calebPH

Howdy. I was told I should start a new thread because the old one was getting difficult to keep track of what I’ve done so far. I’m in a bitter battle with a no/weak spark condition that I can’t seem to crack the code on. Ruled out a vacuum leak, fixed the fuel issue I was having, a weak spark appears to be the final obstacle to keep my ‘84 944 from moving. It was firing briefly, before sputtering to a stop, but now the spark has gotten to weak to even make that happen.

So far…
I’ve cleaned and tested fuel injectors
Checked fuel pressure
Sprayed starting fluid
Compression seems to be good
Smoke test for vacuum leak
I’ve replaced the spark plugs
I’ve replaced the ignition coil from MSD back to Bosch, it’s getting 12v everywhere it should
Swapped out the distributor cap and rotor
I’ve made sure the timing was perfect
I’ve checked/cleaned/fixed all the grounds
I believe the speed/reference sensors are gapped properly, as the car was running briefly before and I was pretty careful with gapping them


Ummmmmm… I think that’s all— it is getting tough to keep track of everything hahaha

T&T Racing

My first test is to spray starter fluid into the inlet of the JBoot with another person cranking the engine. Does it start????

calebPH

Did that-- did not start, unfortunately

calebPH

I will add, that since the car has stopped firing completely. The tach isn't bouncing as much. Previously, I'd start it and the tach would move a lot, like the car was starting. Now, it's just barely wiggling when I turn the key and the MPG gauge kind of slowly raises to the middle position.

Also, dropping this video here again so everyone can see the spark:

Zirconocene

I don't remember the year of your car, but is there any chance it's a factory alarm issue?

ETA: One other thing that is critical in all of that is, are you able to test things out with a known good DME? If you have someone that will let you borrow one for testing, that may help narrow things down.

Good luck

harveyf

You would think that since it ran at one time, the DME is ruled out but stranger things have happened. I did have a no start issue once that was a bad DME unit.
You might want to edit your profile to list the city where you live. Help may be right around the corner. And yes, also do a signature line that lists the year car you are dealing with. You probably mentioned it in the other thread but that is the first thing folks need to know. They are not all the same.
OK, just saying. Are you sure the spark plug wires are sound and are hooked up correctly? Double verify that they follow the correct firing order and that #1 is firing at TDC. TDC from the mark on your cam housing that you use to align the timing belt.

calebPH

I mentioned it briefly in the first post on this thread, but it’s a 1984 944, so I don’t believe there’s an alarm system. Cannot for the life of me find the spot to update my signature, but did add my location. Would love help from anyone in Los Angeles, I will give you anything your heart desires.

I think I am going to pull the DME to inspect the solder joints today. But before I try that I’m gonna swap the DME relay and reverse the speed/reference connectors— just to see what happens!

Spark plug wires are 100% connected correctly, I’ve checked 20 times thinking surely I’ve done something simple wrong, but nope. My only hesitation is that I don’t technically know the age of the wires. They could be old, but they do look fine (the 3rd wire has a small little indention in the middle of it, but it’s receiving the exact same spark as the rest of them). I forgot to mention that I did swap out the wire going from the coil to the distributor cap, but that didn’t change anything.

harveyf

You might want to start a thread "Need to borrow a DME for an early 944, in LA"
I know where one is near San Diego.

walfreyydo

You still have not ruled out speed and reference sensors (despite this being mentioned on numerous occasions) - have these previously been replaced with new?

See engine speed/running signals (check the resistance, check the wiring and connections):
https://www.clarks-garage.com/shop-manual/fuel-16.htm

I can appreciate that you said you checked the gap on the sensors - but thats not going to help you much if the sensors/wiring/connections are bad.

harveyf

@walfreyydo I agree that the speed and reference sensors are always an area of suspicion, especially without a tach bounce. That said, I would be surprised that they are the problem if he is getting a spark.
Whatever, Clarks test is intimidating, as it requires an oscilliscope. A simpler method, if one has a decent automotive meter, is to check for AC voltage while cranking the engine. More specifically, go to the 36 pin plug, insert small leads at the two terminals for say the speed sensor, set the meter on AC voltage, set the meter on the closest range above 2 volts, and set it on Peak Capture. Crank the engine. You should catch a peak AC voltage of 1 to 2 volts. Repeat for the reference sensor.

walfreyydo

He says he barely (or has none) has tach bounce and has never actually tested them. I have not seen a confirmation of spark, only switching around spark plugs somehow helped? This is something that should be sorted before making assumptions about faulty DME's and less common issues. Its part of the diagnostic procedure on Clarks (which everyone should follow before coming to the forums and making assumptions about this part or that part).

