Alternator fuse keeps blowing

[Jonas]

Hi all. 2009 Pontiac Vibe GT 2.4L (Toyota Matrix XRS). I’ve discovered the blown fuse (ECU-IG 2) is associated with the field coil of the alternator (wire labeled “F”). The fuse blow consistently, but sometimes when the car is turned on (no start), sometimes right after the engine is started, or occasionally while driving and in motion. Any ideas on how to diagnose this?

Thanks,
Jonas

 

[eljefino]

I'd unplug the small wire(s) on the back of the alternator then turn the key on. If the fuse pops, it's the wiring. If it doesn't, the alternator has an internal problem.

 

[Mechanician]

You need to determine which load is drawing too many amps.
Could be something wrong with the back-up lights, or rear defroster wiring.
Start by isolating the alternator by unplugging it as described, that's the easiest route.

 

[Jonas]

I'd unplug the small wire(s) on the back of the alternator then turn the key on. If the fuse pops, it's the wiring. If it doesn't, the alternator has an internal problem.

You need to determine which load is drawing too many amps.
Could be something wrong with the back-up lights, or rear defroster wiring.
Start by isolating the alternator by unplugging it as described, that's the easiest route.

Do you mean the harness that pushes/snaps in and not the post terminal wire? I tried that and the fuse still blew.

I would think the back-up lights and defroster wouldn't energize unless I put the car in reverse or turn the defroster on. Is that correct? The fuse usually blows when the car is turned on (no start.)

 

[Mechanician]

Do you mean the harness that pushes/snaps in and not the post terminal wire? I tried that and the fuse still blew.

I would think the back-up lights and defroster wouldn't energize unless I put the car in reverse or turn the defroster on. Is that correct? The fuse usually blows when the car is turned on (no start.)

Unplug what ever the green wire is in.
You're right it's probably not the other circuits, but it could be something in the wiring. Just keep in mind the alternator isn't the only suspect.

 

[Jonas]

I tested the alternator connector for continuity with a multimeter. There are 4 pins in the connector. One of them got a continuous beep, the other 3 got nothing. I can't tell which one is the green wire because of the rubber boot. Does that tell you anything? Maybe the alternator itself has a short?

 

[Mechanician]

Having continuity does not tell you much. Charge the battery well.
Then you can run the engine long enough to determine if the fuse is still going to blow, by running it without the alternator plugged in.

 

[eljefino]

Maybe the alternator itself has a short?

That's what I'm thinking, so if you disconnect the small wire harness that goes there you take it out of the equation. With a charged battery the car will run without the alternator for a bit.

The big post is the +12V alternator output, not a concern.

The other thing to try is to replace the fuse, idle the car, and have someone wiggle various wiring harnesses while watching for the battery light, which should come on when you blow the fuse.

 

[willbur]

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