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post #1 of 8 (permalink) Old 04-14-2019, 06:10 PM Thread Starter
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Charging System Oddity

I've been lucky to get through 50 years of owning cars without ever having any charging system problems. As a result of this dumb luck I have no real hands on experience with alternators and the like.

My S4 voltmeter typically shows just over 12 volts upon start up and measures
12.6v at the battery with my multimeter at idle. After driving about 1 mile the voltmeter jumps up to the 13.5v range and stays there for the duration of my drive. When I'm back in the garage the battery voltage at idle shows 13.6 which I believe is just right.

The p.o. mentioned this to me and said he'd never gotten to the bottom of it.

Anyone know what's up with this?

Bill

1991 S4 Spider "Gabriela", 2017 VW Golf R, 2018 VW GTI,

Previous obsessions; 2006 Mustang GT, 2006 BMW Z4M, 5 other Bimmers, Capri V6, Renault 5, Lotus Cortina, Sunbeam Alpine, Ducati Diana Mk 3, Norton Commando
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post #2 of 8 (permalink) Old 04-14-2019, 06:15 PM
But Mad North-Northwest
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Sounds like the regulator is taking a bit to kick over?

Turn the key to "on" but don't start the car. Does the battery light in the upper right come on? If not you've either got a burned-out bulb or a disconnect in the wire. The regulator uses the bulb circuit as the voltage sense line, so no continuity there means it won't work right.

Tom

1963 Giulia Spider (1750 engine)
1974 GTV
1991 Spider
Former: 1987 Milano Gold
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post #3 of 8 (permalink) Old 04-14-2019, 06:27 PM Thread Starter
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Thanks for the speedy reply Tom. I just checked and, indeed, the alternator indicator does not light up. I'll check the bulb tomorrow.

Bill

1991 S4 Spider "Gabriela", 2017 VW Golf R, 2018 VW GTI,

Previous obsessions; 2006 Mustang GT, 2006 BMW Z4M, 5 other Bimmers, Capri V6, Renault 5, Lotus Cortina, Sunbeam Alpine, Ducati Diana Mk 3, Norton Commando
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post #4 of 8 (permalink) Old 04-14-2019, 06:43 PM
But Mad North-Northwest
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Check the engine bay too. There is a green wire that runs from the alternator to a junction box on the left side of the engine bay, where it's connected to another green wire that goes to the light. Make sure they're not disconnected or broken.

If the bulb is burned out there are instructions how to get to it reasonably easily in the first few posts of this thread (though getting the trim ring back in is kind of a PITA.)

https://www.alfabb.com/bb/forums/spi...4-spiders.html

Tom

1963 Giulia Spider (1750 engine)
1974 GTV
1991 Spider
Former: 1987 Milano Gold
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post #5 of 8 (permalink) Old 04-14-2019, 06:46 PM Thread Starter
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Thanks again, I'll check that junction and bulb and post my results tomorrow.

Bill

1991 S4 Spider "Gabriela", 2017 VW Golf R, 2018 VW GTI,

Previous obsessions; 2006 Mustang GT, 2006 BMW Z4M, 5 other Bimmers, Capri V6, Renault 5, Lotus Cortina, Sunbeam Alpine, Ducati Diana Mk 3, Norton Commando
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post #6 of 8 (permalink) Old 04-14-2019, 09:27 PM
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Quote:
Originally Posted by Jack Straw View Post
My S4 voltmeter typically shows just over 12 volts upon start up and measures 12.6v at the battery with my multimeter at idle. After driving about 1 mile the voltmeter jumps up to the 13.5v range and stays there for the duration of my drive. When I'm back in the garage the battery voltage at idle shows 13.6
I agree with Gubi's diagnosis. Your symptoms are classic for an alternator that isn't receiving the "begin charging" signal through the indicator light.

You can get an alternator to "self excite" by revving the engine. That's what's happening when you drive that first mile. Racing the engine right after starting it will get the alternator going, but doing that while the oil pressure is still building and the engine is stone cold isn't a great idea. Better to get the warning light working.

Jay Mackro
San Juan Capistrano, CA

'65 Guilia Sprint GT
'67 Duetto
'91 164L
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post #7 of 8 (permalink) Old 04-15-2019, 05:24 PM Thread Starter
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I checked and cleaned all the connectors at the junction box. When I attempted to check the alternator light bulb the socket disintegrated in my fingers. As a test I swapped in the oil pressure warning light which I knew worked but it would not illuminate. Obviously there's a problem in the circuit that powers the warning light but I'm stumped at present.

Bill

1991 S4 Spider "Gabriela", 2017 VW Golf R, 2018 VW GTI,

Previous obsessions; 2006 Mustang GT, 2006 BMW Z4M, 5 other Bimmers, Capri V6, Renault 5, Lotus Cortina, Sunbeam Alpine, Ducati Diana Mk 3, Norton Commando
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post #8 of 8 (permalink) Old 04-15-2019, 06:40 PM
But Mad North-Northwest
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It's possible the places where the bulb connects to the board are damaged.

+12V comes in to the cluster through fuse 8 (that's all good if your voltmeter works.) It goes through the lamp then out through terminal 1 of connector C (green wire.) That runs to the junction box in the engine bay, where it connects to the other green wire to the alternator.

You'll need to do some electrical troubleshooting. Start by making sure the green wire is connected at the alternator. Oh, and be careful to disconnect the battery negative terminal if you're poking around with stuff.

Tom

1963 Giulia Spider (1750 engine)
1974 GTV
1991 Spider
Former: 1987 Milano Gold
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