Alternator Help Please!

[Trouper]

Newby here, please help!

I just bought a 1985 Scottsdale to tinker with, but am confused about a problem with it. The previous owner took out the 6cyl and installed a 350 4bbl with a 350 turbo tranny, and it has had electrical trouble ever since. The alternator is not charging the battery. Isn't there supposed to be a wire from the back of the alternator going to the battery? If so, mine doesn't have one. The alternator has a plug on top with a red and white wire, and one plug on the back with one red wire. When jumped, the truck runs great, and has power for the lights and other things, but when you turn it off, it won't restart. The battery is fine, been charged several times but won't hold a charge. Please help this poor newby out. Pics would be great.

[junkyardpartseeker]

I don't have any idea. Just wanted to bring you back to the top so someone who will know can help you out.

Tommy

[44th Miss Inf]

Trouper,
You said the battery is fine, then you say it won't hold a charge. Which is it? Not grumbling here, just need to know to try and help.

Virgil

[CT-Truckster]

hey somebody else from tn on here, i think that makes 4? welcome to the site. from what your sayin i get the battery doesn't appear to be charging while the truck is running? get a volt meter and check the voltage at the battery while its running, should be around 14 volts i think ? what does it do when you try to restart it? does it turn over real slow like the battery is to weak to crank it up, or does it not do anything? sounds like maybe your just running off the alternator. try unhooking the + battery cable while the truck is running and see what happens. just trying to throw a few idears at ya to try and narrow the problem down.

[Trouper]

When I jump it off, it runs fine. If I take off the cable, it keeps running fine. I think the battery will be fine once it gets a charge back in it, but it just won't recharge while in the truck. I will try some other stuff and get back with ya'll. I was hoping someone could take a pic of their alternator, and post it, so I could see the wiring off of it.

[44th Miss Inf]

I would go somewhere and have the battery 'load' tested. Some of the big 'chain' stores will do this (Autozone, Advanced Auto parts, etc)
Crank the truck and take a screwdriverand hold by the rear bearing of the alternator while the truck is running. You should feel a 'magnetic' pull. This means the alt is putting out. (at least some) WARNING--- BE SURE NOT TO TOUCH ANY CONNECTIONS WHILE DOING THIS.

Virgil

[77bowtie]

I gonna make a few assumptions. You have a 1 wire Alt sys, the starter relay is where the battery is connected to the alternator. Check for proper connections there. As I see it your Alt does work, as shown when you disconect the Batt, the engine still runs. Which is right. Its just the Alt is not feeding the Batt. And that connection IS at the starter relay. Good Luck....Jon

[Krosati]

While the truck is running and the battery is disconnected, your alternator is charging.
Check voltage at the alternator, should be over 13-14 volts. Now check the battery (still connected) if it's less than the first test, you have a blown fusable link or a wire is disconnected.
Try running a wire from the BAT post on the alternator to the + on the battery. If the battery does not hold a charge, you have a bad battery.

[SCOTI]

For what it's worth, the old 'pull the cable/if it runs it's ok theory' is only partially correct. A vehicle can run w/o the cables hooked-up when 1 or more of the diodes in the alt are bad. Since the diodes are what convert the a/c current to d/c, one or more means less useable charge making it to the battery.

Get the load test @ Autozone or whatever local parts house that's close to you so you can get an accurate volt/amp reading & diode check under load .

[N2TRUX]

Scoti speaks from experience on this issue. I was going to add that if you are field testing, and pull the battery cable, try turnning on the lights, a/c, radio, or anything that draws power. This will give you an idea of how much output the alternator has. If it can handle all the load, it's "probably" in good shape. If it dies as you turn things on, it may have a partial output as Scoti describes, from blown diodes...

[AirMale]

one more thing....I recently swapped out my alternator and put a new one in. It needed a jump to get going but started fine after the battery was charged. When the truck sat for a day the battery would die. Turns out after I chased down the draw on the battery it was a bad diode in the alternator draining the battery. This is something that won't show up on a bench test at ADVANCE ect., the unit may test fine but still draw current when not running. That was a one of those weeks that made me crazy trying to figure out where I had a draw.