Issues with starter

[hpmad]

Hello,

I have a 1980 911 3.0 SC and it's not starting anymore :-(

I would really appreciate any help on fixing this as I am at wits' ends :-(

Over the last years, it has had more and more problems starting.

The symptoms are:
When battery is fully charged and all is good, when I put the contact, the starter would run and the car would starts, but more and more often, the starter would not start turning...

The starter is good, I have verified it (I have actually changed it thinking that it was bad)...

I have tried to trace through the contacts from the key to the starter, opening connectors, cleaning them, but it did not help (and I broke stuff that were a pain to fix after :-(

I have checked the voltages on the 3 wires going to the starter:
1: If I "breach" the main 12V from the battery to the "starting trigger", the starter runs, no problems. So this is not the issue...
2: the "small yellow wire" seems to always have 0V on it.
3: the slightly larger gauge yellow wire seems to have around 10.9V when the key is in "run mode", and jumps to around 11.5 when in "starting mode", but no sounds from the starter (not even a click from the electro-magnet).

Thinking that the "start" wire just does not have the "power" to start, I tried driving the starter through a relay (see drawing)... (Note, this is a solid state relay, not a mechanical one)
But as soon as the key is in "run mode", the starter turns (probably cause of the 10.9V which is there when the key is in "run mode").




Any advices?
Thanks,
Cyrille

[Dpmulvan]

Check your grounds and battery if you havnt already. Just because a battery reads voltage doesn’t mean it’s good.

[yelcab1]

I would suggest that you remove the non-stock relay modification for now.

Take your battery to a parts store and have them load test it. 90% it is going to be bad. Replace it.

[76FJ55]

that relay is a real mystery box the way you have it drawn. does it have any terminal designations or schematic so we can understand how it is wired in? regardless I tend to agree with yelcap1, that it is probably better to have it out of the system for the time being.

the yellow wire shouldn't have any voltage in the run position, so I would probably start troubleshooting at the ignition switch.

[Machala]

Tough spot, sounds like you’ve done some solid troubleshooting already. The constant 10.9V on the yellow wire in run mode is a clue, could point to an ignition switch issue bleeding voltage. A mechanical relay might be a better test than solid state, just to rule that out.

[911obgyn]

So you replaced the starter, was there any dirt, oil, grease build up where it mates to belhousing? I put in a new starter quickly without cleaning up surface and it was intermittent until i took it out and cleaned it up. Also ground strap at transmission and ground at battery cable shouldbe cleaned.

[PeteKz]

Agree with the others about testing the battery and removing the relay.

The problem you describe is most likely caused by a bad battery or poor battery connections.

1. Remove and clean the battery terminal connections, put some grease on them, and reconnect. remove the ground strap to the chassis and clean that. Remove and clean the ground strap under the car between the transmission housing and the chassis. Then try starting it again.

2.. If you have battery jumper cables and another car or good battery, then try jumping your car from the other car, or swap the battery. If it cranks the engine every time, then you have a bad battery.

3. If you don't have jumper cables and another car to jump from, then take the battery to an auto store and have it tested. NOTE: the electronic hand-held tester that many stores use does not always identify a bad battery, because it does not put the battery under full load.

4. How old is your battery? If it's 5 years or so, that's about the most life you can expect from it (unless it is a LiFePO or other special type battery).

5. Even if you have a bad battery, the voltages you measured on the yellow wire still don't make sense. There should be zero voltage on the yellow wires when the key is not in "Start" position. After you figure out whether the battery is good, then check voltage on the yellow wire again. NOTE: The larger yellow wire that connects to the starter solenoid goes back to the ignition switch. The small yellow wire goes to the fuel injection and powers the cold start valve and thermo-time switch. Both wires should have voltage only when the ignition switch is in the "Start" position.