Regarding my 1987 944 NA:

So I've had this ongoing electrical problem pretty much since I bought the car. I've posted a few times here and on Rennlist as things develop. I post now because the problem has changed once again.
To summarize the story so far, the car has an intermittent electrical failure that prevents it from starting. It always starts after it has sat overnight or for several hours. 95% of the time, it starts within 5 minutes of being shut off. After I drive it, it can sit for 30 or 45 minutes, sometimes more, and it will not start. I've discovered that the no-start is due to a lack of spark. The starter turns over, but the engine does not fire.
Things I have replaced:
Ignition wires
distributor cap
distributor rotor
removed old aftermarket alarm system

I have a new ignition switch, but have not yet been able to get it in.

So up until last week, the car had never had a single problem while it was running. The only problem was starting the car within the particular problem interval. Last week, I was in the drive through line at the bank on an 80 degree day here in Minnesota. The car died in line while it was idling and would not start again. I popped the hood, put a spark plug on the end of the coil wire, and sure enough, no spark. This is the first and only time this has happened while the car was running, but the symptoms were identical to any of the other no-starts.

I dropped it off with a reputable import mechanic later that night. They have run the car, driven the car, and started it at various intervals since shutting it off. Of course, it starts perfectly every time. I have had problems nearly every time I have driven it for several months now. I am getting extremely frustrated. Does anyone have ANY suggestions? I'm about to give up on this thing.

 

Do you have a leaking battery tray? I had a similar problem and tracked it down to water leaking from the batterry tray, under the dash, to my DME. You may want to take a look. I removed and opened my DME and found water internal to the enclosure. Do not forget to disconnect the battery before removing the DME if you suspect this is your problem.

Good Luck

 

The first thing I would test/replace in ANY car with that problem is the ignition coil.

 

Two quick checks/fixes: try swapping/jumpering the DME relay (beat everyone else to it) and clean your grounds.

 

^secound that change the coil. Same thing was happening to my 67 and my 85e30

 

Engine refrence and/or speed sensor could be the culprit also. Without the speed sensor signal, the DME won't deliver fuel or spark. Without the refrence sensor signal, you'll get a spark, but the ignition timing is off.

 

CHECK YOUR GROUNDS

 

I'm inclined to say that it's not something that become detached sporadically or loses contact. Before last week (so for the past 9 months) the car has always been great once it is running. Also, the interval in which it won't start is really predictable.

 

Sounds like coil to me also

 

I have never seen a coil fail in a 944. The most likely problem from the symptoms listed from most likely to least likely.

1. check/replace dme relay
2. check/replace speed/reference relay.
3. very unlikely but your chassis to bell housing ground or your battery to chassis ground is bad or not connected.

 

Speed/ Reference sensors, or bad dme are my guesses.

 

I had the exact same problem. The car wouldn't start at the most inconvenient or embarassing times. All other times it would start right away. Almost always started when ice cold. If it didn't start, it would always start within 5 minutes of cursing. My fix was the speed/reference sensors. A clue that it may be the sensors is the tach won't bounce when cranking the engine. The tach bounces a tiny bit on a normal engine being cranked.

 

Could it just be that I need a new DME?

 

It could be anything that was mentioned so far. Have you started testing components?

This isn't a car where someone is going to say "x is your problem - replace it and everything will be OK". They're simply too old at this point, even for issues that could have been solved that way when they were new. Electrical problems can be, for example, the sensor itself, the wiring in-between it, the DME harness making a poor connection, or the DME itself in a lot of cases.

 

Make sure the tiny little ground wire is bolted tight onto the crossmember

And try a new coil like the others said. Check the wires to it from the battery also.