1979 doesn't start at all, tried many things, now stuck :(
 

Hi Guys, new to rennlist but longtime Porsche nut.
I didn't find any introduction thread so not sure that is common here, anyway;
Semjon from Holland, currently '76 924, '79 928 and '65 911 in restauration (long story which I won't bore you with right now.)

As the title states I am having some trouble with my 928, 4.5 automatic from 1979.
Car started fine, no problem at all. Then it got more difficult to start when warm and suddenly no start at all anymore so I investigated few things also through tips found here.
Actually nothing happens when turning the key, no crank, no fuel pomp only one slight click from relay board.

What have I done?
- Checked battery and cables, all good there
- Checked for ground issues, reading 2ohms between negative and body which in my opinion is good
- At the 14 pin terminal near the jump start terminal in the engine bay, I connected a short piece of wire between the yellow wire and the jump start terminal > car cranks! and when done while car is on contact it even starts which tells me battery starter, selenoid etc are not to suspect
- Cleaned the 14 pin terminal and put it back together
- Changed the fuel pump relay, at least now the pump primes when I put the car on contact but still no start.
- If I take relay XV out and connect pin 87 and 30 with a small piece of wire (bridge) the car starts perfectly!
-Changed relay XV for new one, no luck :(
- With the bridge in place at the relay XV spot I am measuring 11 volts on socket for pin 86 and with running car this goes up to 12 - 12,5 which seems a bit low to me.
- I am not getting any reading on the socket for pin 85, no voltage and infinite resistance but not sure if this is correct since I can't find the pin setting from the relay.

So now I am stuck.. any tips? Relay XV has a resistor inside which makes it difficult to test (at least for me ;)

And another question, can I use the car with the bridge in place? It is only necessary for starting and while running I can take it out and the car keeps running (as excpected).
928 are rather new to me so any help is appreciated.

Thanks and regards from Holland!

Green wire?

Has the ignition switch ever been replaced? The fact that you can start the car with a jumper from the hot post to the 14-pin connector but not with the key says to me that the ignition switch is defective.

I will second the idea of the ignition switch. They are known to fail.

Welcome. Intro threads are pretty common. "Hi, I'm new and this is my car" is all it needs. And pics. Post pics.

Unless, by "intro thread" , you mean a thread for new people to read through.
Look at the top of the forum. There are several 'sticky' threads, including one titled "New Visitors, Read This First". Read that.

Thanks for the fast replies!

green wire, you have to help me one that one.. I read something about it and that it is also a weak spot but I lost the context..

ingnition switch has not been replaced as far as I know.
Does that also match with the bridge of relais XV?
with the bridge in place car starts right up with turning the key.

and with intro thread I indeed ment like hi I am and this is my car.
will work on that :)

What does XV do on the 79?

On my 85, it's the kickdown, and that likely isn't an issue.

The "Green Wire" is the signal wire that runs from the front of the motor to the distributor. It's a shielded cable with very special & unique connectors. If it is compromised, the car won't get spark and won't start.

Since your issue is 'no crank', not 'cranks but won't fire', I doubt you have a problem with that.

No worry on the intro thread. Not a requirement, but many folks do.

I would also check the resistors on the fender well check that one of them is not broken and that they both give the proper resistance that's stamped on them one is 4 ohms and one is 6

Since the car runs the green wire is good for now.

in a pinch put the key to run then put the 14th pin to the hot post
the engine will run so you can drive it
reconnect the 14 pin connector
And it will shut off with the key

If XV is the starter relay (and from your description it is) then check this thread: https://rennlist.com/forums/928-foru...-location.html

If you’re getting infinite resistance on pin 85 of the start relay, and if it’s an auto and you’re in P or N, then you have a bad ground. Check connector in spare tire well and ground point under right rear quarter panel. See above thread for more.

First of all a few pics;
https://cimg0.ibsrv.net/gimg/rennlis...955524bdcf.jpg
It featured in a Dutch Classic car magazine in May :)
https://cimg8.ibsrv.net/gimg/rennlis...f6ba4816b3.jpg
https://cimg9.ibsrv.net/gimg/rennlis...149ea2bacb.jpg


Back on thread; so I suppose the ignition switch is off the hook for the moment as a suspect and I should aim for the ground wires? Especially in the right rear quarter?
I expected the ground for the relay board a bit closer but like everything on the 928 this also appears to be over-engineered ;)

:bowdown::thumbup:

Ugh, I read through his initial post twice and the 'no crank' didn't penetrate my brain. Thanks, Joe.

Small update, still working on it but very short on time.
Driving around in the meantime with a wire bridge instead of the relais.

That depends on how you used your meter to determine terminal 86 was 'open' at the relay socket. Not at all off the hook for me. In fact, I'd bet that it's either a faulty ignition switch or broken yellow wire (terminal 50) at the ignition switch.

This seems to be the big clue to me...in my experience, check the temp sensor for proper resistance in the front of the water bridge. Once this starts to go bad (e.g. out of resistance), it causes a gradual hard start/crank symptoms. Also check your fuel accumulator under the passenger rear wheel well. Sniff/Visual inspection fuel smell or leakage. If either, that has gone bad. That will also cause 'warm' start issues...

Issue is confined to starter circuit based on info from OP. Bridging power across terminal 50 at 14-pin connector to jump post and across terminal 30 to terminal 87 of relay socket for starter relay results in the car cranking and running normally from all that I discern.

The issue can be one of three things to my knowledge:

1. Ignition switch
2. terminal 50 yellow wire
3. Selector position circuit

Selector position circuit and switch function are quick to diag with an ohmmeter. If it were me, I would rule out an issue there first. Then I would just order a new ignition switch. Worst case scenario is that you end up with a spare switch if the problem is with the yellow wire instead. FWIW I keep a spare switch because of their failure rate anyway.