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Voltage surging during idle

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Hello. I have a 1998 ROW C2 coupe tiptronic that I just replaced the alternator on. The new alternator has been in only for two days but now it will surge between 13V-14V when idling in gear. If I am driving normally, it is just under 14V, steady. If I turn on the Ac/Radio etc. the voltage will drop slightly (as one would expect) but it holds steady. As I am sitting at a stop light, the voltage will surge up and down and the rpm will surge with it, not much but enough that you can hear the engine sound change. As I pulled into work and put the car into park, the surge stopped and held steady just under 14V. I put the car into D just to test, the surge can back. Back into P, the surge stops.


From what I am reading, this is more than likely a problem with the voltage regulator, but why only when it is idling in Drive?

Edited by longrowe
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I suggest you test in this order:

- weak battery (do both a load test and specific gravity test on each cell)

- alternator (many auto parts shops can test alternators and regulators)

- regulator (inside the alternator housing can be tested at the same time the alternator is tested)

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I agree with Loren.  Do those things first.  I will assume that the car did not exhibit this behavior prior to replacing the alternator, so the first rule of mechanics is to go back to the last thing you did.  I know it will be a pain to pull the alternator once again and have it tested but that is what you need to do.  Also, it is possible to get a bad one out of the box.  I see you say "new" alternator...was it brand new or reman?  If it were me, I'd pull the alternator and take it back to where you bought it and get another new one.

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Had the battery tested, came out at 84%, had the alternator tested with the AC and lights on, showed as "Good". I have the VR off of my old alternator that I can try and swap in. The old one had a mechanical issue (bearing) so the electrical portion should still be good. Never had any trouble with the old one charging correctly. They did not bench test the alternator, simply hooked up to the cables and did a test.


The one I ordered was marked as "new" not re-manufactured.

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