D-Jetronic articles series by Dr-DJet

7.1 Trigger contacts function

Trigger contacts are located in lower body of your ignition distributor. Bosch D-Jetronic ignition distributors are therefore a little bit higher than others. Trigger contacts are mounted by 2 screws to distributor. After removing them you can take out trigger contacts easily. It either has a 3 or a 5 pin connector or a short cable on the outside. Trigger contacts are inside and are operated by cams in located on distributor axle. There is either 1 cam or 2 cams in a 90° angle to each other which open and close trigger contacts. These contacts are very similiar to igntion contacts, just you have 2 or 4 of them. 4- and 6-cylinder engines operate with 2 trigger contacts opposite of each other, while 8-cylinder engines have 4 contacts in 2 levels. To be clear about it: Trigger contacts are wear and tear that will last roughly 60'000 km. They used to be thrown away like old igntion contacts and replacement was just 60 DM (30€). Nowadays Bosch has found a gold mine and sells new trigger contacts for 140 to 400€ or repair sets for 8 cylinder engines at more than 400€. In 2012 2-pole trigger contacts were just 100€. Such hyper inflational price increases cannot be explained with decreasing demand of less D-Jetronic cars. Luckily you can only buy new trigger contacts for BMW, but it fits also elsewhere. They seem to be that last ones in stock. But don't worry: Luckily there are other solutions than just Bosch itself.

What do these trigger contacts influence? Via pin 12 they are linked to ground and they forward this ground level via these 2 or 4 contacts to ECU. ECU reads 2 informations from these contacts:

1. Engine revolutions. Depending on type that means one or 2 pulses per crankshaft revolution.
2. Time of injection of 2 or 4 injector groups following ignition sequence

An injection group is a group of two (4- and 8-cylinder) or three (6-cylinder) or even six (12-cylinder) injection valves. They share a common driver in ECU (or amplifier in Jaguar) and inject at the same time. Grouping is according to ignition sequence and commonly is as follows. Beware that especially 4-cylinders have various igntion sequences, you will find deatils in Pinout of ECU per car.

Exact timing of injection versus cranksahft position is not reqeusted. Fuel is injected into intake manifold and waits there until intake valve opens and it is sucked in with air into cylinder. ECU however doses fuel amount exactly.

Citroën SM (not Citroën DS) and Jaguar do not have Bosch distributors and therefore have their own solutions. Citroën SM uses trigger contacts built by SEV Marchaval, that are fixed with clips to distributor. Jaguar has a no wear and tear solution with reed relais or later hall sensors. It is surprising that Jaguar's 12 cylinder only has two injection groups.

7.2 Compatibility and changes over the years

An important finding after research and foto comparisons in various forums was, that trigger contacts for BMW, Citroën DS, Lancia, Mercedes-Benz 6-cylinder, Opel, Renault, Saab and Volvo are all compatible despite of different Bosch numbers.

During the years following improvements were made, but none of these makes a basic change in functionality:

• Greaser on trigger contacts and behind distributor axle, reducing wear.
• Partionally a felt as sealing between trigger contact and distributor housing, later again omitted
• Small 90° 3 pin connector instead of wider 75° connector. Both are compatible.
• Bent instead of welded noses on carrying steel sheet
• tranparent protective caps over contacts
• Pertinax cams mounted via rings instead of rivets

Additionally some manufacturers used a special 3 pole connector on wiring harness with 2 latches. As long as you do not use 0,8 instead of 0,5 mm blad receptables the normal 3 pole connector should reliably connect.

7.3 Maintenance

Pertinax cams show a wear. In order to minimize it, you should grease where they contact the cams on the distributor axle. ATTENTION ! Do not use normal grease. It would start to drop due to engine temperature and finally stick to your trigger contacts, disabling them. maybe that is the reason why BMW recommended not to grease them at all. You need to apply a special high temperature grease. Early version did not have the gresing felt, they need regreasing every 5 to 10 Tkm. During maintenance you just add a LITTLE high temperature grease to distributor cams, that is enough. Using Norbert's special plug gauge you can verify at the same time, how much it is already worn and readjust them. You can also check whether oil has made it on the contacts and clean them. If so clean them with brake cleaner. Todays trigger contacts have a protective cap over the contacts. You cannot adjust trigger contacts otherwise.

So please don't worry whether you have to readjust trigger contacts when remounting them. That is not the case. However you should readjust ignition timing if you removed your distributor. Don't forget to verify revolution and vacuum adjustment of your distributor at the same time. Remember you have a contact operated direct or transistor igntion.

7.4 Verify and repair

Many workshops immediately call for new trigger contacts. Sometimes they are right, but often they aren't. You can use a duty cycle measurement tool or a oscilloscope to verify during engine run (or trying to start) whether your contacts still open and close properly. Those 2 or 4 injection groups are supposed to close one after the other (that is the trigger, it does not really matter how long they close) and keep closed for 135° (theory) to 160° of distributor revolution. Pin 21 closes roughly 15° before piston 1 upper dead centre mark on distributor. Trigger contacts are not adjusted like distributor contacts. If you do not have a duty cycle tool, use your multimeter and measure voltage levels on each trigger contact. If they are continuously 0V or 12V, your contacts do not operate. Values of 2 ... 6V should be seen here. And they should be the same on all trigger contacts.

Bosch offers a repair set for 8-cylinder trigger contacts for 400€ (2014). In fact that replaces the whole trigger contact without connector and housing. BEWARE: You should mount this repair set straight to the back or you risk to have different opening times on left and right side.

If you need a cheaper solution. ask me here for Günter's replacement cams (unfortunately he passed away) operating the contacts. Or you just use Norbert's ingeniuous plug gauge to readjust your contacts properly if there is still enough cam left. But one day they will be completely worn. That is why I recommend you to properly grease them! Another possibility is to put some spares in stock. Trigger contacts have a hyper inflation price develeopment. If that continues I will be able to buy a house in 10 years from my stock ;-). New cams on trigger contacts have 2.5 to 2.6 mm, normally they are worn when they reach 2.0 to 2.1 mm. Bosch has ceased to sell 4 and 6-cylinder trigger contacts in 2017. They are harder to repair than 8-cylinder T.C. Cams on 8-cyl T.C. are fixed with a lock washer. On 4- and 6-cyl T.C: cams are fixed by border crimping on the axis. That is why I contructed something to replace the complete axis as well. But still on all T.C.s one has to remove the rivet that holds the cams to the spring brackets and replace it with a new one.

Your Dr-DJet (Volker)