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post #1 of (permalink) Old 07-07-2003, 09:51 AM
OLY29
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Charging System

I am working on my 77 Chevy truck and I am having problems with the charging system. I have two alternators that have both been checked by a electric shop and both are good. I have replaced the cables from the battery to GND and to the starter. The battery is only 3 months old. When I measure the output of the alt. it measures 12.50 volts. Both wires to the alt. measure 12.50 volts when the motor is running. I have charged the battery for over 8 hours on the trickle charge (about 2 amps). All of the fuses test good. Where can I check next to fix this problem.
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post #2 of (permalink) Old 07-07-2003, 01:31 PM
ratlover
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Re: Charging System

did the shop test them on the truck or did they take them off? if you bring the rpm up to 15-18 hundred you still get 12.5?
post #3 of (permalink) Old 07-07-2003, 02:15 PM
OLY29
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Re: Charging System

Both units were tested out of the truck. The person testing them put a load on both and said they were good. I ran the motor up to 3000 and still 12.50 volts on the output.
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post #4 of (permalink) Old 07-07-2003, 03:10 PM
kd7kmp
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Re: Charging System

Have you checked the regulator wires? What about the condition of the chassis, engine, etc. where the ground wires make contact. Clean wire lugs on rusted metal is no better than a bad wire. What series alternator is this? a 10si, 12si...? Were the alternators checked at a real automotive electrical shop or the local Checker? Need some more info to help.

Kevin
post #5 of (permalink) Old 07-07-2003, 03:48 PM
OLY29
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Re: Charging System

Both cables were replaced and both places that they connect to were not rusted or covered with grease. The GND wire from the battery goes to the bracket that holds the alt. The positive wire goes directly to the starter. I took both of the alt. to a shop that rebuilds alt, gen. starters, and also fixes problems like this. He said to make sure that the contacts that plug into the alt. were clean so I removed them from the plastic housing and cleaned the contact surface with emery cloth. The alt. is the stock one that came on a 77 Chevy pickup. Two wires in and one wire out. The alt. is self regulating. The only other GND wire is from the valve cover bolt to the frame and both connections are free from rust and grease.
post #6 of (permalink) Old 07-14-2003, 11:58 AM
**DONOTDELETE**
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Re: Charging System

I just finished dealing with a similar problem with a '79 Suburban I just bought. Charge regulation was all over the place, especially when things got hot. Also had intermittant headlights and no horn. All these systems come together at the junction block on the firewall. I pulled all the wires loose from the j-block, cleaned everything up. It also felt like one of the wires had a break in it, so I clipped and butt-spliced back together. No problems since, but not totally convinced that was the problem. AC compressor had been removed, but lots of wires loose in engine compartment. Removed excess wireing, as well. Could have been a contributing factor, or just coincidence. Whatever, do check out the j-block and wires going to it. There's a fusible link in there, also. Hope this helps.

Dave
post #7 of (permalink) Old 07-14-2003, 10:23 PM
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Re: Charging System

Does the idiot light work? Key on, engine off, it should come on. A burned out bulb can do your complaint. The bulb provides initial field current.

Test to see if it can charge on the velhicle by bypassing the internal regulator. At 12.5 it isn't charging at all - the light should be on.

Look on the back of the alternator - you'll see lots of vent holes but only one in the shape of a "D" about 3/8 diameter.
While running at idle watch the votage while you push an allen wrench or small screwdriver in through the "D" hole (no, you won't hit any rotating parts if you are in the right hole.) It'll go in about 3/8" then touch something - the connector tab for the regulator. Touch the tab and lean the wrench so it touches the tab and ground at the same time. That bypasses the regulator and should make it charge. Voltage should go above 13, maybe as high as 15 at idle. That proves the output is reaching the battery OK.

Then check the side connector.
Engine off, key on, unplug the side connector. Measure the voltage with key on. Both wires should have 12 volts on them.

The red is the "sense" line for the regulator - to decide whether to charge or not - should go to the "hot" terminal block on the firewall or to the starter "big" terminal.

The white or brown wire, goes to the idiot light. Key off it should read 0, key on 12 volts. That wire goes to the light, the other side of the light goes to switched ignition.

And - look close at the big wire from the alternator - sometimes there's an in-line fusable link about 6" from the stud connector. Disconnected that wire should also read full 12 battery volts.

It's a simple system, hope this helps.

Let us know what you find.


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post #8 of (permalink) Old 07-16-2003, 09:53 AM
OLY29
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Re: Charging System

Well after tearing the dash apart looking for the idiot light it looks like the only thing I have in the dash is the volt meter. Could this be what is causing the problem? I did all of the checks with the key on and off and everything mesures OK. I still haven't tried puting the pin in the ALT. to see what the voltage by making it run unregulated. Will try that tonight. I will try and connect both wires going to the volt meter together to see if that fixes the problem. Thanks for all of the information received.
post #9 of (permalink) Old 07-17-2003, 06:15 AM
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Re: Charging System

No No - don't cross those wires to the voltmeter - you'll let the smoke out! That would be a direct short - hopefully through a fuse.
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post #10 of (permalink) Old 07-17-2003, 06:31 AM
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Re: Charging System

You did make those voltage measurementsa with the side connector unplugged I hope. Plugged in they would be invalid readings.

If it charges when bypassing the regulator through the "D" slot, next thing I'd do is find which wire on that side connector is the one that switches on and off with the key (I think it's the brown wire, but confirm it.) Then while running, jumper from Batt + to that wire - you can get to the wire connection by backprobing the connector.
If that makes it charge, then it's not the alternator.

I believe the switched voltage source is through a resistor of some kind, either a resistor wire or resistor buried in the harness if no idiot bulb is used.
I'd simply take a brake light bulb and bulb holder - or solder wires to the bulb itself - Wire directly to that wire on the side connector, the other side of the bulb to switched ign (+). Poof - You just created an idiot light.
Caution, you have to insulate the bulb holder from ground, as both sides are hot.

If it doesn't charge by that jumper trial, then take the alternator somewhere else to be tested = Autozone, Kragen etc. The rebuilder's tester or procedure may not be working properly - yes it can happen.

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