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So I had my 1988 951s out for a track day several weeks ago, and after 70 event free miles over 3 or 4 sessions, the car suddenly lost power and began bucking badly. The tach was bouncing around, and it was impossible to rev past 2k or so. I was able to nurse the car off the track, and then it died and would not restart.
No tach bounce and no spark, so it would seem to be ignition related. I swapped out the distributor cap (rotor looked fine) and the coil to see if that was the suspect, but change. Same for the DME relay (I tried both a stock and a Focus9 solid state). No tach bounce and no start.
I pulled the spped and reference sensors and I did see a nick on the speed sensor, so I replaced both the speed and reference sensors. Earlier this year I replaced the reference sensor harness with the Lindsey racing one, so that should not be the issue. I’ve also re-gapped the speed sensor to 0.8mm, and still no spark.
I’ve also gone through the ignition diagnostic procedures from Clarks Garage (Fuel 16), and all checks out OK. The only thing I could not do was check the reference sensor outputs with an oscilloscope as I don’t have access to one.
As all else checked out, I concluded that perhaps the DME itself was flaking out, and so I decided to upgrade and bought Focus9 unit instead of having the original repaired. That came back 3 days ago and I finally had a chance to plug it in (with high expectations) and …nothing. Still no start and no tach bounce. Quite frustrating as you may imagine.
The only thing that I can think of that perhaps the threaded pin on the flywheel is damaged or missing (given the nick in the reference sensor) and is not triggering the reference sensor? I’ve pretty much checked or replaced every other component in the ignition system, but before I remove the reference sensor bracket and start looking for the stud, I wanted to see if anyone had any thoughts as to what else I may have missed.
"Need to diagnose before just swapping parts". Amen to that! Especially on these cars. I do miss my '65 Corvair at times like this where all you have is points and a condenser, although fuel injection >4 carbs any day.
I (thought) had done as much as I could along that route. I replaced the harness as it was crumbling and the speed and ref sensors because of the nick, and also because they are often the culprits. I also went through the ignition diagnosis per Clarks, but the end of the line, after all else, is that maybe you have a bad DME.
I just watched the Specialized ECU Repair video regarding checking the voltage output at the DME plug. I actually tried that before I replaced the DME, but I had not realized that the VOM should be set to AC voltage. I tested using DC and (unsurprising in retrospect) got an unsteady reading. So I will try that test using AC voltage tomorrow and see where that takes me.
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Last edited by Porvair; 10-27-2018 at 07:02 AM.
Reason: typo
I am assuming after replacing all of the above mentioned parts, there is still no tech bounce. It appears that there is no power to the DME/ECU and testing the ignition switch is the next step.
To test, remove the DME plug from DME unit. I believe Clarks Garage has a ignition switch test procedure and a key to the individual pins 1-35 or Google for a key diagram. Turn the ignition key to the start position and determine with a volt-ohm meter the voltage supply to those designated pins. It should read 9 plus volts not 12 volts as stated in the Clark Garage procedure to test the ignition switch. It would read 12 volts with the ignition key in the run position.
Also try bypassing the KLR. You disconnect the KLR plug and short pins 9 and 16 iirc (check the pinout diagrams to be certain - what you're trying to do here is connect the KLR's ignition signal in and ignition signal out together). If the car starts in that configuration, the KLR is the issue. Check power to the KLR too.
And don't rule out the possibility of getting a faulty speed/reference sensor out of the box. We've had several cases of that (for example this one and this one if you're looking for a few days of light reading )
And don't rule out the possibility of getting a faulty speed/reference sensor out of the box. We've had several cases of that (for example this one and this one if you're looking for a few days of light reading )
Learned over 40 years ago that just because it's new doesn't mean it's any good. Now days it's even worse with components made in one "Worker's Paradise" or another and sold under an old respected brand name.
So I have confirmed that the DME is receiving power (pins 18 and 35 of the DME plug). I checked my notes and I had confirmed that a couple of weeks ago before I replaced the DME. I also jumped pins 9 and 16 of the KLR to bypass that, but no dice. No tach bounce.
