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2001 VW Golf mk4 Voltage regulator or bad wiring?

The battery isn’t charging at all, but I’m getting 15v+ testing between the alternator casing and the battery ground when running and just over 12v when off. Battery was recently recharged. Battery reading just over 11v when running and at 1500-2000 rpm, so I think this may mean the battery isn’t charging but the alternator is working.

Checked connection between positive out from voltage regulator and positive post on battery and got -0.00v to–0.01v when I think it should be -0.05v?

Battery test at Auto Parts store suggested bad voltage regulator. Could a bad regulator cause the battery to not charge (rather than over-charge) or is this more likely bad connections/wiring? Suggestions welcome! Thank you!

“between the alternator casing and the battery ground” that should be zero, ground to ground. If you are actually reading 11 to 15 volts, one of the grounds is missing or bad.

If the alternator is providing 15 volts and you read 11 volts at the battery, that indicates a high resistance (or open) between the two. They should be directly connected with a fraction of a volt between them.

But then you state: “between positive out from voltage regulator and positive post on battery and got -0.00v to–0.01v”
which is good, lower the better.


Okay, when I get home I’ll check the readings again. I’ll try alternator casing to battery ground then lead output from alternator to positive battery post, on both of these there should be a low voltage reading (less than a half volt). If I’m reading battery levels between casing and ground I should be looking at the ground connection on the alternator?

Edit: How does an alternator connect to ground anyways? Is it just a direct connection to the engine block or should I be cleaning all the wire connections and looking for frayed wires and whatnot?

By alternator casing…I presume you mean that "From the Positive lug on the back of the alternator to the ground. That lug on the back is not a ground. This is the cable that carries the electricity to charge the battery.

The first test is the battery voltage…engine off 12.5 volts …and engine running 13.5 to 14.5 volts.

From that lug to ground, you should read 13.5 to 14.5 volts with the engine running.
If that lug reads good voltage but the battery pos to neg reads much less with the engine running…then that wire that charges the battery is bad. This wire is a “fusible link” and if burnt out the entire wire is replaced with another fusible link.


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The ground is the body and engine, connected to the alternator case by the physical contact of the alternator case and its mounting bolt to the engine, or brackets attached to the engine.

As Yosemite implied, the positive lug on the alternator case is electrically isolated from the case. If they short circuit, something has gone wrong, or will in a fraction of a second.

Okay, here’s what I’ve got:

With the car off I’m showing battery levels, around 12.2V, between the lug on the alternator and ground. I’m showing close to zero between alternator casing and ground.

With the car on I’m showing the same. Between the lug on the alternator and ground I’m showing battery levels 11.8V. Between casing and ground I’m showing next to nothing, a just 16 millivolts.

I’m assuming here that if the alternator were good and the voltage regulator bad that I should be reading higher levels with the car running. Could this still be a problem with the regulator, is there further tests to be done, or is this the alternator?

Older VWs (like my mk1 GTI) are known for developing poor grounds from engine (and, therefore, alternator) to the body. Add a separate ground wire from the engine block to body (make sure it’s metal to metal connection) and see if that helps. And check all other connections to and from the battery while you’re at it, along with the cables.

The voltage regulator is likely part of the alternator assembly or a function of the control computer. Sounds to me like the alternator is dead but you could remove it and have it bench tested at a parts store.

The test Yosemite mentioned above doesn’t require any probing of the alternator. Just the battery posts. Like he says, between the + and - battery post, you should read about 12.6 volts before the first start of the day, and about 13.5-15.5 volts after you start the engine and let it idle. If you are reading 12.2 volts at the battery before starting the engine, the battery is in a very discharged state, or is faulty . If you also read 12.2 volts when the engine is idling at the battery, the alternator is probably faulty. As mentioned above, parts stores often have fixtures that can test the alternator independent of the rest of the car, once it is removed from the engine compartment.

I’m probably looking at a bad alternator. I’ll remove it and have it tested as suggested. I’m really thankful for everyone’s help on this. I’m looking forward to trying this out! I haven’t done much beyond oil changes and headlights, time to step it up.

If there’s a local auto electric shop they may be able to give more expert testing and advice and repairs than a parts place, whose only option is to sell you a new or rebuilt alternator.

So it took me a bit, but today I finally fixed the car. I double checked the readings everyone suggested, pulled the alternator from the car, ran the check at a parts store to confirm, bought a new one and all is working well.

I don’t think I’d have tried this on my own without finding this forum first. I’m really very grateful for everyone’s help here and I’m happy to know y’all may will be here when the next problem pops up.

Thank you!

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Thanks for the update, happy it turned out ok.