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I had this problem a few weeks ago and replaced the alternator (which had presented problems several times after being rebuilt a year ago) with a used alt from a boneyard. The original symptom was that the charging system was putting out around 18+ volts and the battery boiled, then popped the tops. I did notice a motif, though. It seemed to do this when running under 2500rpm, then leveled out above 2500rpm to normal levels. (First realized this when the fan blower motor kicked into super high gear at low revs, then lowered to normal levels at higher revs; unfortunately, I don't have a voltmeter installed inside the cabin of the car).
After the new Alt was installed, I checked all my ground points along the charging system path. No problems that I could find -- cleaned and retightened several of them. I chased the wiring loom (which was replaced with the newer loom about 3 years ago) under the crankshaft pulley and didn't find any chaffing or blatantly visible signs of damage as might be suggested in the 700/900 FAQ. The car ran for several weeks with no problem, then today it started freaking out.
It got really bad tonight while driving home in the dark. The low beams were more like high beams and though I only had to drive about 10 blocks, after 2 blocks, the battery popped and I heard the cap bounce around inside the engine compartment and then saw it in my rear-view mirror on the street. It never popped that bad before -- not completely and violently off.
What should I check next? In the morning, I'll inspect my newish voltage regulator. Perhaps I'll need a new battery? One tiny sentence in one buried paragraph in the Haynes mentioned that a bad battery may cause the system to overcharge, but this seems like a lot of voltage. It does seem to be the same cell that keeps popping its top. (Outside cell, nearest the positive post -- engine side is positive, not the fender side, correct? Can't remember right now. But perhaps that is taking the brunt since it is on the positive side.) Current battery is less than a year old.
Is there a diode in that system I don't know about or can't see on the wiring diagrams that I should suspect? Is there some blatantly obvious ground I should have noticed? Grrr. TIA.
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