hjalmarsson41

Hi everyone,

New to this whole thing with forums and such, hope this makes sense what I'll be describing.

I recently aquired a 86' Porsche 924S here in sweden that hadn't been driven since 2017. I got it home and started work with it, changing out almost the whole brake system and much more. After a couple weeks of work I started it on the stands and it started right up after 1-2 seconds. All good! or so I thought. I went on a test drive and it ran beautifully, no problems. When I got home I washed it, tested the lights and put it to rest. BUT, when I came back the morning after it wouldn't start no matter how hard I tried. I changed out to a known working DME relay, no difference. However I had no spark when testing the #1 plug against the block.

After this I tried changing out the coil for a spare(know it works) and it made no difference. I've tried starter cables with another battery, no difference. Speed and reference sensors checked and they are up to par. I've also checked for continuity between the green cable at the coil to the #1 pin at the DME harness and it shows 0 Ohms. Tried to shortcut the ignition via a cable from the positive terminal of the battery directly to the black cable on the coil and it made no difference either.

My biggest concern is the reading on the coil. When I measure the voltage between the black cable and - it read 11.4 V. Black cable to + read 1.0 V. Green cable to - read 9.0 V and green cable to + reads 3.4 V roughly. That seems strange...

All these readings were with the terminals attached to the coil which I have also checked the resistance on.

Any Ideas what the next step should be? Kind of puzzled what it could be, hoping not to be the DME computer... in that case I have a 944 85.1 which I may be able to switch between to test, if anyone knows if that is OK?

All help appreciated!

931guru

The DME computer for the 85/1 is not compatible with the 924S computer. Different generation.

There is an ignition coil test here:

https://www.clarks-garage.com/shop-manual/ign-04.htm

Jfrahm

Do you see tach twitch?
Can you see negative pulses at the coil? DMM on AC or Hz, or maybe a test light.
Test for spark at the coil vs, one of the plug wires? This points you to the distributor rotor.

As it has been sitting some concerns are the fuel system collecting debris on the drive, or the timing belt breaking. I understand you are not seeing spark however it's sometimes difficult to test for spark.
The timing belt breaking or stripping teeth will make for a "sewing machine" sound when cranking.
Plugged up tank pickup, injectors, etc. mean it should start on starting fluid, carb cleaner, etc.

hjalmarsson41

Good to know, sad to hear...

I've tried the procedure from clarks garage about testing the coil and the weird thing is that when I do all the steps 1-8 I get no spark at the plug. When I do the same on my 944, I doen't spark either. Only sparks at the contact which I am tapping on the battery ground to see if there comes any spark from the spark plug. And the 944 starts and works very well...
I tried the procedure with 2 different plugs so that shouldn't be the problem and as I said, I get the same results for the 924S and the 944, even though the 944 starts and drives perfectly. I guess the facts that the "tapping" contact sparks at the battery ground is a sign that the coil is OK?

hjalmarsson41

I can see the tach twitch and I've measured both speed and ref sensor.
I tried putting a test light on the ground terminal at the coil while cranking, but I could see no pulses. I also tried using a strobe light on the wire from the coil to the distributor and it showed no pulses.

Timing belt is OK, I've changed both timing belt and balance belt. Fuel should be OK since I drained the tank 2 times and put fresh fuel in it afterwards. New fuel filter and hoses aswell. As I stated before, it started up and ran beautifully just the night before all of this...

Jfrahm

Next I would look for the negative pulses at the 9 pin connector and failing that the DME itself
https://rennlist.com/forums/924-931-...ngine-bay.html

Green wire I think. Maybe disconnect it in case there is a short downstream.

orig944

If you have an alarm on the car, you should bypass it per the instructions on Clark's.

hjalmarsson41

I see... That was basically my conclusion aswell to look into the green wire, so I've already tested for short both from the coil to the 9-pin aswell as from the 9-pin to the DME pin #1 and couldn't find any, so that's nice atleast.

When I check voltage with a multimeter at the 9 pin connector I get these readings:

Ignition turned off:
+ (at the battery) to Green Wire(From DME): 12.8 V

Ignition turned on:
+ (at the battery) to Green Wire(From DME): 12.3 V

Guessing I've found my fault... permanent ground at #1 from DME computer or what do you guys say?

If there still is some reason to check, how would one go about checking for pulses at the 9 pin connector? test light? I'm unsure...

orig944

The alarm system grounds that wire to immobilize the car.

hjalmarsson41

Interesting... is that some kind of aftermarket alarm system u are refering to or is it mounted from the factory?
How do I go around this caveat?

I can see no signs of it having an alarm since the past, except that the electrical system is a bit of a rats nest in general. Aftermarket speakers/radio installed in the past....

orig944

I'm talking the factory alarm. If you have a keyswitch in the body panel just behind the driver's door, you have one. If it isn't there, you don't. Bypass instructions are the first item in the electrical section of Clark's Garage.

hjalmarsson41

I see, the 924S that I have does not have the extra keyswitch you are mentioning, so i'm guessing the problem is not with the supposed alarm

orig944

This is a what we call a red herring (not sure if that translates to Swedish.) Without the coil providing a current source to the wire, the wire will be pulled down by both leakage in the DME, and the tach circuit.

Can you explain how you measured the 9V at the green wire in the original post? Were you cranking, or just ignition on?

hjalmarsson41

I measured 9 V on the green wire with the ignition on and both the black and green wire attached to the coil.

orig944

If you unplug the DME connector at the DME, what are the voltages at the black and green wires? This will determine if you have a DME problem, or a partial short of about 20 ohms to ground on the green wire.