High Alternator Voltage

[tc212]

My old 91 LS400 (>200K miles) is showing signs of an electrical problem. The dash warning lights will all light up when the rpms exceed 2500. At idle, and low rpms everything seems OK.

Checking voltage at the battery, it's a little over 14V at idle, but at the point the dash lights appear, it jumps to over 18V.

Looks to me like the alternator isn't regulating voltage very well, and the high voltage confuses the warning lights (?).

The dash light symptom is the same as I had a couple of years ago and had the alternator replaced then (except at that time, it wasn't charging the battery at all).

Ideas?

Thanks,

Tom

[95LS400Bob]

I would pull the alternator and take it to Napa or somewhere where they can test it. Probably has an internal regulator that has a problem.

[SKperformance]

Sounds like a bad regulator

buy a new one and replace it, unless you have a bad wire which is breaking off contact causing the vdc to spike

[obergc]

I would get the alternator checked out ASAP, those high voltage spikes are not doing your electronics any good.

[tc212]

Thanks, everyone. I replaced the alternator yesterday (now showing 14.2V consistently). The alternator was in stock, and the voltage regulator was special order, so I opted for the alternator. Unfortunately, the battery didn't like the high voltage, and got very hot -- to the point of slightly melting the case. Just proves this problem can't wait long for fixing.