scarceller

I have been chasing a intermittent problem in my 84 Carrera for about a year.

I've had the following intermittent issues and they don't stay around long enough to diagnose:

- While driving the engine has died 2 times, when this happens the tach drops like a rock to 0 RPMs then the engine instantly catches and fires back up.

- Then recently while at idle right after startup the engine would shutdown just for a moment but enough to hear it then suddenly catch. Sometimes it may just die at idle.

- Then while sitting in the car I noticed I heard a relay clicking when this happens, turns out it's the DME relay. I put in a new one same thing. So I decided to see what was actually happening in the relay and found that only the Fuel Pump relay within the DME was being inactivated. The other main relay within the DME relay was always staying active - or at least the test light did not show it turning off at time of failure. I had 2 LED 12vdc test lights measuring the output of both DME internal relays. When the condition occurred only the Fuel Pump lost power and the tach dropped.

I know tons about the DME internals and how it works (I've dis-assembled all the program code) so this gave me a solid understanding that what is most likely happening is the DME is simply losing power for just a brief moment and then re-booting. I confirmed my suspicions and know 100% that the DME is simply re-booting.

Then just the other day another issue came up, I turned the key to 'RUN' but engine not cranking or started and noticed the TACH was pegged. Now I'm really wondering what is going on. I check all the tach wires things look good. I left the car overnight. I come back next day and put key in 'RUN' tach is fine now.

I start the car and the engine cut out is back this time worse than ever.

On a whim I decided to bang on the bottom of the dash just under the ignition switch and all hell breaks loose! The tach is all over the place the engine dies!

Start it back up and this time I tap directly onto the Ignition switch from under the dash with a socket extension, once again engine dies!

I just ordered a new Ignition switch should be hear in few days.

I just wanted others to consider the ignition switch when you have sudden unexplained cutout issues.

Has anyone else seen this? a flaky ignition switch that sort of works MOST of the time?

User 4221

I have seen this issue mentioned on 928s quite a bit. If you tapped the switch and was able to reproduce the issue, then I think you found it.

The engine dying dead and easily re-starting is clearly electrical. If the switch is not the culprit, then I would suspect a coil or connections to the coil.

Good luck.

Oh, and thanks for your thread on mixture/idle adjustments over on the bird. It was very educational.

rusnak

Steve commented once that you can pull forward on the electrical portion. Presumably, to align the contacts or see if they'll become un-aligned. I haven't tried it, but that would be one way of coming to an definitive answer early on.

dshepp806

I plan to get mine outa' there sometime in the near future,..just to provide fresh copper to the switching,..trying to , preventively, avoid the gremlin world......

Good info and thanks for sharing....

Doyle

scarceller

Doyle,

What I learned is that at the very least you should tap on that ignition switch with the car running to see if it has any issues. I think it's a good idea to do this and have it fail during your tap test rather than wait for it to fail on the road.

These switches are 20+ years old and it's amazing they still work as well as they do.

Lorenfb

"So I decided to see what was actually happening in the relay and found that only the Fuel Pump relay within the DME was being inactivated. The other main relay within the DME relay was always staying active - or at least the test light did not show it turning off at time of failure. I had 2 LED 12vdc test lights measuring the output of both DME internal relays. When the condition occurred only the Fuel Pump lost power and the tach dropped."

Actually the ignition switch affects both relays, so there's probably another
problem too or the test results are incorrect.

scarceller

Loren,

I'm well aware of the wiring setup for the DME and you are correct that the power to the relay also comes from the ignition switch. What I think may be happening is a very small spike (loss of voltage) that I can't detect on the LED test light on the main relay. If this where to occur the 8051 processor would not tolerate it and simply re-boot. When it re-boots it will instantly shutdown the FP ground to the FP relay within the DME relay.

At this point I know that tapping the ignition switch makes everything go crazy the tach flips out and then the dme re-boots quickly and the engine catches or the DME does not re-boot quick enough and engine stalls.

I know it's re-booting because I have a chip emulator in this DME and I'm running the 89 code with all the program in the chip. I then monitor for the re-boot routine with the emulator and can clearly see the routine being executed when this failure occurs.

I have also not ruled out the speed sensor since the program also constantly looks at speed pulses and if it misses 10 or more of them it will also re-boot the DME but I have no evidence at this time that 10 pulses have been missed.

dshepp806

scarcellar,..where's that damned (STORAGE) o'scope? (kidding).

Although mine isn't presently problematic, I do plan to change it out (as mentioned before).

I'm afraid if I tap it, it will start to go bonkers on me,.....and "timing" is EVERYTHING!!!!!!!!!!! (kdding, again)....

Will you take pics of your changeout?

would be cool...

Thanks,

Doyle

scarceller

The change out seems simple, I already can easily get to the back of the switch and have already removed the harness plug from the switch. The hard part is that you have to get 2 screws out, the bottom one seems real easy but the top one will be a challenge. I will not remove the switch till I get my new one later this week. I don't think I can take a good picture up in those tight quarters, but if I can I'll try.

rusnak

I've had to remove the ignition switch a couple of times on separate cars. I usually like to start with a spring loaded center punch, then use a bolt extractor with a cordless drive screwdriver. If feel that it is more precise than using a chisel and striking the bolt head at an angle.

scarceller

I don't need to replace the mechanical portion only the electrics at the back, can this be done from behind with the mechanical portion still in place? I looked at the setup behind the dash and it seems like maybe it could be done IF you can get the top screw out. Have you ever tried this?

rusnak

Some guys who are really really good can do that. Others like me have to take the long way. My mechanic friend Tony can do it. My friend Salvador also got his out that way. It depends on how loose that screw is, and what you're using.

ron mcatee

It takes a short Phillips screwdriver and patience.

scarceller

Update: I replaced the ign switch and yes it can be done from behind the dash. Pain in the butt to get top screw out!

But I still have erratic tach, sometimes it pegs at 7000RPMs and sometimes it does not read correctly. Then I also have the issue of the DME simply deciding to re-boot, this causes the engine to sort of mis-fire and re-catch within a second or 2. Sometimes the engine dies but can be restarted.

So it's not the DME relay or the ign switch these are both new and I still have issues.

After reading several other threads it's very possible I could have a faulty voltage regulator causing an over voltage condition. Several threads mention the vdo tachs don't like to see voltages above 16vdc as this causes them to act up. I also suspect the DME would not like voltages >16 as well. I plan to monitor system voltage at the DME relay with a DVM while I have these issues.

Will keep you posted.

I do however think the ign switch was somewhat faulty because when I tapped on it it caused DME to re-boot, now with new switch I can tap on it all day long and no ill effects. So I think I may have more than 1 issue here, this is not un-common in 20+ year old cars.

Ed Hughes

Could there not be a problem with the DME board's wiring? I mean 20+ years of vibration could take its toll. There have been more than one report of some re-soldering needed.