He can at least run the very simple resistance checks (without needing an oscilliscope) and verify the speed/ref connectors aren't disintegrating (common for the plastic connectors to fall apart). We also know the OP "re-gapped" the sensors but unsure if it was done correctly - incorrect gapping could correlate with the weak or no tach bounce, and given the OP's knowledge this is certainly a possibility. Bad/failing sensors would also correlate with this.

Diagnose and verify the critical systems first. Speed/ref sensors are critical to both spark and fueling.

Also it wouldnt be a bad idea to replace spark plugs just to be sure (they're cheap).

orig944

If it is sparking, the reference sensors and wiring are good enough. The DME is processing the input and driving the transistor that grounds the coil.

Since you have both weak spark, and low tach bounce, I would suggest looking at the common area.

1) The green wire between DME pin 1 and the splice for the wire that goes to the tachometer.
2) The connection at DME pin 1
3) The splice
4) The entire ground path for the DME (DME connector pins 5, 16, 17, 19,) condition of wires, and ground connection to the frame.
5) And, it could be the DME itself. Specifically, the output driver transistor could be damaged or have a bad connection internal to the DME, like solder cracks.

orig944

Also, check the black wire to terminal 15 of the coil from the fuse panel. It should stay at battery voltage while cranking. Since the battery voltage drops while cranking and you are going to be looking for pulses, it would be difficult to find them looking at just the voltage. A better way is to look at the voltage differential from the battery positive. To do this, put your voltmeter negative lead at the coil pin, and the positive lead at the battery positive terminal. Since this will be pulses as it cranks, an old analog meter would be better, if available (an oscilloscope would be ideal.) Small voltage pulses (maybe half a volt) are expected due to resistance in the wiring. If you get big ones, there is something wrong with that wire path. Actually, I would do this before the above, since this is much easier than getting under the dash.

Also, also, if your car has a key in the body next to the driver's door, it has an alarm system. If so, I would jumper it out per Clark's instructions just in case. It is unlikely to be the problem, but the green wire leads there as well.

Good Luck!

calebPH

Spark plugs have been replaced twice (most recently a few days ago, still in fine condition) and the speed/ref sensors are both new because the connectors on the old ones were starting to fall apart when I rebuilt the engine (and the gapping was triple-checked). I can pull them again and quadruple-check, it's just a pain in the butt to get my hands back there and I'm already pretty certain they're solid. I did have a solid tach bounce up until the day after I replaced the ignition coil with the OEM coil, when the car was still firing, then tried it the next morning and it won't fire at all.

@orig944 thanks for the diagram, gonna inspect all of that when I get some free time.

Took yesterday off from working on the car because I needed a break, but gonna pull the DME now to inspect for cracked solders. Good idea on the thread for borrowing a DME, gonna do that now.

951Dreams

If your feeling brave, since you already have it out, you can reflow the joints. I just did this with my cruise control board last night and fixed my 6+ year old issues with the cruise. But of course, there is always risks using extreme heat on boards. I would for sure try it before buying a new DME however.

Have you tried reinstalling the MSD coil to see if your tach starts acting right again? Seems strange that swapping the stock coil back in caused new issues.

Since it was running, and just stopped, I have a feeling some wire somewhere shorted/melted. Something the DME needs. My rear wiper switch melted and burned about 6" of the ground wire insulation off. Burned the carpet as well under the shifter trim. I'm lucky it didn't start a fire. My 28 fuse (I think it was 28, the one for the rear defrost) I found MELTED into the fuse box the other day. Yet, it wasn't blown. These things are getting old and doing crazy things. And there is something about German wire insulation, as my BMW has the same issues with wire insulation crumbling. And it's a 2001!