Alternator light on with key off Options

[Joec]

On my non running 74 2.0, the alternator light is on all the time. If I turn on the key it goes out. Any ideas?

[Joec]

Could it be the relay box?

[GregAmy]

One side of the lamp goes to the alternator (via the wiring harness and relay box).

Other side goes to the battery (via the underdash harness and associated sources wires, same circuit as other indicator lights).

Light comes on when there ~5 or more volts between the two. When the key is off there should be zero differential (0V - ground - on both sides); with the key on, battery voltage differential (~12V on one side, ground on the other). With the car running, zero differential (alternator output of ~13.5V on both sides).

First, pull the gauge and see if the right wires are connected. One should be blue, the other red/white.

Second, check your fuses. Your lioght is powered by S9.

Third, are all other gauges working correctly? Are there any other electrical anomalies?

Finally, with the car off, use a VOM or a test light find out which side of the light has power; is it the blue alternator wire or the red/white dash wire? Neither should have power with the key off.

Troubleshooting electrical issues over the Internet is soooo fun.... (IMG:style_emoticons/default/wink.gif)

Greg

[Pops71Por]

Had similar issue and the voltage regulator had went bad. You could quickly unplug VR from fuse box in engine compartment as cheap and simple test. otherwise ground issue could be in play

[Dave_Darling]

I believe the typical cause of this is a diode in the alternator failing.

--DD

[GregAmy]

^^^ That would follow. Diode pops, dropping the alternator side to battery voltage, and with the key is off it grounds that other side through whatever path it can find. Then with key on, both sides see battery voltage so light goes off.

Easiest way to tell is to see if the blue wire has +12V (battery voltage) with the key off. It should not.

[Joec]

Of course the blue wire has power with the key off or the light wouldn't be on. And I did check and yes there is power there with the key off. Seems properly wired but car was sitting for a very long time and has had some disassembly.
I can't check much until it is running and I am waiting ( patiently) for new fuel lines to arrive.

So alternator diode? replaceable part or whole alternator? Any other possibilities?

[Dave_Darling]

Replaceable part if you're up to doing an alternator rebuild. Some of the replacement ones you get can be problematic. Things like a stud inside the alt being long enough to short on the rear cover, or various other quality issues.

--DD

[whiterabbit]

I recently dealt with this same issue. Turned out to be a faulty alternator. Kinda sad because the the alternator had been recently replaced with a factory rebuilt bosch unit. Here's my thread on the issue. Hope it helps.

http://www.914world.com/bbs2/index.php?sho...=352501&hl=

[914e]

Here is a link on replacing a bad diode.
Any of the diodes that have continuity in both directions is bad. It is the most common part to fail in an alternator.


https://www.thesamba.com/vw/forum/viewtopic.php?p=8869239