Alternator Weirdness

[OK-944]

My 85.5's alternator can go weeks behaving "perfectly" - which means reading just over 12V at first key position...then reading rock-solid at 13.8 while driving, with only slight downward blips while using blinkers, brakes, etc.

...But then, for no obvious reason, the alt. will suddenly dip to 12.5 or so - with the gauge needle wavering as if the regulator is about to bite the dust.

The biggest power draw by far is when I push in the mileage reset button, which causes the alt. reading to go well below 12V with the car idling. But sometimes pressing this reset button actually seems to "cure" the alternator if it had been acting up, as the gauge reading often returns to normal after this, and will, once again, be fine for quite awhile - only to misbehave yet again at some random point in the future.

Alt. was rebuilt a couple of years ago, and I've also carefully cleaned/restored all grounds. Belt seems pretty tight. Fuses seem fine, but there are some accessories which don't work (driver's side mirror, blower motor, sketchy sunroof, etc.) - but wouldn't any power draws from any of this stuff show up as a consistent deflection of the gauge reading?

Car otherwise has been extremely reliable - but I hesitate to take it on longer (200 plus miles) trips before I can tackle this issue. At any rate, any and all advice is welcomed!

[dgcantrell]

Check the voltage regulator mounted in the back of it. The brushes can get worn and stick from time to time. I had a gravel get sucked up into mine one time (dunno how THAT happened) and fell out when I took the regulator out. I put it back in and ran that puppy another 10 years without issue. $40 for a regulator. Takes about a half hour to change from that I remember.

[Jfrahm]

You can also replace just the brushes, there are youtube videos on the process. You need a soldering gun or similar. If the brushes are worn out it's probably a good idea to pull the alternator and repack or replace the bearings also. Or plan to do it soon.

[djnolan]

A good test of the charging system is to first fully charge the battery. Then idle the engine and turn all of the accessories, brights, fog lamps, A/C, blower, RRdefog, etc. Volt gauge should hold steady and only drop a few tenths of a volt. If it drops down near battery volltage then the alt or the regulator or the belt is suspect.

Also what year car do you have?

[djnolan]

I had some weirdness on my 85.5 and it turned out to be the alternator bracket was loose, yet there was no belt squealing. I had even ordered a rebuilt alternator and all it was was a missing bolt. I still have the alternator if it turns out you need one. It is a 150 Amp model, too, whereas stock is only 115 amps.

[88Silver924S]

When you say the trip odometer causes voltage spasms it makes me think that the problem is in the instrument cluster not the alternator. What voltmeter are you using i.e. is it factory installed or aftermarket installed or handheld? If it is installed in the instrument panel then maybe it's grounding or wiring is sketchy? I have a manual trip odometer, I assume you have an electric/electronic one?

Resetting a trip odometer shouldn't noticeably affect the voltage. If it does (assuming you have an electronic odometer), then the instrument cluster/voltmeter wiring is where you need to focus, nicht wahr? I suspect dirty contacts, spray the connectors with electronic cleaner and work them on and off a few times. Good luck.

[dgcantrell]

Mine does. Always has.. It's activating an electromagnet and will cause the needle on the voltmeter to bounce.