Does anyone know what the symptoms are of a bad ingnition control unit (the little gray box on the passenger fenderwall in the engine bay labeled BOSCH) are on a K-Jet 928? Would a bad unit cause no start, or does it affect how the cars runs, like during idle or accelleration?

 

Quite an open ended question .... but if a box of potted electronics has a failed component/solder joint/ .. , it is more likely to die than be intermittant ( not alyways true, just 'more likely'). Regards the early 16v ignition system, any intermittant issue is more likely due to the 'green wire' that carries the activation signal from the distributor to this Bosch module.

 

Mostly no start - however, some failures are heat or cold related. So it may operate fine until things get hot.

The real test is when you have a no-start condition, or if it stops running, do you have spark? The "green wire" (which may not be green due to age) that conducts the Hall effect sensor in the distributor to the spark box is a common failure point. It is a shielded lead, so the outer skin must not be damaged.

Since spark voltage is very high, you need to use caution when testing for spark or you can be shocked out of your shoes!

 

The TSZ Ignition control box used 78-84 (USA) seldom fails and nearly always is complete failure. Very rarely does one become intermittent (cutting out when hot) although that type of failure is far more common on 911s which used a very similar spark box. The two series resistors mounted to the inner fender near the radiator can fail and cut the ignition. Note that they are used to cut down the voltage to the coil the small wire to terminal #15 the + on the coil. When starting power is passed only through one resistor but in the run position of the ignition switch power passes through BOTH. So the coil always sees less than 12 V but in the start mode gets MORE than in the normal run mode. These resistors sometimes fail. As a test you can briefly put full 12v power to the #15 + terminal but that will overheat the coil !!

 

Okay, thanks guys! I don't have this problem, I was just wondering.

 

Hi guys I know this thread is quite old but can you tell me if the green wire is polarity conscious? My car refuses to start now. I am not sure if it related to the green wire to the Distributor. Cheers Carl PS The special plug at the end had disintegrated so I have connected the wire via crimp links to test if I could start the car. Hence I am not 100% which polarity is which cheers

 

Yes I believe so, one wire carries the signal, the other is a shield wire. There was just a recent discussion about this here https://rennlist.com/forums/928-foru...tion-wire.html
It is an expensive rite of passage but there is no alternative but to replace it.

 

Ritz its best to install a fresh green wire,
dont get talked into adding a new end to a marginal wire.
Its dumb idea and will lead to other issues down the road .

 

Cheers for the comments! I will check out the recent thread as well. Regards

 

​​Years ago, for a period of about 6 months, new green wires were assembled incorrectly, directly from Porsche (obviously supplied by Bosch.) You could get the engine started (barely) but the timing acted like it was way, way off (it was.) As I recall, the timing was extremely retarded.

 

This could never happen!! I mean, that would be like one of their suppliers for engine blocks for the Cayenne having the improper silica content and the engines failing after 40,000-50,000 miles. Pure lunacy.