1982 911SC electrical issue

[trojwl]

I have been having starting issues with my 82 911SC. It has now gotten to the point of not starting at all. And I do know what the problem is -

lack of 12vdc output to fuel pump from FP relay (pin30).

Below are voltage readings from each relay pin (relay inserted into socket) in OFF, ON, START, and "Engine running". After several failed

attempts, enough raw fuel accumulates in cylinders enough to initate combustion and engine will then start and idle. Once running, the 12

vdc output appears at pin30 (and only then) thus feeding more fuel to sustain combustion.



pin OFF ON START Engine running

86 0 12 0 12

87 0 0 10.6 0

87A 0 12 0 12

85 0 2.5 0 12

30 0 0 0 12


The zero volts at pin30 appears to be the most likely culprit. Also, there could be a problem with ignition switch.

I really appreciate all suggestions above "did you check the fuse?" I think this will take someone with more 911 experience than me.


Also, the site won't allow alignment of readings making them difficult to read - sorry.
Thanks

[boyt911sc]

Have you tried to test and verify the condition of the FP relay? Search for Dklever48's thread about FP relay test. Everything you need to know is discussed in that thread. I suspect your FP relay is the culprit and could be verified by testing it. Keep us posted.

Tony

[trojwl]

I verified voltage to relay coil, voltage to the other pins, and pin30 output. That's pretty much how you test a relay. Like I said before, all my spare relays do the same thing. The most logical explanation is that all relays are bad. They appear to be the Chinese variety so that is indeed a possibility.

While the relay is obviously the most likely cause, what is throwing me a curve ball is the fact that once engine starts (from the accumulated fuel in cylinders, the 12 vdc now magically appears at output pin 30 (which now enables engine to continue running). Can anyone tell me if pump is energized whenever key is in the ON position or is it energized only when you are cranking engine (i.e. START position) and then a latching circuit keeps pump energized until motor is turned off? Knowing this will help tremendously in t-shooting.


If the new OEM relay that I have purchased from PP works (should see it in a day or so), I will tell PP that they need to do something about these defective units they are selling.

The ignition switch would be suspect #2 and will be pursued if necessary.

I will keep all posted on progress.

Thanks for any replies.

[timmy2]

Yes, the air flow safety switch circuit disables the fuel pump in the run position if the engine is not running.

Porsche used reverse relay logic so that the relay is actually de-energized in the run position. If the air flow through the air flow metering plate stops, it closes the ground circuit to energize the relay opening the current path to the fuel pump.

My suggestion to you is to print out the color 1978 current flow diagram available on Pelican and tape it all together and study the diagram to see how it works.

[trojwl]

Just realized a mistake in my thinking. My mind read "Hot in Run or Start" as "Hot in On or Start". That explains how pump power circuit is "latched" with engine running . When that 12 vdc goes away (i.e. engine stops running for any reason), the pump voltage disappears shutting off fuel flow.

It's looking more and more like I have a small handful of sick relays.

[fred cook]

Check the 6th fuse from the firewall in the trunk. That should be the fuel pump relay fuse. They are notorious for getting quite hot and losing spring tension on the fuse holder. If you are not getting power across this fuse, the fuel pump won't run. There should be a red jumper wire coming from the 11th fuse from the firewall going forward. Good luck!

[trojwl]

Don't get me wrong, I really do appreciate the suggestion, but it kinda saddens me that you think I haven't been all over the fuse box and would have never noticed a overheating fuse or defective fuse holder.

That was like step 2 in this prolonged t-shooting process. Step 1 was opening the hood.

[timmy2]

trowjl,
Feel free to PM me with a phone number and a time to call if you want to discuss your situation.
Happy to help troubleshoot with you.

[trojwl]

The new OEM relay just came in. The car started up immediately. I've also ordered another from PP (always gotta have a spare!). Also got an RMA # to return a small handful of never-used defective relays. This is why I always recommend Pelican Parts - along with very reasonable prices and lightning fast delivery.

The two major obstacles in my efforts were -

1. Not knowing exactly what RUN means on fuse labeled HOT IN RUN OR START (Bentley manual). Is it with key in ON position or is it when engine is actually running? My DVM showed 12 vdc only after engine was running. Still not 100% sure but it doesn't matter now.

2. The fact that it exhibited same symptoms with every relay I tried really threw me a curve ball!. What are the chances that all would be bad? The only explanation I can come up with is insufficient awg for relay's internal wiring or possibly a problem with mechanical spring for contacts. Also, I never blew a fuse so I doubt anything got "hot".

Anyway, if it weren't raining (Florida - the "Sunshine State"), I wouldn't be here pounding on a keyboard. But it is - so here I am.

[timmy2]

Did the ones you sent back say URO on them?

[boyt911sc]

Originally Posted by boyt911sc

Have you tried to test and verify the condition of the FP relay? Search for Dklever48's thread about FP relay test. Everything you need to know is discussed in that thread. I suspect your FP relay is the culprit and could be verified by testing it. Keep us posted.

Tony

Trowl,

Recently, another long thread was on top of the technical forum for very similar problem. I suspected the FP relay after seeing your data because new from the box FP relay have been exhibiting very similar characteristics. Defective FP and FV relays are now flooding the market now days. It is for this reason that buying these products from a reliable or dependable supplier like Pelicanparts is a very prudent choice.

Tony

[trojwl]

Two of them are URO and three say "made in China" on them, And, believe it or not, the ones from China are the OEM units! Of course, the URO's are probably also Chinese and could even be same manufacturer with different label.

They are from purchases made over the years and all came from PP.

However, since I never saved the little white boxes they came in, I am basically SOl.

Soon I will have an available spare in the car at all times.