no ignition pulse from the coil?

[drift a 944]

Im not very familiar with the whole ignition systems yet on the 944's but here is my problem:

My car would not start. I checked the spark on the black wire from the coil with a test light and there is a constant light, but no pulse. I believe this should be pulsating if its working correctly...

Other than that, everything else seems to be fine (fuel system, timing, compression).

What does this mean when there is a constant power (according to my test light), and how might I fix it, so that it is pulsating like it normally should?

Thanks in advance for ANY help on this problem, I need this fixed ASAP as this car is my daily driver .

[pelaeon]

I'm having the same problem on my '87 924S daily driver too man. So far I've replaced my Coil (stupidly, without doing proper diagnosing) DME Relay (needed it), and Speed/Reference Sensors.

SoCal has a good method for testing your ignition system:
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Go through the five minute quick test.

Check for power at the coil and at the injectors with the key on.

No power at the coil it's the ignition switch or related wiring.

No power at the injectors it's the DME relay.

Power at the coil and injectors points to a bad reference sensor. The connector may be corroded and/or not plugged in all the way or cracked and may have broken the wires inside.

Check if the sensor is working by pulling a plug wire, inserting a spare plug, grounding the plug to the intake and cranking the engine. If there is spark then the reference sensor is working.

Could be the second set of contacts in the DME relay. These are for the fuel pump. The fuel pump should run for a half second after you stop cranking. Have someone listen for it behind the passengerside wheel well. Check the fuse for the pump too.

On the early 944's you can use an alligator clip jumper between Aux fuse #2 and #3 to run the fuel pump directly. Count from the drivers door. Don't leave the clip on.

Come back with your findings.
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Here's some info on your Speed/Reference sensors that I've slowly memorized over the last few weeks... :-/

Clarks-Garage Speed/Reference Sensor info

[drift a 944]

awesome, thanks a ton man... i'll try it out tomorrow and see what happens.

somehow though, i have a hunch that it's the reference sensor on my car... we'll see.

thanks again!

[SoCal Driver]

Everything is simple -- once you understand it!

[SoCal Driver]

As you get further into the test there are several other items that will keep the spark from happening. One is the HV wire from the coil to the dist cap. This can get corroded inside the weather boots and broken from changing the oil filter. Either by removing the wire or just twisting it out of the way. The bolt holding the rotor to the cam shaft can come out and let the rotor twist just enough to change where the spark jumps. The last thing that can keep the spark from the plug is a broken timing belt. If cranking the car sounds like the high whine of a sewing machine then the timing belt is kaput.

In most cases it's the reference sensors.

You can bypass the DME relay on the later models with a bifurcated wire harness. Check out www.clarks-garage.com

[Razorback1980]

drifta944, check the spark at the plug wires. Place a spark plug in the wire and lay it on the intake...do you see a spark?

[drift a 944]

Razorback - i did try that and there was no spark.

I took the plastic cap off the coil and checked for power where the wires connect to. I thought my test light should be pulsating when i cranked the motor, but it just stayed on solid.

cap + rotor are new, and spark plug wires are new. I have not replaced the coil or either sensor.

Tomorrow im going to try out a few things. If it ends up being the sensors, is there a way i can just clean them or do I have to replace them completely?

thanks again to all!!!

[bazar01]

I took the plastic cap off the coil and checked for power where the wires connect to. I thought my test light should be pulsating when i cranked the motor, but it just stayed on solid.

+12V on the coil positive terminal should be constant.
12V at the coil negative (green wire) should be pulsating.


Try disconnecting the coil negative green wire.
Run another grounding wire to the coil negative with the other end free (do not let it touch the ground).
Put a spark plug on the coil plug wire distributor end and ground the plug to chassis or engine.
Turn ignition switch to ON. Test to make sure coil has +12V.
Touch the grounding wire free end to any grounding point.
Did you get spark?

If you did, your DME computer is not providing grounding pulse to the coil.
What is missing? Could be ref sensor or DME itself.

If you did not get spark, coil is bad.

[SoCal Driver]

I've used a spark plug wire directly into the coil then the extra plug grounded on the chassis. With the key on and 12 VDC at the black wire, grounding and then releasing the terminal where the green wire from the DME connects should cause a spark across the plug. You can run a jumper from the battery to the coil and not turn on the key to do this. With the power jumper you are just checking the coil. With the key on and cranking the car you are checking the coil, the sensors and the DME's reading of the sensor pulses.

There have been problems on the older DME's where the large transistor that fires the coil has come unsoldered from the PC board. Search for the thread on it.

If you have an older volt meter with an analog meter you can see the plus 2 volt pulse that the front sensor puts out when the pin passes by during cranking.

Check that the coil is good first. Then check at one of the spark plugs -- dist between the coil and plug. Could have a burned out rotor.