No Ignition light = No Charging ! Options

[63acornwall]

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 ?

[GregAmy]

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