924Board.org

Min · Posted: Mon Sep 13, 2004 8:58 am Post subject: Voltage Oscillation.

About 3 days ago the 924 started showing its voltage was changing drastically. By this I mean, normally the car was running at around 12.8 volts (a little low) but 3 days ago, is started jumping to 14 volts or higher quite suddenly. It also developed a alternator belt squeal, so I figured I'd repair the alternator belt, which I did today. After doing that it started running at around 13.8, I figured great!, I also fixed the volt gauge in the console today as well, so we took it out for a spin, and found it, its still jumping voltage, but now instead of 14 volts, its jumping to 17+ volts, which I confirmed with a digital multimeter, on the way back home, suddenly it stopped charging at all. Alternator light popped on, voltage guage dropped. I'm thinking that perhaps the wire that tells the alternator what it should be doing has been broken, can anyone tell me were that wire is located, I know it runs from the alternator into the car someplace, any help locating it would be appreciated.

Min

Paul · Posted: Mon Sep 13, 2004 9:43 am Post subject:

Time for a new alternator or at least a new voltage regulator. Its on the back of the alternator and is held in by two screws.

wdb · Joined: 02 Nov 2002 Posts: 2024

my ford mustange did something similar , the headlights would get bright then really dim , turned out to be a bad ground for the volt regulator . tightened it up and was good to go

Min · Posted: Mon Sep 13, 2004 9:58 am Post subject:

how about I check everything before replacing the alternator ... like the wire that tells the alternator to actually charge. Anyone know were that wire goes? which was my original question. The rest was backstory.

Min

numbbers · Posted: Mon Sep 13, 2004 10:03 am Post subject:

Make sure the ground strap is in place on the alternator.
_________________
1980 924 Turbo

Paul · Posted: Mon Sep 13, 2004 10:33 am Post subject:

There is no wire since the voltage regulator is mounted on the back of the alternator. It also contains the brushes.

Min · Posted: Mon Sep 13, 2004 10:55 am Post subject:

uh, so your telling me that the D+(I believe thats what its called) wire doesn't exsist? ... your alternator only has 2 wires coming off it?(ground and power) Perhaps I'm hallucinating ...

Min

Min · Posted: Mon Sep 13, 2004 11:09 am Post subject:

nope, I'm not hallucinating, you just don't know what your talking about paul.

notice the B+ (power) the shaded grey wire (Ground) and the D+ wire, which detects how much the alternator should be charging the charging system (since when you put a small resisting device inline on that wire such as a diode, it increases your battery charge) I believe this wire has broken off somewere, I know it goes to C2 (the big plug at the back of the fuse box) but I don't know were it goes from there, and that is my question.

Min

Min · Posted: Mon Sep 13, 2004 11:16 am Post subject:

Smoothie · Posted: Mon Sep 13, 2004 11:26 am Post subject:

Yes D+ exists. It's a small gauge green wire on my 931. It's called the "exciter" wire - tells the alternator when to start/stop charging and controls the dashboard alt light. Sorry - couldn't tell you where it goes. It's a low voltage that passes through it. Maybe you could disconnect it from the back of the alternator and check it for voltage. That should tell you if there's a break in it somewhere without needing to know where the wire goes. As I recall it should only read something like 1.5 volts.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

marky522 · Joined: 02 Nov 2002 Posts: 335 Location: Port Huron, MI

Its the voltage regulator, this is what happens when it goes.

Mark

Min · Posted: Mon Sep 13, 2004 12:41 pm Post subject:

strange, I've had a voltage regulator die, and this is in fact not what that one did. Does anyone know what sort of signal D+ requires so I can attempt to provide that too it. From what D+ is supposed to do, what my car is doing actually suggests to me that its more of a high resistance/broken D+ rather than a dead voltage regulator, or dead alternator (I've had both of those items die on me in the past neither was anything like this)

Min

Min · Posted: Mon Sep 13, 2004 2:16 pm Post subject:

tested from D+ to C2, with the good old ohm meter, 20 ohms resistance for that length of wire.

Min

John Brown · Joined: 07 Nov 2002 Posts: 903 Location: Leesburg VA

Min. You answered your own question. You published a portion of the wiring diagram which clearly shows where the D+ 'goes'.

Since you found C2 I wouldn't think you'd be stumped by the relatively simple task of following the blue wire through the firewall to the white plastic 3 connector plug roughly behind the cylinder head or distributor. From there a black wire to the D+ stud on the alternator. Or you could go the other way and trace it back through the alternator no-charge indicator light. Also on the diagram.

Having said that; loss of D+ will not cause a voltage spike such as you described.
_________________
John
80 931 - #931 44Cup
99 Escalade - tows track cars
gone but not forgotten: original 924.org car - 82

Min · Posted: Tue Sep 14, 2004 4:24 am Post subject:

hrm, any idea what would cuase the voltage spike?

Min