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DaddyGlenn DaddyGlenn is offline
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Join Date: Dec 2004
Location: Pittsburgh
Posts: 2,325
Alternator stops charging

I’ve done a search but did not find anything helpful. I’m sure it’s out there, just can’t find it.

Here is my situation. 1978 911sc, battery is a couple years old, alternator is much older (12 years w/internal regulator). I drove from Pittsburgh to Indianapolis without a problem, then on to St. Louis the next day. Still no problem. The car sat in STL for a few days while I attended a conference and fired right up on Thursday morning when I headed to Florida.

In the afternoon I stopped at a rest stop in north Georgia and there wasn’t enough battery to start it back up. Multimeter showed just under 10 volts at the battery. I found the battery negative ground wire was a tad loose and tightened it up. AAA was called and 3 hours later, got the car jumpstarted. I drove to a nearby auto parts store and had the battery tested. Battery was good but needed a charge.

It was only about 15 minutes of driving to get to the auto parts store but the battery had enough juice to start the car without any problem. I drove another 15 minutes and stopped for the night. Hoping the problem was the loose wire.

The next morning, the battery was showing 11.8 volts and the car started easily. I continued south. Shortly after crossing into Florida, my radio would not stay turned on. This had happened the prior day before I found the issue. So I took it as a sign the electrical issue was back. I powered off the vent fan and unplugged my phone from the charger to save power. With a lot of drafting of 18 wheelers, I stretched that tank of gas and made it to my destination (Melbourne, FL) 3 hours later.

Generator light only came on when first starting the car. Other than the radio not staying on and my phone not charging like normal, there were no indications of a problem. I can only correlate the issue with running duration of 4-5 hours and heat. Both days were cool in the morning when I started out but got really hot.

Question ... is the alternator failing when it is hot, a typical failure mode?

I have ordered a rebuilt Bosch alternator to be delivered Monday and hope to be driving back to Pittsburgh on Tuesday. I just need to boost my confidence that replacing the alternator will solve the issue.
Daily Driver - '78 911SC RoW, Track - '85.1 944
Support vehicle - '00 Suburban 2500 4x4
Wife's - '11 Mini Cooper
Son's - '85.1 944

Last edited by DaddyGlenn; 10-06-2018 at 11:30 AM..
Old 10-06-2018, 06:56 AM
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