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Old 01-14-2009, 08:12 PM

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Alternator / Voltage Regulator / Battery drain

I have a 1980 Trans Am. Originally a turbo car, I bought it with a Pontiac 400. I am nearing completion of the project and have run into a bit of a brain teaser regarding my alt/volt reg.

I kept losing my battery charge overnight. The battery, starter and alternator are new (alt is the 2nd one due to a blown VR in the first). I pulled the neg and put a meter between the neg cable and bat post. Sure enough, there was a drain. over the next couple of months off and on, I pulled everything.

I checked and cleaned all of the ground wires & added one to the alt bracket. I pulled the fuses one by one, then all at once, I went over all of the wires and found no "compromises". Finally, I narrowed it down to the plug in the back of the alt (one red and one brown...believe it is to the tach, or horn).

I found that if I leave the battery connected and the wires to the post on the back of the alt and pull the plug, no drain. However, if I leave it plugged in, the alternator gets almost too hot to touch in about 45min. This occurs with the ignition OFF. the next morning, the battery is completely dead. I checked the volts at the battery with the car running @ about 2000 RPM and I am getting about 14.9 - 15.6, so it is charging the battery.

I am sure that it is probably something very simple, but my eyes are crossed (figuratively speaking) and I am really not good with automotive wiring to begin with.

1980 Turbo T/A (now Pontiac 400)
Automatic TH 350

Any suggestions on next steps? Thanks for any input guys-


Last edited by eastboundanddown; 01-15-2009 at 02:55 PM. Reason: Clairification
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Old 01-15-2009, 04:46 PM
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Unplug the two-wire plug on the back of the alternator.
-- With the car turned off (or on), the red wire (F or field sense terminal) should show 12 volts. This terminal always has 12 volts and in some cases it is jumpered right over to the BAT terminal (the single threaded terminal).

-- The other wire (R or relay terminal) should not have power when the car is turned off.
When you turn the car on (but not to start), this wire should show about 12 volts and when you plug it back into the alternator light should go on in the dash.
When you start the car the wire should still show about 12 volts, and the alternator light in the dash should go out.

How the dash light works
The alternator light in the dash has 12 volts applied to one terminal - powered from the ignition switch. The other side is connected to the line going to the alternator R terminal.
-- When the alternator regulator is bad (or the car is off) it provides a ground through the alternator R terminal, and the dash light comes on when the ignition is on.
-- When the alternator is charging, it provides 12 volts on the alternator side of the dash light, which turns the light off. (if you apply 12 volts to both sides of a bulb, there is no voltage differential and it will not light).

My guess would be that your dash light circuit has 12 volts applied, even when the ignition is off. The wire might be connected to the wrong place, or it may be touching another wire in the wire loom. Someone may have bypassed the dash light circuit when the engine was replaced.

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Old 01-16-2009, 03:20 PM

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Thanks Bruce

Checked the F (field sense) wire and found it to have about 12V constantly, no matter what position the ignition was in. However, the R (relay terminal) did not have any juice with the ignition in the ON position. I pulled apart my dash and got up close in the wiring again. I rechecked the relay at the junction of the R terminal and the CHOKE light and found it to look good (translucent green plastic cover was not darkened anywhere). After swimming around for a few hours last night and this AM, I decided to open up the relay in order to varify that it was OK (new one was $10). Sure enough, the thing was bad. The copper winding was blackened and swollen and the wire was broken where it connects to one of the blades. I am on my way over to Checker to pick up a new relay. I will let you know how it comes out.

Thanks again for the push in the right direction---hopefully we have it solved!
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Old 01-17-2009, 05:22 PM

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Relay was bad, but the big issue was that someone had reversed the wires on the alternator. It looks like they took them out of the plastic fitting/plug and reversed them. switched them back, changed the relay and all is working fine. Once again, thanks
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Old 01-18-2009, 09:09 AM
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Alternator problems

I like the Delco Remmy alternator that you have, they are good and reliable. I have used them on a lot of different cars and heavy equipment. Check everything that Bruce is talking about, it is right on. I have always referred to the connections that he is talking about as 1: always hot (battery supply) 2: switched on hot (hot when switched on) and 3: battery lead (the big wire). But if you you still can't figure it out then you are either going to have to go into the harness and run down the wires as Bruce states or replace the wiring harness. However there is an alternative and it works real good! Take your alternator to your local automotive electrical alternator shop and tell them that you want them to convert it into a single post. They will install a module with a diode in the alternator and then it only requires one wire from the battery. I have done this countless number of times and installed the standard Delco Remey on everything from fords, volkswagen beetles to Franklin log skidders.

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