fiftyfive

Apologies for multiple posts, learning as I go.

PET5 MG=1 SG=7 shows item 13, 930 606 101 00, as a boost pressure switch. Is this what drives the overboost relay? In other words, a false alarm from it could kill the engine.

There's also item 14 930 606 103 00 boost pressure transmitter which I assume drives the waste gate and boost gauge?

JFairman

There's a small aluminum color relay on the console in the engine bay that keeps the ignition on for about a second after you turn off the ignition key. The idea is to burn up any residual gas in the intake ports after you shut off the car, and fuel pumps with the ignition key.
Maybe that's the "ignition cutoff" relay you're referring to.

fiftyfive

Yes, that's it. I got a chance earlier to explain things to the very experienced guy who takes care of my cars. He noted what struck me, that the tach needle dives hard to zero when the engine cuts off: Even with the car rolling and clutch engaged the tach was dead. Sight unseen, his prime suspect is the CDI box and/or coil. Only reason the car's not at his place now is that I live fifty miles away.

I'd guess the ignition cutoff relay is controlled by the ignition switch, with some type of internal delay circuit in its secondary to maintain ignition voltage after the key is turned off. If the relay supplies the CDI box, it would seem to be in a critical path.

nathanUK '81 930 G50

I would say you are right about the ignition delay relay being in the path, but I've never heard of one failing yet. It is always the CDI itself... $£$£$£

If you search me on Pelican I posted lots of data of meter readings I took of my CDI to help people out. Not sure if I posted the data here or not.

I suppose you could hook up a fuel gauge and have it in the car to make sure it's not a fuel problem, I've done this in my 3.2 and had it ready to throw out of the window if it burst or anything.

fiftyfive

Nathan - Nice hearing from you again. Don't know if you remember, but you helped me a couple of years ago with some things. I'm in Virginia.

Long story short, the car wouldn't start at all today - which makes finding the issue easier. The CDI box turned out to be a Perma-Tune. I got it out and checked terminal resistances per the 930 6-pin troubleshooting guide at perma-tune.com (Tech Support link). It and the 930 8-pin guide have good info on the ancillary relay functions, what the distributor's trigger circuit should read (600 ohms), etc.

I've yet to hear the box whining in the car, but I don't trust my hearing. Next is powering it on the bench with 12 volts. As Sinatra said, if it doesn't whine there it won't whine anywhere.

JFairman

The Permatune is silent. Never makes any squeal or whine. I have one too.

fiftyfive

Good point. Its inverter frequency is probably above the audible limit. The Bosch OEM one was designed a long time ago, possibly at a much lower frequency.

Bruce Eyring

Stephen is right...the yellow relay in the rear fuse area is the place to start. Same thing happened to mine.....put in a new relay (after 220,000 miles) and no problems since. If you can't hear the fuel pumps when you twist the key...you ain't going anywhere.

Chuck Jones

"......If you can't hear the fuel pumps when you twist the key...you ain't going anywhere. "

That is precisely how all my troubles started....and an excellent place to start when the car won't. I have to admit that this crowd excells at long distance "braille trouble shooting" considering the fact that most of the problems are being described in text, and not always very clearly. In my case, they were dead on right.

nathanUK '81 930 G50

Glad I could help!

Did your CDI pass all the tests they have for them?

You could always check your coil with the figures I posted if it's a stock one.

fiftyfive

Nathan,

The CDI box, magnetic pickup resistance, and coil primary resistance are all within Permatune's specs. Car was competely dead on Sunday. On Monday I took the CDI box out, cleaned up its grounds and replaced. Car fired up immediately, thought I was in good shape, and then it died after a couple minutes. Happened two or three more times. Last night the car showed no problems.

Because the tach needle drops hard and stays there when shutoff happens and because of its sharp on-off nature, my technician and I are leaning toward a shaky CDI unit. Even with terminal resistances ok, if the CDI's inverter stops working, thyristor doesn't fire, or one of a hundred other internal electronic issues, the unit won't output pulses to the coil.

On Sunday when the car wouldn't start, there was a raw gas smell and at one-point quasi-hydrolock hard cranking. This seems to indicate fuel delivery.

Mark Houghton

Chuck Chuck Chuck....here we are at now 57 posts and still you're not happy! Why is it that we collectively can't seem to find the problem? I'm leaning toward your MSD doing some sort of internal failure. Know anyone that has a spare they can lend you to help maybe take that out of the equation? If it aint that, then either the coil (I doubt that, but it's possible), or that damn yellow relay. Did you ever try swapping the yellow relay with another, or with a red fuel pump-type relay? You've definitely got something electronically failing after is heats up a bit.

Chuck Jones

Mark: I wasn't disagreeding with anyone....I was simply saying that after all the trouble I had, the fellows who first diagnosed the problem I was having as the yellow relay were correct...my car is running fine, although the yellow relay was sort of jerry-rigged to run and I should really replace it with a new one. I think you misread or misinterpreted my post. At this point, I've replaced my CDI box with an MSD, and re-soldered the base receptacle for the yellow relay...I'm good to go. I do carry extra red and black relays, and will be ordering an extra yellow one.

911rudy

I had the same problem with my yellow relay. But I was lucky and pulled it out, used a rubber eraser on the prongs and reinserted it. No more problems. That was two years ago.

Mark Houghton

Chuck, I didn't enterpret your message as disagreeing with anyone; quite the opposite. What I meant to convey was some general group embarassment for not yet helping to solve this problem sooner.

Now that I look at it, I guess I'm one of those fellows who pointed you toward the yellow relay early on. But here's the real brain blaster: I just realized that I was responding to an issue that was successfully closed way the heck back on 7/13/07!!!! No wonder I wasn't tracking with what you were saying. Jeeze, where was my head at?

Anyway, all of us...you included...are always eager to help each other. That's what's so great about this forum. It's not the forum, but the people that it's comprised of.