I also checked the AC voltage at the DME plug per the Specialized ECU Repair video. Speed sensor seems OK as that is registering a strong signal of about 4v, which is well above the "over 1v" mentioned in the video. The reference sensor measured 0.1v initially and then dropped to 0.04v, which are within the "less than 1v" mentioned in the video.
As I said, I've also had previously followed all of the other DME test procedures per Clarks (i.e., testing voltages at the DME relay) and no faults.
I can't think of anything else in the ignition system that could be faulty. I may check the coil resistance again, but if that were bad (and the only problem) I believe that there would be at least some tach bounce.
I had my son rotate the engine while I checked the flywheel threaded stud. Looks like it is in poor condition, so the reference sensor is not picking up a good signal. There is a good thread here about a similar problem https://forums.pelicanparts.com/pors...t-944-a-2.html
Given the tight constraints and condition of the stud, I can't see this as being an easy fix. I cannot imagine that the stud can be extracted other than a rather lengthy disassembly and pulling the flywheel.
So I have confirmed that the DME is receiving power (pins 18 and 35 of the DME plug). I checked my notes and I had confirmed that a couple of weeks ago before I replaced the DME. I also jumped pins 9 and 16 of the KLR to bypass that, but no dice. No tach bounce.
I also checked the AC voltage at the DME plug per the Specialized ECU Repair video. Speed sensor seems OK as that is registering a strong signal of about 4v, which is well above the "over 1v" mentioned in the video. The reference sensor measured 0.1v initially and then dropped to 0.04v, which are within the "less than 1v" mentioned in the video.
As I said, I've also had previously followed all of the other DME test procedures per Clarks (i.e., testing voltages at the DME relay) and no faults.
I can't think of anything else in the ignition system that could be faulty. I may check the coil resistance again, but if that were bad (and the only problem) I believe that there would be at least some tach bounce.
What else could I possibly be missing?
Yes you would definitely see tach bounce regardless of the condition of the coil - assuming the tachometer is working of course.
It might be time to beg, borrow or steal an oscilloscope and start taking a closer look at the signals.
When testing the crank sensors, however you do it, I'd advise removing the cover on the DME plug so that you can probe the various signals while it's still plugged in to the DME. I don't know if you read those threads I linked to earlier - they are very long - but in at least one case, the sensor tested fine on the scope with the DME plug disconnected, despite being faulty. This is because the shield and signal wires were shorted together somewhere inside the sensor cable. Since they're both still separate from the ground, the signal looked fine with the plug disconnected. But once it's connected up to the DME, the shield is grounded, and so the signal was grounded too. Probing the signal with the DME plug attached will tell you for certain if you're getting the signals you need.
Thanks John - I'm reading those threads now and there are so many possibilities. I may need to pick up one of those mini oscilloscopes from Amazon and start probing.
The Stud should stick up 5 mm from the Flywheel. to sheck that you could gotot the hardware store and get a collar with a steel rod.. (Picture) This is what I use to gap, you could also use it to check the height of the stud. If the stud is too damaged (not the correct height) maybe you can remove the bracket to get to it. The Stud and the Gap need to be correct.
Did you test the reference and speed sensor at the DME connector plug pins? This step verifies the integrity of the harness wiring, open, short, or good
Yes - I tested the resistance of both sensors at the DME connector plug, and they were within spec. Its a new harness as the old plugs at the back of the engine were crumbly.
I have also swapped in two new and the two existing reference sensors in every combo, with no change. I also confirmed that the metal external sleeve of the sensors did not have continuity with the middle pin of the sensor plug as I've read that could cause a problem.
I'm going to pull the starter to see if I can get a better feel for the condition of the reference pins. I imagine that those would be a nightmare to change if they are all damaged.
I did check to see that I had not revered the two sensors. The upgraded harness is clearly marked by color.
Also still not sure why the car died in the first place.
I did buy a mini oscilloscope and hope to get some readings this weekend. That may eliminate or at least help narrow things down a bit.
10-26-2018, 11:41 PM
Car crapped out and no spark
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