Hi all , I have a recurring problem with ignition light not coming on when starting up , and when this happens the voltmeter also shows no charging taking place - blip of throttle does not alter volt reading

When this first happened 10 months ago , it was transient - did not happen everytime .

Then it became permanent , I checked the ignition light bulb with a circuit tester and it was fine , just not seeing any current . Took car to an aircooled 911 specialist - its a 1975 914 retrofitted with a 1975 2.7 litre 6 from a 911

My local Porsche specialist replaced the alternator and all was good again , then the problem recurred 3 months ago. This time it was a broken brass contact tag on the earth side of the bulb circuit - replaced tag and all working

Now the problem is back . I pulled out the dial and studied the spaghetti of wiring, the new brass tag still there ok , bulb still ok , but the live connection to bulb not live when ignition on

The electric fuel pump starts up , the starter motor starts up , the dash lights all work , the indicator lights etc all work . I have not checked the generator drive belt , its a bit hard to get at , assume if it was ok when alternator replaced the still ok !

I have a Haynes manual with 1975 circuit diagrams as my only guide to the electrical circuit , not sure where to look before getting towed back to another specialist

Any suggestions ?

Can you explain "ignition light"? I'm going to work on the assumption you mean the "G" generator light.

In the 914-4, that bulb is connected to two places: the buss/fuse bar under the dash (fuse 9) and the diode D+ pack on the alternator. That bulb comes on when there's 12V+ at the fuse box but no power at the alternator, causing a delta-V.

It's an incandescent bulb so it will also come on when there's too much voltage (like, 17V) at the D+ (e.g., improperly-working votlage regulator) but still only ~12V at the fuse box.

First, are you still using an incandescent bulb? If you've replaced it with an LED then go back to the regular bulb. There's a long history of these not working correctly with LED.

Second, when you're having this problem, is the alternator charging OK? If so, you'll have ~13.5V or so at the battery with the engine running.

Finally, if the bulb is not on, and bulb is fine, and the alternator is charging the battery, it indicates a wiring problem.

Clarify and we can offer more guidance.

GA

Thanks Greg , yes I mean the generator light . There is only a single supply wire to the light on the 'live' side , on the other side it seems to be linked to the other panel bulbs which I assume to be the 'earth' side

The generator light bulb is the original incandescent , its a bit old and blackened but still working

The generator light does not come on with the ignition switch

The generator (alternator) does appear to be charging the battery

correction -finger trouble - the alternator is NOT charging the battery

You noted that this was a conversion from a /4 to a /6. Did the installer offer any wiring diagrams for that conversion?

Where is the "live" side going to? Where is the "earth" side going to?

On the stock /4 car, one side of that bulb goes to Fuse 9 via a red/white wire; it will be +12V with the key on. The other side goes back to the D+ alternator circuit via a blue wire; it will show grounded when the engine is not running, and alternator output voltage with the engine running and the alternator properly charging.

I cannot speak to the original stock /6 installation or alternator, as I do not have a wiring diagram. But I'll reiterate that the bulb's purpose is to compare voltage between the fuseblock and the output of the alternator; if they aren't identical or very close - which it should be with a properly-functioning alternator - then the bulb will light.

Further, the alternator needs that circuit connected to work, as 12V from the Fuse 9 "excites" the alternator to start. Without that, you'll generally not get the alternator to provide charge (and why it doesn't work with LEDs). You can actually mimic this initial "excite" by briefly connecting the red voltage regulator wire to batttery power through a resistor (a bulb), but I don't recommend doing that unless you fully understand it.

I don't know if the /6 alternator works the same way, but I'd not be surprised to find it does.

I suggest you need to trace wires and see where this stuff is going. See if you have 12V on the red/white with key on, and see if you can trace the blue all the way back to the voltage regulator (it goes through the T14 connector on relay plate, then to the VR, which could be another point of failure...you do still have that relay plate installed, yes?) - GA