D-Jetronic articles series by Dr-DJet

12. Maintenance and Adjusting

Normally injection system D-Jetronic is considered maintenance free. But there are 4 tasks that you should complete during normal car maintenance.

12.1 Trigger contacts: Grease cams and check their weat

After removing trigger contacts from ignition distributor, you can check the contacts and grease the cams. If contacts show a coating or are oily, clean them carefully. As we have a very low current going through them, even a little coating can destroy signals from trigger contacts. to avoid oily contacts, do not use normal oil or grease for the cams. You should use special hot bearing grease. If contacts are oily, clean them with brake cleaner and check the source of oil. If necessary check also O-ring in foot of distributor. Now check the wear of the cams operating your 2 or 4 (8-cylider) trigger contacts. New trigger contacts cams have a nose of 2.6 mm that operate the contacts. When they reach 2 mm, it is time to replace them. Now you have three choices. Either you pay 400€ (in 2014) to Bosch for an 8-cylinder repair kit, or you ask new cams from Günter or me, or you use Günter's special measuring gauge to readjust the cams. which will keep your trigger contacts running for a while.

You can reduce the wear of your trigger contact's cams by greasing them regularily. tis is especially necessary if your T.C. does not have a grease felt. Bosch recommends regreasing those every 5000 km (3000 mls)! Use hot bearing grease and distribute a little on distributor shaft cams and grease felt. And as you have already removed the distributor, it does not harm to to do the same with the ignition contact. If you already have transistor ignition, also this contact does not carry much current and will not any more burn down. That's why qou will also find a grease fet on this contact. If you use oil or normal grease instead, you risk to get oil or fat onto the contacts. Such contacts will not trigger injection!

After greasing you need to reinstall your distributor and readjust its timing. I strongly recommend to verify dwell and ignition timing verifying speed and vacuum adjustment with manufacturers refrences. More often ignition is the cause of problems than your D-Jetronic. Professionals use an ignition oscilloscope to check secondary voltage.

12.2 Idle run contact in throttle switch: Verify adjustment

Now it is time to very adjustment of your throttle switch. It is important for idle-run mixture adjustment via ECU potentiometer and for over-run shut-off. To verify it, pull off the plug from your throttle switch (carefully, rubber sleeves are expesnive!) and attach a multimeter to pin 12 and 17. When you release the throttle, resistor should be close to 0. If you push down throttle a bit (1 degree or half a tick mark), resistor must become open. If that is the case, you are okay.

If it does not fit, you have to readjust your throttle switch. Loosen the 2 screws that fix it to throttle (on V8 it helps to remove vacuum pipe first) and turn it until it just starts to show 0 Ohm on your multimeter. Then continue half a tick mark (see picture) further in that direction. This half tick mark or 1° is a safety margin for temperature expansion. Verify again that you have 0 Ohm or open when moving gas pedal. If your gas linkage shows signs of moving heavily, open all connections, clean and regrease them. Then repeat adjustment of throttle switch.

There are 2 alterntive ways of doing this job. Instead of looking at tick marks you can use a 0,4mm feeler gauge between idle-rest of your gas linkage. Again this ensures that whatever temperature your engine has, the idle-run contact will always be closed when you release gas pedal. Remember without it, idle-run mixture adjustment and over-run shut-off will not work.

Tricky people use the over-run shut-off to provoke a sawing of the engine. Therefore you open the air pipe from auxiliary air valve to engine, creating massive false air. If your idle-run switch is closed, this will trigger over-run shut-off to cut off injectors when your engine has reached a definite speed. After that speed will immediately fall to something like 500 rpm before engine starts to un up to previous speed again. As this can only happen if your ECU has over-run Shut-off and your idle-run switch is closed, you can use it to adjust your idle-run switch.

At the end do not forget to properly reapply your connector and rubber sleeve to throttle switch. Replace broken rubber sleeves!

12.3 Check idle-run mixture and adjust it

Note: Very early ECUs do not have an idle-run mixture potentiometer. After having adjusted ignition and throttle switch, let your engine run warm. To adjust idle-run mixture, you can use an easy infrared measurment based exhaust CO measurment tool. Very precise 4 gas testers are unnecessary. By turning the potentiometer left, mixture will get leaner, right will make it richer. When going to biannual TÜV checkup of my car, they are always surprised that I can adjust the emissions from inside the car. If your mixture will not change by turning this potentiometer on ECU, you either have oil in cylinders or your idle-run contact is not properly connected to ECU or misaligned. Manufacturers tell you how high Carbonmonoxide should be, typically it is between 1,55 to 3.5% . In Germany up to 4,5% for classic cars without catylitc systems is allowed. Be aware if your car has a catalytic system or other exhaust eduction systems like secondary air injection. Either you must disable these or you have to measure at exhaust manifold before your catalytic system. Sometimes with old oil it is also useful to remove crankcase breather hose for a meaningful value.

To get rid off the misbelieve that this ECU potentiometer will influence fuel / air ratio also in part-load or full-load conitions: NO, this potentiometer is only active when idle-run switch is closed. As soon as it is open when you push down your gas pedal, it is disabled and has no influence on mixture. These were the early days of US emission control when they only measured emissions in idle-run.

12.4 Verify fuel pipes and fuel pressure in fuel hose

Especially the few cm of fuel hoses crimped on injectors like to leak fuel over time. Your engine will start to smell fuel and these hoses get a litlle wet. ATTENTION. This creates the risk of fire and you should act immediately. Reade more in chapter injectors how to solve this. After verifying all fuel hoses we also look at the hose from intake manifold to MAP sensor. Also it is a fuel hose type, it only connects vacuum. Now we adjust fuel pressure on the fuel pressure regulator. Turning right will increase the pressure, left wil decrease it. We need 2.0 Bar with a tolerance of +0.1 Bar. This means between 2.0 and 2.1 Bar, not 1.9 Bar and not 2.2 Bar (eception some Volvo cars have higher fuel pressure). I use a 2.5 Bar glycerin dampened manometer with a fuel hose connection gland for this task. Glycerin damping is nice to have. I run the engine in idle-run and adjust to 2.0 Bar. Remember, it shall not fall below! If necessary I go up to 2.1 Bar. When you are doubtful about the performance of your fuel pump or your fuel hoses, you can attach a 2 meter fuel hose and watch the manometer whild driving.

There is no more maintenance on D-Jetronic, but of course on ignition, engine, battery, ...

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