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Voltage Regulators and German Cars... Doh!

So, who can tell me what causes a voltage regulator to go bad? Aside from normal time wear and tear factors? If it’s of any use this is an Audi TT I’m dealing with, 2001 225 hp version.

Please bear with me for a moment, I just want to throw all the relevant information about the car out there so if there is something I missed maybe one of you will catch it. I did a timing belt replacement about 4 months ago. For about a week after it drove just fine, and then I noticed the Yellow battery indicator was coming on intermittently as well as the red battery indicator in the speedo area. I noticed that revving the car caused my lights to brighten and letting it idle normally caused dimming. After a bit of a search online and some tests at my local Autozone I decided that the issue was the voltage regulator mounted right on the back of the alternator. During the timing belt replacement we had to knock the alternator pretty hard to get it back into it’s bracket, I worried about the electronics at the time and chocked the voltage regulator issue up to the abuse it required to get it back into the bracket. So, I replaced the voltage regulator and everything seemed great for a couple months until a couple days ago. Same symptoms again. Lights pulsing brighter and dimmer at idle, charging failure, etc.

So, I’m getting ready to do a voltage regulator replacement again, but I’d really like to know if this is really the problem again or if there is something I’m overlooking, what could cause a voltage regulator to go bad, and if you think like I’m beginning to that I just got a faulty part somehow. I did get it at Autozone after all… but it was Bosch and I thought they did a pretty good job?

Crap… I forgot. I’ve already pulled the voltage regulator off. I inspected the points where the brushes make contact with the shaft coming out of the alternator. There does seem to be some dust/debris in there, like very fine metal shavings, but not so much that I would think it would have a hard time making contact, and even at that It doesn’t seem like the problem would be so consistent to the point that it would kill my battery in about two drives.

Failing battery, failing alternator, poor wire connection between the alternator and battery are a few possible causes.

The regulator may not even be the culprit but without hands-on it’s impossible to tell what’s going on with the car. Bosch makes good parts so if regulators are actually failing consistently then they’re being killed by something else.

I’m almost 100% that the regulator is at least part of the problem. As far as I can tell nothing else will cause that pulsing light effect. I did read on another forum about the wire harness failing between the battery and alternator, but I’m a little skeptical that this is the source of my problem, but at least there is a test that I’m going to try out for that one tomorrow.

Bad wiring or connections to the regulator can cause the problem too. The regulator needs a good connection to provide a reference voltage to do its job. If the voltage it ‘sees’ is fluctuating, it will be all over the board trying to adjust alternator output.

Or you could have just got a bum replacement regulator, or the alternator could be at fault. These things happen.

Thanks oblivion, I’m starting to think that it does have something to do with the wiring harness. I guess if I’m being honest I’m 1/2 trying to figure out exactly what the problem is and 1/2 trying to avoid pulling and replacing the alternator, because that is quite a bit of work on this compact engine compartment. I might as well be replacing the timing belt at the same time.

Just so I’m covering my bases here, and I really don’t even want to open this can of worms, but I’m also wondering if there is any chance that this problem could be related to the computer? I really, really don’t want that to be the case, but again I’d just like to ask if it’s possible.

Also, just to cover this one, I do know that the battery is good. It’s pretty new and was tested and charged at Autozone. It’s been through some abuse lately with all these issues, but I know it’s in good shape.

The problem is probably the alternator itself. In my experience, the alternator and regulator work together, and problems with one always translates to the other. The voltage regulators are built into the alternators, so it always seems best to just replace the whole thing as a unit rather than try to replace the bits and pieces. I know it costs more, but it results in a more permanent fix. Since the regulator replacement, with new, attached brushes, only fixed the problem for a couple of months, the rest of the alternator is highly suspect. A damaged field windings or stator can burn up a regulator pretty quickly.

BTW, the computer on this car does NOTHING to regulate the voltage for the alternator/regulator. However, a faulty alternator/regulator can damage the car’s computer and other expensive-to-fix electronic modules.

I suspect a bad ground. jump a cable between the engine block and the battery neg. post. let me know!