realporsche

Hello Everybody, I own a 1987 944 turbo and I have a huge problem:
The car won't start anymore. Everything checked, all sensors and followed all check-point from the Porsche repair manual. So I placed a used DME of a friend in my car and it runs!! For about 2,5 weeks, and after that it was problem again. Car stopped while driving and that was it. No reaction anymore. So I checked all things again, but without any result. Today I found out I get no spark at the sparkplugs, and i get a blockwave signal in my DME to KC/P unit (I measured with a scope) and all sensors and signals are ok. But I didn't check the output of DME connector 1 (to ignition coil). So I think it's a problem into the output from DME to ign.coil, and that's because I have no ignition at sparkplugs. My questions are:

1) does anybody know or this is the right way I am looking for?
2) Does anybody have had suchlike problems at his 944/951?
3) Who knows wich sheets belong to the specified Bosch parts at the analog-part from the DME? (so I can change those parts)
4) Does anyboy know a company (with a site) who revising and repairing Porsche DME's ??
5) Who knows more about the Bosch parts and know or any information can get for it?

So thanks, hope it will help a bit, cause I have already 2 broken DME's and I am afraid that if placing a new one it will be damaged soon again!

greez and
Dick

Tom M'Guinn

Something must be taking out the DME. Search for my recent thread called "DME Too Hot". That thread contains a link to a home-made schematic for the DME and lots of technical info. In that thread you will read about the DME's driver for the ignition and how it can be overloaded by a faulty coil. Have you checked the resistance of the coil between the primaries? If it is out of spec, then you can overload the ignition driver circuit on the DME board and blow it out. Also, when I was having problems, I learned that the faulty engine grounds can blow out a DME. The factory manual even says not to start the car without the motor grounds installed, or else "immediate" damage to the DME will result. (If the grounds are bad, the current will run through the DME ground and overload it.) Hope that helps. Let us know what you find. And, welcome to rennlist!

P.s., Vertex in Miami, Florida sells rebuilt DME boxes for a price.

Edit: here is the thread:

https://rennlist.com/forums/showthre...ht=DME+too+hot

Peckster

Hmmm. Yet another reason to talk to Iceshark about his grounds kit.

NeedPorscheSpeed

realporsche, I'm sorry you are having difficulties with your ECU. I found a company online for you that rebuilds ECUs. They are located in the UK, not too far from you. Here is a link to their site:

http://www.carelect.demon.co.uk/inspect.html

Best of luck to you!

realporsche

Mmm, thanx all for comment. I will give it a try what I read from some post a few years ago, when Mike was fixing his DME with the 1012 Darlington, I guess I get the same problem. I have called for the part and it will be in the shop tomorrow, so I am curious!!

Does anybody know what the problem with this transistor cause? My ignition coil is good, my car ground and DME ground are good, cables all are o.k. I have changed the voltage regulator on my altenator some months ago, maybe that will be the cause?? Maybe a too high voltage come into the DME or sth.??!

How is the alternator working? Is it just loading the battery or does it active work at the total motor running process (to give power to all kind of electrical equipments?

greez
Dick

realporsche

Oh yeah, I checked the resistance of the coil connections, all as it should be (as my Porsche repair manual told me), and I get another coil of a friend of mine, so I guess this isn't the problem.

How can I best measure theground points, and whhich ones do I check ?
-- Dick --

realporsche

Yessssss, I found out the problems with my DME unit.

Thx to the help of Mike's earlier posts, I find out the 1012 Darlington power transistor was broken. So I changed it in a BU-931, and put it first outside the DME housing to give it a try. I placed it on a coolerblock from a pc, complete with fan! Cause I tried the first time just placed the transistor on the block with tie-raps, and the car started immediately, but when driven 2 km the transistor burned out by overheat. So I placed it better on the block with bolts, cooler-paste and a fan, so now it works fine. And after a week or more km's driving I will put the transistor again into the DME unit and place it under the carpet.

I also found out that my power supply of the DME (a transistor who stabalise 12 V to 5 V) for giving 5 Volts to the IC units on the analog part (the under print and the digital-part (the upper print) So I get 12 Volts, but no 5 Volt. And that's not the way it will work

So I changed that part and yess, my beauty will drive again, hope that this will be the last DME problem, cause I spend sooooo much time on it (measuring all components and sensors and stuff like that)

So now I take some rest, put my car in the wax for near summer and take some well deserved

Mike

I'm really gratified to think I may have helped someone out with this problem

Not sure what causes this part to blow up - mine was 150K miles and 15 years old so I guess it didn't owe me anything...

Generally, semiconductors fail in their infancy, and later in life from some catastrophic event like too much heat (for too long a period) or some sort of overvoltage/ESD (electrostatic discharge) condition. Of course, it's possible that the device was running its entire life at or near its (intended or otherwise) maximum ratings in which it would simply fail due to "old age"...

This ignition driver is electrically right at the output of the DME computer (no protection to speak of - like a fuse) and driving a nasty load. I can imagine various electrical issues in the car giving it some grief.

The good news - the BU941Z device was only a few $$ and has worked just fine for 3 years now. The repair, once begun, only took an hour or so and you don't even get your hands dirty! So I guess it's just one of the car's weak points we should all learn/know about (though apparently not as common a failure point as the DME relay).

I bought two BU941's (so I have a spare device - somewhere) and based on the chatter on this list, I also just bought a new DME relay (though I haven't needed that yet).