Ignition Noise with key out?

[boostfix]

Went for a drive this morning and the car ran great. Came home and put it away and started chores around the house. I just walked by the car and heard a high pitched whine coming from the engine compartment. (1988 930) I took the cover off the relay and fuse tray and stuck my head in as far as I could to listen. Then I felt around with my hand and found the Bosch ignition control module to be quite warm even though the car has has been sitting for over 4 hrs since the morning run. This is also the area the high pitched sound was coming from.

I checked for an ignition fuse - none. I am trying to study my scanned wiring diagram to figure out the power supply to the igition control module...

The car still runs fine.

Has anyone else experienced this before?

Thanks

[boostfix]

I just read an interesting paragraph from the ignition section of the manual....

"when stopping the engine only the fuel pumps are turned off at first. After a delayed-action time of 3 to 5 seconds the power supply for the CDI control unit is broken automatically. In this manner an orderly combustion of the residual fuel until the engine stops completely is guaranteed."

Perhpas this is my problem??? A fault in the delay circuit 3-5 seconds has turned into hours and counting.

There is no mention if the delay circuit is internal to the CDI unit. Anyone know where the delay circuit is?

I'll have to unhook the battery tonight....

[JBL930]

It's one of the relays in the back fuse/relay area near the CDI, can't remember which one it is, there was a thread on rennlist about it some years ago. Some people never experience the engine running on after the key is turned off but it is supposed to, if it doesn't then the relay is bad, yours sounds like it might be stuck on? Try pulling each one out until the CDI stops humming. I'll try and find the rennlist thread

[boostfix]

Thanks JBL930! Did a search on RL and probably found the thread that you referred to.

Thanks to A930Rocket for his picture:


The suspect relay is the small rectangular one behind the round black relay.

Prior to this search I had unplugged the CDI and the hum stopped. So on the right track. I plugged the CDI back and it started to hum again with the ignition off. Then I pulled the suspect relay and the CDI stopped humming.

Now going to take apart the relay and figure out what is wrong with it.

[OSI930]

That is the ignition delay relay you are looking at. I think you are on the right track. I had to replace one several years ago and bought one from our host. Problem solved.

[NathanUK]

Yep, that's the one. Maybe you should check it's not getting powered open before you take it apart.

[boostfix]

Took the aluminum cover off the relay and then plugged it back in. (Nathan I saw your post after disassembly) Now you could see the relay triggering to power the CDI with the ignition off. There is a small circuit board controlling the relay….a capacitor, transistor, resistor and a couple of diodes. Now I had help from my father who is the electronics guy….all the components checked out with the exception of the transistor. The collector/emitter junction was not crisp on the tracker scope like a new transistor would be. So he replaced the transistor and the delay relay is back in the car and working with ~3.5 second delay for ignition shutdown.

My father bench tested the relay as it came out of the car and the delay was variable 6 seconds, 8 seconds, 20 up to 30 and then also staying on. The latter is what happened to cause me to hear the CDI being energized at rest with the ignition off.

[rsrmike]

Congrats, Man. That's really cool. I love fixing the unfixable. What was that transistor, like $.20??

It's even cooler to have a father/son project like that, hopefully you dad got a mild ego trip too!

Cherish those moments, you have no idea how cool they were till years later...

[NathanUK]

Good Job!

[boostfix]

JBL930 - Thanks for getting me on the right track.

rsrmike - Yeah my dad had a big grin on for sure when he soldered in the new transistor that cost less than a buck! It feels pretty good fixing something like this even though it's only a $57 part. Besides I had the car running faster than ordering and shipping a part.

Some useful info:

• In the Porsche Workshop Manual on page 28-13 there is the description of the delayed ignition cutoff circuit (as I mentioned in post #2 above) Now that I know what to look for there is a small arrow pointing to the delay relay.
• In the Porsche Workshop Manual this is called the Delay Relay - this is why I never found it because I was looking for something ignition..... That said, on page 90-2b there is a picture and legend that shows all the relays on the regulator plate.
• Lastly, this relay could have failed the opposite way by cutting off power to the CDI. This would of course leave you stranded. In an emergency you could remove the delay relay and jumper out pin '30' and 'L' on the relay socket. Which socket pins are these???? When you remove the relay, there is a diagram imprinted on the side of the relay labeling each pin.
• What you need to do this: 8mm socket to remove relay, and a short wire with a spade connector crimped onto each end. I'll be having one of these in my tool kit!

[OSI930]

Nice work!

[JBL930]

Originally Posted by boostfix

JBL930 - Thanks for getting me on the right track.

Glad to help, nice job with the fix