Trying to confirm a bad alternator

Driving home today the battery light turned on in my '01 Golf. I checked the voltage when I got home and it's around 12.5V with the car running. I checked the large cable at the alternator and it has battery voltage as well.

I pulled the little connector from the alternator and with the key in run I measured ~11.5V on each pin.

I then pulled the belt off and checked the pulley, it seems to be nice and tight and the clutch seems ok.

At this point I'm thinking the alternator (or the regulator, which amounts to the same thing) is done.

Is there anything else I should check?

Jim

Mine just went suddenly the same way, so I brought it to get tested and it gave no reading on FLAPS machine. I thought I could try replacing the regulator but it smells a bit fried. You reminded me to check the fuse. It's difficult putting a new one in without knowing what happened, but I think I will.

Yes, if you're only getting 12.5V as a system voltage with the engine running the charging system has quit.

You can remove the regulator/brush assembly with the alternator still in the car... worth a check to see if the issue is simply worn brushes. They are a wear item and pretty common point of failure.

That said, good idea to check the slip rings too if you find short brushes... if the rings are deeply grooved as well no sense buying a new regulator/brushes; you'll want to spend that money on an entire alternator.

When my alternator died it went the same way, sudden battery light.

Well, since the car has 231,000 miles on it and I think the alternator is original (I've never replaced it and I've had the car since 80,000 miles) I think I'll just go ahead and change it out. Chances are the brushes and slip rings are pretty worn at this point.

Kind of annoying since just 2 weeks ago I replaced the clutch and flywheel and now I have to dive right back in again.

Oh well.

Jim

Yeah, it’s no shame for the miles though.

Dieselmonkey02 said:

Yeah, it’s no shame for the miles though.

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Not at all, I just kinda wish it had failed 2 weeks ago.

Did it fail? Sounds like it, but I would have it checked, pretty simple test if you got he machine. Troubleshooting battery/charging system with just a volt meter can be problematic.

BobnOH said:

Did it fail? Sounds like it, but I would have it checked, pretty simple test if you got he machine. Troubleshooting battery/charging system with just a volt meter can be problematic.

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Unfortunately a volt meter is my only tool here. Plus I figure if I have to take it out to test it I might as well go ahead and replace it. At 231,000 miles I think I can let it go.

I watched a video where the tech tested both DC and AC voltages. The latter was a check on the diodes; should be almost zero volts AC. He also did a voltage drop test on each of the battery cables; can't remember the maximum voltage allowed, but it should almost be 0.

Double check your pulley hasn't failed and it is allowing it to slip to the point of no charge. They either lock solid or they don't grab. I had this issue and it was only the pulley. Take the belt off, put a screw driver into the internal fan blades to hold it still and see if you can move the pulley either direction. It should allow for movement in only one direction. Neither direction is bad as is both directions.

Check the connector right above the starter too. I have seen an alt wire pull out of the connector there. Replaced the alt on a buddies car for no reason. Found that afterwards.

ghohouston said:

Check the connector right above the starter too. I have seen an alt wire pull out of the connector.

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I’ve wondered about opening that up, how annoying it pops out!

I had 160k on my alternator and a new pulley at 155k. 5k later the alternator died. I could try new brushes but an all new alt frees me up to field a failure on something I haven’t messed with.

I have a Brazilian made vehicle and I still recommend taking the pass light cover off when reattaching the upper air pipe, It was the least fun of the whole job. I couldn’t do it until i could see everything. My hose clamping pliers don’t open wide enough for a fully open clamp.

The rebuilt Bosch alternator makes the tensioner pulley real smooth, even though the recent replacement pulley was still good.

Thanks for all of the ideas, I got my new alternator from idparts yesterday and put it in. Car is now charging like it should.

I did check all of the wiring and the pulley before deciding to replace the alternator, so I was pretty confident that it was the culprit.

I'm pretty sure I have replaced the pulley on this car before as it looks a lot newer than the rest of the alternator.