Internally regulated Alternator no dash "idiot" light...

[BMAN212]

Feeling like an "idiot" for posting about the appropriately name dash light issue I'm having. Posting here in the 911 section because of the alternator involved but the rundown of the project is this...
1960 356 b Coupe former race car converted to "outlaw" style street car. I did not build the car but have been assigned to get it running and streetable again. Engine is a 2700CC Type 4 VW engine from FAT performance with upright 911 Style cooling. The car had an externally regulated alternator that would not light the dash light. fought with it. Replaced the regulator with a new solid state unit and still no light and no charge when running. 12 volts into the alternator at the battery side and only about 7 volts at the D+ wire when running. When not running the D+ wire sat at .56V with 12V on B+

Took this alternator to local rebuilder for test that showed what he felt was a "week diode". Replaced with Valeo 13208N internally regulated alternator and am having the same issue.

12V at B+
12V on positive leg of dash light (shared + leg with Oil Light which works as expected)
dash bulb is 4W new bulb (per 1982 TSB) that is good and shines bright if Negative leg is grounded. With key on I have 12v at the junction block in engine compartment (track car) where dash light negative side hooks to "blue" D+ wire

The alternator negative lug has been grounded to the fan housing and that has been grounded to the engine case new ground straps added from starter to chassis and transmission to chassis with cleaned connections and star washers. Grounds are good!

when power is turned on the dash light will not light and the voltage from the alternator will not climb above battery voltage (no charge is taking place). the alternator was removed and taken to rebuilder to be checked again where he showed a properly working alternator on his bench. 12v at B+ and a working indicator/exciter light. he spins it up and 13.9 volts at B+. bring it back to the shop and install in car and NADA...

What the heck am I missing? Hoping someone here will chime in with the "hey dummy did you ..." message that will get me back on track, or "road" rather.

Thanks in advance!

Branden

[john walker's workshop]

The blue wire is not grounding internally with just the key on. Trace continuity between the dash light and the alternator. I assume there is 12V to the dash light already, just needs to ground.

[BMAN212]

Yes 12V@ the + leg of the lamp and Neg leg of the lamp runs to D+ as per wiring diagram. My understanding is that the ground side of the lamp receives ground through the Alt until the Alt spins at a speed which causes resistance needed to signal the Alt to charge above Battery voltage. This then sends 12v+ through the D+ post causing balance across the bulb making it go out?

[john walker's workshop]

Correct. Ground the blue wire and see if the light comes on.

[BMAN212]

Grounding the blue wire causes the light to come on by completing the circuit but does not cause the alternator to charge nor will the light go out when started.

[john walker's workshop]

That was a test for continuity between lamp and wherever you grounded it. it obviously is not completing the ground inside the alternator. Usually it's because the brushes are not contacting the commutator. Take the thing apart and find out, or just get a fresh one. May just need brushes.

[timmy2]

Measure the resistance from alternator housing to negative terminal. Had a case where stainless steel washers were used at transmission to ground strap bolt and it was enough resistance to cause a no charge situation.

[al lkosmal]

Not quite the same issue....but, when changing from external to internal regulators on the 911, non-charging issues are common. there was a bulletin regarding the fix. not sure if this is the issue with the 356....but a likely suspect.




regards
al

[BMAN212]

Timmy2, gotcha. Will measure resistance again in the morning at all grounds.

Al, I’ve seen the TSB and will recheck again tomorrow. Was also told that we may need the resistor across the bulb per the TSB

[Porsche 935]

Don't mean to high jack your thread, but I need one of those resistors. Any source for them?

[BMAN212]

I ordered them from Amazon. Shoot me a DM with your number and I can text the link.

[timmy2]

https://www.mouser.com/ProductDetail/YAGEO/SQP500JB-91R?qs=sGAEpiMZZMtlubZbdhIBIAik%252Bt0yKDKe5YHH%25 2BatdrX8%3D

[dannobee]

Does it charge if you power up the blue wire?