Date registered: Aug 2005
Vehicle: '81 280E turbo M119 project, ‘85 Audi Coupe Quattro, ‘71 BMW turbo 2002, '73 BMW 2002tii +others
Location: Hawai'i (Maui)
Mentioned: 0 Post(s)
Quoted: 22 Post(s)
D Jetronic Components Function
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 restictive 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.Also 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 noticable 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 noticable at all when the engine is warmed up.The symptoms would be more like a carburetor 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.
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 carburetor.
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.
The D Jetronic pressure sensor is known as a linear variable displacement transducer (LVDT).It consist 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 center 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 ata 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 servicable 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.
Last edited by Nutz 4 Benz; 10-09-2006 at 08:22 AM.