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Old 02-20-2013, 01:39 AM
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Alternator problems

Hi, all. First time here. Got an 85 GMC truck, three wire alternator. Alternator went out on it couple days after I got it, replaced with a new one... still doesn't charge. I've checked the connections, all seem right as far as I know. Batt wire goes to starter post that the battery is on, #1 wire is switched 12 volt, #2 goes to fuse block. It will run the truck with no battery at 12 volt (sometimes), but never goes up to 14.6 or so like I want. Any ideas? I'm completely stumped. Also, the wiring isn't stock in this truck. Not exactly butchered, but definitely not stock.

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Old 02-20-2013, 03:42 AM
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Sounds like you've got it hooked up right. Check your connections from the batt to the starter, starter to the lg lug on the alt. Check your grounds, from the battery to a clean, bare metal on the engine block, block to the frame, fenders and fire wall.

I'd use at least an 8 gauge wire from the alt to the starter and a 2 gauge from the starter to the battery and 2 gauge for the ground.
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Old 02-20-2013, 04:40 AM
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All of that seems to be in order. The ground was on the intake manifold, but I moved it to the alternator bracket suspecting that was the culprit, but no dice.

I've heard of a diode or 10 watt resistor put in line with the exciter wire, seeing as I don't have a dummy gen light, I have the gauge.

I've seen 50 million ways to wire one up, and none of them seem to work on the old alternator or the new one. They all tested good at O'Reilly's, but that machine didn't tell me any sort of output voltage, just a "passed" symbol on the screen.

edit: I have battery voltage (12.2) at the exciter wire.
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Old 02-20-2013, 11:27 AM
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How many amps is the alternator and what accessories are running when the engine is running? External regulator?
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Old 02-20-2013, 02:31 PM
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Is the alternator bracket painted? Better to include a ground wire directly connected to the body of the alternator.

The tech section at MadElectrical.com is very helpful, especially their write-up of "remote voltage sensing".
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Old 02-20-2013, 03:53 PM
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What size wire do you have running to the large stud on the alternator.
What is the charging voltage.
Dont rule out a defective replacement alternator, if it is an Autozone etc rebuilt there is about a 20% chance it is junk. Before you kill yourself chasing wires, pull it and have it tested.
Your wiring sounds ok if the wires are actually going where you think and have good connections.
If this is an 10si style alternator there is a hole in the back that has the regulator tab exposed. If you short this regulator tab to the case of the alternator it will cause a healthy alternator to put out at max rating. The hole is d shaped. See pic below. A small flat blade screwdriver will work perfect.
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Old 02-20-2013, 07:52 PM
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The alternator is a 78 amp. No accessories, maybe radio. Internally regulated GM 3 wire.

Yes, the alternator bracket is painted, but I've wire brushed where the ground connections are made.

8 gauge, no charging voltage.

I do believe it could be a defective alternator. Their little test machine said it passed, but it didn't give me any voltage readout, thus I don't trust it.
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Old 02-21-2013, 03:51 AM
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Assuming you measured at the lug on the rear of the alt.....If you are getting zero volts on the charging lug and the alt is checking out good, IMO, you have a couple of areas to look at:

1 - the exciter wire isn't making a good connection with the pigtail or there isn't 12v coming thru the exciter wire.

2 - the sense wire isn't connected into the elec system, therefore it's not telling the alt to produce juice or the pig tail isn't wired correctly (wires reversed)

You would only need a diode in the exciter wire if you had run-on (truck wouldn't shut off with the key)

you stated that the truck would run without the battery sometimes........ so your system is working? but just not as good as you'd like?
BTW that puts a huge load on the alt and is very bad for the alt, the alt wasn't designed to handle the entire load of the elc system all at once etc....

Last edited by EOD Guy; 02-21-2013 at 04:02 AM.
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Old 02-21-2013, 01:18 PM
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So, I found out I needed a diode in the exciter circuit because I don't have a dummy light in the truck. Did that, got it charging... but it every time I turned the headlights on it, it stopped charging. My buddy came over, and saw that the belt didn't have enough tension on it.

Sometimes you just need a fresh set of eyes to look at the problem, was so stuck on electrical that I overlooked the mechanical side of it. Problem solved, charges 14.6V all day long. Thanks for your help.
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Old 02-21-2013, 09:47 PM
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Good to hear it was a simple fix.

BTW you do not need a diode unless you are getting run-on....... I am running a gauge and not an idiot light and I have a plain switched 12v wire running directly to the alt, the diode won't hurt anything but probably isn't needed.
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