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Old 02-13-2014, 04:22 PM   #1
Busted Knuckles
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Default Alternator shutting off?

I've noticed this winter, with my headlights and wipers on, that my alternator seems to shut down while at idle. This appears to be leading to damage to the battery, and I had to replace one this past summer, but got a pro-rated refund. Now the new one is showing signs of weakness.

As I'm warming up the engine, the voltmeter shows 13.5-14V, idle speed around 1,200 RPM. This is normal and healthy. But, as soon as the engine warms up, the RPMs at idle drop to 750-800 RPM and the voltmeter drops to 11.5 to 12V. Revving up to 1,200 RPM sometimes gets the volts to 12.5V, but I don't see 13V again until I'm driving with RPMs above 2,000.

I noticed this with my old alternator before it bit the dust a couple of years ago. The new one did the same, so I thought it was normal. But, because I spend a lot of time in commuter traffic, I've seen the voltmeter drop to as much as 11v while stuck in line at a stop light.

I'm wondering if anyone else has experienced this, is it normal, or do I have a problem?
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Old 02-22-2014, 05:15 PM   #2
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It's normal (or perhaps I should say common... there's more to it but I don't have time to discuss it now), even higher output alternators provide very little juice at low RPM. The only real fix would be an underdrive pulley. A more powerful alternator (such as the 110A from a MKII MR2) helps recover quicker though when the engine is running at a higher speed.
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Old 02-25-2014, 04:17 AM   #3
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Well, the guys at supramania.com suggested I run a voltage drop test. I did, and wound up with a 1.0V drop between the alternator and the battery. So, I'm tracking that down now. I found a bunch of issues, like a big drop across the fusible link, so I replaced it. Then, the connector from the fuse block the the battery was frayed, so I trimmed the wire and put on a fresh connector. I'm at 0.65V now. Also, the alternator voltage bothered me, so I took it for a bench-test. It failed, so now I have a replacement alternator, thanks to a lifetime warranty. I hope I can track down the rest of this voltage drop. Thanks for your advise, tho. Even with the voltage drop at idle, the battery was suffering from lack of charging power at running speed that wasn't allowing the battery to fully charge at all. That lead to the dimming lights that was bugging me at stop lights.
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Old 02-27-2014, 03:57 AM   #4
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Yeah, 1v drop is pretty big. This is a common problem among all older cars, wire oxidizes and resistance increases, as resistance increase things heat up which further increases heat and can also cause wires to oxidize faster... vicious cycle. The old wire type fusible link used from '86.5 up to the mid '89 model is famous for causing voltage issues. It can be swapped with the sealed FL used later on.
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Old 03-04-2014, 01:57 AM   #5
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Default Solved!!... for now.

Well, I went on the hunt for the last of the voltage drop this weekend. I wound up re-wiring the wire between the fuse block and the battery. I found the fusible link wire for the battery connection was trimmed to about 2 inches long! Everything I've read is this wire should be no shorter than 9 inches. I picked up a 14 ga fusible link at the parts store and wired it in. I also re-loomed that part of the wire harness and really cleaned up the area. I also found the 80A sealed FL from my old Toyota Pick-up fit in place of that fusible link in the fuse block.

After double checking all the connections, I re-ran the voltage drop test. Now I'm at .442V drop total! Today, we had cold and rain. I drove to and from work with headlights and wipers on. At the stops in traffic, I no longer am seeing a dimming of the headlights and a slowing of the wipers! I feeling good, now!

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Old 09-23-2014, 11:27 AM   #6
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For any of you still having problems, CP Generator sells overdrive pulleys to bump up the alternator speed at idle. Now, my alternator maintains 13V at idle for less than $30.
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