Motronic control unit problem
 

Hi guys,

89 3.2 carrera..
I think the unit has a fault, (problem with car starting) have checked everything else and having an ECU check on Friday.. i thought id pre empt the case of having to get a new one... my part number is 91161811119..obviously checked this out cost wise and WOW!!! Is there an alternative?

buy a used one. Various sources in the US will save you many $. You can also have yours rebuilt, most likely.

Can you find a friend to borrow theirs for a bit to ensure that is the problem?

I think it's fairly rare to have a problem with the Motronic unit itself. When you say you have checked everything else, could you be a bit more specific, and have you replaced the DME relay? I second the idea of trying a known 'good' Motronic unit first if you can and potentially avoid an unnece$$ary purchase. Good luck!

hi guys, thanks for the info, yes ive replaced the dme relay, had the starter rebuilt, cleaned all grounds..car will start in the morning, if i run it for 10 mins then try to restart it turns over but wont fire, no spark to the dizzy.. if i push it 5metres it starts...!
I think ill look at some used units if the ECU shows its the box thats faulty.. i really hope it isnt!

I can't see how the Motronic unit would be affected one way or the other by this, so I doubt it is the fault. Maybe the pick up sensors next to the flywheel?

"if i run it for 10 mins then try to restart it turns over but wont fire, no spark to the dizzy.."

You need to check not only for spark when it doesn't start but also the injectors:

1. Always check for spark at the coil wire first & not at the spark plugs to
eliminate the distributor as a problem source.
2. Get a test light and check for a spark signal at the coil terminals.
3. Use a noid light (like Pelican sells) to check for an injector signal.
4. Then check for the proper DME ECM powers.
5. Then check for the proper speed & reference signals..

If you can push-start the car but not crank-start the car then:

1. The starter motor is not turning fast enough when warm/hot, or
2. The starter motor is causing a large voltage drop which may be
enough to inhibit the DME ECM from functioning.

thank you very much for your help its much appreciated, i have a nice check list to follow now tomorrow!

The entire Motronic control system is fairly complex.
Any part can make the chain go bad.
As opposed to buying expensive parts for a swap; why not try to find a fellow 3.2 driver who is willing to assist in swapping the control units.
If the problem switches car, you know what it is; otherwise continue swapping other easy to swap parts until the problem moves with te car....

I found this thread in the archives. Tastytel had the exact same symptoms as my car; would start when cold but once warmed up it didn't work. Just to close out this thread for future readers I contacted Tastytel via email and he told me that he ended up having a garage change out the reference and idle speed sensors at the flywheel and the car never exhibited the problem again. Unfortunately Tastytel sold the car shortly after this thread as well.

You are describing he exact same problem I had on my 84 Carerra, it was the Motronic. A darlington transistor was failing due to age/heat fatigue. I had it repaired No problem. How did I find out it was a heat related failure. It would intermittently fail, I went through the sensor diagnostics and after all said and done, a cold source placed on the Motronic (carefully wrapped to eliminate moisture/condensation build up) and it started right up. You could use a condensed air can held upside down to cool it off two. Don't start it with moisture on anything.

Hi DonMo,

So even though the transistor had failed in the DME, once the car was started it would run fine? I would have thought that if it was an issue in the DME related to a component or solder joint that once the components on the PCB warmed up and begun to operate outside there designed specifications that the vehicle would have begun to run poorly.

Does anyone out there have a layout file or schematics for the DME?

I had already handled the cold solder joints, the transistor would heat up then fail, cool off, start up, heat up then fail, then it finally died.