Hi,

I'm having a no spark issue on my '83 944. There seems to be no power getting through the ignition coil. I have looked through Clark's garage quite a bit and followed the troubleshooting guides on there, but i've met some roadblocks and not sure where to go. So far I've gone through a number of things trying to diagnose:

• Bench tested ignition coil with 12 volts and got spark
• Jumpered ignition coil positive to battery and got no spark while cranking
• Resistance readings on both the speed and reference sensors were in spec but i get no tach bounce.
• Tested resistance between the middle pins on both the speed and reference sensors to check for shorts, but they were good.
• Jumpered the DME relay which got the fuel pump running but still would not start

Unfortunately I dont have an oscilloscope to test the sensors. Ive read on these forums that the engine wont start under 300 rpm, and mine seems to be cranking slow. Might need a new battery. But I should still see tach bounce right? It makes me think that either the sensors are bad, the DME is bad, or there is some bad connection along the way. Inspecting the wires I couldn't see any visual spots of rust or corrosion. I also opened the DME and there was no water damage or corrosion that I could see.

Someone correct me if I'm wrong: The ignition coil is grounded directly to a transistor in the DME, and the speed/reference sensors tell the DME when to ground the ignition coil and send spark. Is there any other component in this circuit that could prevent spark? To me it seems like the DME and those two sensors are the only things that could prevent spark other than a bad ignition coil and bad connections.

 

Doesn't the factory alarm also have the capability to prevent spark?

 

90+% chance this is the sensors. In your shoes I'd replace them, and if it still didn't start I'd replace then again before chasing anything else (once in a while, not often, they're bad out the box)

edit: and definitely replace the battery if there is any question about it. Maybe do that first

 

Have you bypassed the DME relay to be sure the DME is powered?

 

I would pull the speed sensor sensor to see if there is iron chips on the probe. If so, then the gap was compromised.

 

Alright I'm going to try and get the sensors out this Friday.

Honestly I didn't know the car had an alarm system but I will also try to bypass that.

 

The 944 did not come with an alarm system as standard equipment. If it has an alarm system there will be a lock cylinder on the driver's side quarter panel just behind the driver's door. If there is no lock cylinder there then the car did not come with a factory alarm.

The DME relay can fail so I would take the advice above and bypass it temporarily and see if you are getting spark.

The speed and reference sensors are suspect and can be tough to remove if they haven't been out in a long time or have never been out. The o-ring on them dries and the sensors will seize in the mounting bracket. They can be removed with patience. I've found that silicone spray helps to soften and lubricate the o-ring and moving them, allowing the silicone spray to weep down further will help. I've seen people destroy mounting brackets to remove them. Once they are out, you may be better off just replacing them.

 

No tach bounce is a pretty good sign that there is a problem. Before you get new sensors:
1) Pull the multipin plug at the DME and check continuity there, so as to check the complete circuit. The connector for the sensors on the bracket at the rear of the cylinder head can degrade and become unreliable.
2) If you have a digital multimeter, especially one that can capture a peak value, connect the multimeter to the 2 non ground leads from the sensor, set it on AC voltage, start the peak capture, and crank the engine. You should capture an AC voltage between 0.5 and 2 volts. Do this for both sensors.
3) I recently purchased an oscilloscope for about $60 that will read the signal from the sensors. It is a DSO112A.

As always, ask yourself what has happened since the last time the car ran? In particular, have you messed with any wiring?

 

I tried matching up my pins to this diagram and got around 1 kOhm between 27 and 8, and around the same value when connecting pins 25 and 26. Not sure if those are the right pins but it seems the connection is good? Still working on getting the sensors out.

 

I did this too. My sensors were good. Those scopes are beginner friendly. I had never touched an oscilloscope and it wasn't hard. Money well spent.

One thing to consider. Buying the oscilloscope for $50 ish is a good idea. Never can have enough diagnostic test gear. If you do replace the sensors, they can be difficult to remove. So if a o-scope can verify yours are okay, that may save you money and some headache.
Go on Amazon or ebay and search "dso shell oscilloscope assembled"

 

Crank/reference sensors are the most common causes of no spark. Next would be the electrical portion of the ignition lock with a broken housing and not allowing contact to complete circuit. One time a really strange problem caused a no start. The stub on the flywheel that the crank sensor "senses" was broken so there was nothing for the sensors to sense. I believe someone either left a loose bolt in the bell housing that cause the stud to break or someone dropped the flywheel on the stud when the flywheel was out of the car.

 

2 cents after all these years including chasing multiple no start sagas..

blame often goes to the sensors or DME relay but i would bet that at this point, 75% of these issues are flaky old DMEs.
there's enough jostling to put a relay in or through other testing that a faulty solder joint or something might make contact again.

i first went to an aftermarket ECU in 2015 (still using stock sensors and DME relay)...after that, the only times that i ever had trouble starting was when i messed with the tune too much. software issue, not hardware issue.

in short, it's a smart move to either:
1) have your DME fully gone over by ECU doctors or similar
2) buy the NEW Ftech9 DME
3) go to an aftermarket ECU.

 

Get a good Battery
jump the relay
look at speed and ref sensors, as well as the likely very old wires they connect through to the DME.
knock on wood, I haven’t yet experienced an issue with a bad DME myself.


with my own 951 ownership, most of the issues have been with old wiring. It is relatively easy to replace (versus, say, a broken timing belt or a burnt valve) and the wiring is not too expensive. Lindsey Racing offers a good option for this, if it is suspect.

 

I got an oscilloscope and tested the sensors as you all said......... And it seems the sensors are actually working. I got around 9Vpp on the speed sensor and maybe 2.5 vpp on the reference sensor. And that is at the dme plug so the wiring must be good between there. Thats great, because i was having a lot of trouble getting the sensors out and now i dont need to . I tried putting a jump pack on the battery but nothing changed so it looks like i'll just grab a new battery and see where to go from there.

I can really only look at the car on weekends so its been a slow process so far.

 

Bump to let you guys know I got the car running! it was a bad DME.