension. Thus, the injector is open for a total of about 3 to 6 milliseconds.
Very little goes wrong with the D Jetronic injectors themselves. An occasional burned out solenoid winding or restriction from contamination is the most common problem. Another problem is leaking from the hoses that attach the injector to the fuel rail.
Typical symptoms of injector problems include rough idle and poor power.
Fuel Rail
The injectors and the fuel pressure regulator are attached to steel tubing known as the fuel rail. The inbound fuel lines from the fuel pump and filter feeds fuel to the injectors and the fuel pressure regulator through the fuel rail.
As a passive component of the system, very little can go wrong with it except for leaks and restrictions.
Cold Start Injector
Also attached to the fuel rail is a solenoid-operated valve known as the cold start injector. Since the earliest applications of the D Jetronic system were four cylinders, the cold start valve picked up the moniker "fifth injector.” It stuck in some circles, even for six and eight cylinder applications.
The cold start injector receives battery voltage whenever the starter is engaged and is grounded through a device known as a thermo-time switch. The thermo-time switch is a temperature sensitive bimetal switch designed to provide a ground for the cold start injector when the temperature of the engine is less than 95 degrees F. A second circuit in the switch is an electric heating element intended to heat the bimetal as the engine is being cranked.
Consequently, the cold start injector should operate only when the engine is being cranked, the temperature of the engine is less than 95 degrees F. and for a maximum of five to twelve seconds.
