StoogeMoe

Ok, let's not get side tracked. The DME relay controls the fuel pump, so that is not his issue. He has a no spark condition. Pull the center wire going into the distributor and see if you are getting spark there. If you don't get spark there, then it could be the alarm as mentioned, a bad coil, or a bad DME computer. Check clarks-garage.com on the steps to trouble shoot no spark conditions. Here's the link Ignition System Troubleshooting

Tom Rathjen

I think I found it. Bad rotor. Looked all burned in the middle, and no continuity at all between center and tip. That's got to be it. I have one on order, so we'll see when it gets here if that was indeed the (only) problem.

Thanks!

athens944

Bad rotor made mine sound as if it was not getting fuel. Be sure not to strip the small allen head screw. Also, tighten properly they can vibrate loose over time.

Tom Rathjen

Will do!

I did get the old one off successfully.

New one probably won't get here till next week. So until then....time to work on the other project Porsche (928).

mtccl

This was my guess based on you found fuel OK. Sometimes it is the simple things.

Reading the thread I thought fuel...no OK, DME..probably not.....Coil, rotor, rotor cap, coil wire.....you think rotor so I agree.

Burned a rotor on a 88 Trans Am after installing a hypertech system and went through the same series believing it couldn't be anything electrical....new cap...new rotor....new life.

Let us know.

Legoland951

A bad rotor can be identified definitively by spark coming out of the coil but none getting to the wires to the plugs. No spark at all from the coil would rule the rotor out. Is there definite spark from the coil?

Tom Rathjen

Well.....the new rotor finally came in the mail. Installed it this morning no problem. But.....it still won't start. Still no spark. (With a plug connected to a spark plug wire and grounded, and also connected directly to the coil and grounded.) I can induce a spark by manually pulsing a ground to the negative terminal on the coil (with the ignition on).

I'm afraid it is the DME. I have checked the speed and reference sensors the best I can without an oscilliscope: I get the correct resistance at the connectors at the rear of the engine, and at the DME connector. And the tach wiggles when I crank it. So I just don't think they are the problem.

Unfortunately, I don't know anyone with a working 944 to swap DMEs. (Heck, I just moved to a new state so I don't really know ANYONE.) But I sure hate to go out and order a new DME without really being sure. Any suggestions to verify this is my problem?

PS--In trying to troubleshoot further, I verified that the negative wire (green wire) to the coil has good continuity and no shorts to the DME connector (pin #1). And I have tried to see what is happening at that wire when connected to the DME. It is supposed to pulse ground, right? When I test for continuity between the green wire (disconnected from the coil) and ground, I get between 2K and 5K ohms resistance, depending on how the polarity is connected to my meter, when the engine is not being cranked. (Shouldn't it be zero?) Then when I crank it, the needle on the meter jumps around. When I measure voltage at the wire coming out of the coil (going to the distributor), I get a constant 12 Volts.

Thanks!

Legoland951

I can ship you a 87 DME which is slightly different 944.618.121.04 versus the .05 version but will work along with a couple of reference/crank sensors from a running car. PM me for info.

Tom Rathjen

Hey, can anyone explain mechanically how the speed and reference sensor work? That is, what is really happening in there at the flywheel? Is there "something" attached to the flywheel that these sensors "sense" as it spins? Or is there a reference "mark" that is detected by the sensor in some electro-mechanical way? I'm wondering what, if any, failure modes there could be related to these sensors that do not involve the sensors themselves failing. i.e., is there something that could have broken off of the flywheel or something like that?

Thanks!

Legoland951

One counts the teeth on the starter ring gear and the other read "stubs" sticking out of the the flywheel.

PeteL

Watching this one closely, as I have the same problem.
I am leaning towards the computer too.

Papamurphdog

Did you jumper the alarm system yet? Try that... Also, did you R&R/clean all the grounds on the car? The three on the back of the motor get really cruddy and corroded. With your recent posts it may just be a grounding issue or a simple short somewhere... jumping the alarm will serve to eliminate one possibility... you can build a simple DME relay jumper also, so you can listen for the fuel pump and also eliminate a variable during trouble shooting... Just trying to help...

roman944

here I am too, merge 3 threads into 1!

Tom Rathjen

It turns out that my car is not equiped with the factor alarm. So that is not the problem. And I believe I have checked and rechecked for shorts and good grounding, and come up OK. Thanks for the suggestions, though.

At this point I'm not sure what more to do until I get the spare DME (lent by an extremely nice member of the forum!) in the mail to try.

I did run across a business on the internet that advertises it will diagnose and repair Porsche DMEs. ECUDoctors.com. Has anyone had experience with them?

Legoland951

I sent out the dme from my track car so I know it works. In the same box are speed/reference sensors out of a running car. You should be getting it shortly via usps priority.