Alternator draining battery

[Scratcher]

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I have a '79 with a 350 from an '81 Corvette. It has HEI ignition. A few months ago I replaced the battery and alternator (3 wire type). After that, the battery would drain overnight. I added a switch on the negative battery post as a temp fix. Then recently I traced it to the alternator. I disconnected the exciter wire. That fixed the problem. When I turn the truck on, the alternator does not kick on until I rev the engine. I fixed the problem, but don't really understand how that fixed the problem. And will running it with that wire capped off have any negative effects?

 

[Charlie]

 

[bucket]

I would assume it's a defective part in the alternator. But I only know enough about them to be dangerous.

 

[Matt69olds]

If it’s draining with the heavy wire connected to the stud most likely you have a bad diode. If you disconnect the small plug and the drain goes away the regulator is bad. There are tons of YouTube videos on testing and rebuilding GM alternators. It’s not hard, basic hand tools and a cheap meter is all that’s needed.

 

[Scratcher]

Its not the heavy wire on the back. The one I have clipped is the #1 wire on the connector (connector has two wires). I believe it is the "exciter" wire. If that means my regulator is bad, wouldn't the voltage shown on the gauge in the dash be fluctuating? It reads a steady 14.5 V.

 

[Matt69olds]

Not necessarily. All it means is the regulator isn’t going to “sleep” when the engine isn’t running. Be thankful the regulator failed this way! I had a regulator go bad on a built up 87 442, stout 403 engine, etc. the regulator wouldn’t control the voltage. My ex-wife was driving the car, she came home and told me the turn signals, stereo, and lots of other things weren’t working. Not to mention a horrible rotten egg smell. I started it up, immediately noticed the volt gauge pegged. It excessive voltage burned out most of the bulbs, fried the nice Kenwood stereo, boiled the battery, and later on the ignition module. Lots of expensive damage caused by a cheap part.

 

[semperfiguy98]

I've seen this before when the wires are hooked up backwards. May be worth looking in to.

 

[Jim G]

I just joined on a few minutes ago and I am 100% stumped on my issue... over a year ago with factory wiring off the battery I changed my alternator to a single key power with battery leg alternator, part number 7861-7an, it is off of a 94 model chevy 1500 105 amp regular every day alternator (stock) 2 weeks ago I cut the factory wires that had those little white cylinder deals off and extended the wires just so I could make it look cleaner under the hood, and now I am on my 5th alternator. I have completely simplified my wires down to just one key power and ran one wire off the lug to the battery and once the key is turned to the on position (does not have to run just power on) and then turn key off unplug key power wire but leave battery wire on lug the alternator is back feeding from the battery. Does anyone have any idea what is going on...?

 

[hatzie]

I just joined on a few minutes ago and I am 100% stumped on my issue... over a year ago with factory wiring off the battery I changed my alternator to a single key power with battery leg alternator, part number 7861-7an, it is off of a 94 model chevy 1500 105 amp regular every day alternator (stock) 2 weeks ago I cut the factory wires that had those little white cylinder deals off and extended the wires just so I could make it look cleaner under the hood, and now I am on my 5th alternator. I have completely simplified my wires down to just one key power and ran one wire off the lug to the battery and once the key is turned to the on position (does not have to run just power on) and then turn key off unplug key power wire but leave battery wire on lug the alternator is back feeding from the battery. Does anyone have any idea what is going on...?

The sections of wire from the ring terminals to those white nylon baluns are fusible links. If you replaced them with regular wire and you develop a short it'll burn down your truck.

I have a '79 with a 350 from an '81 Corvette. It has HEI ignition. A few months ago I replaced the battery and alternator (3 wire type). After that, the battery would drain overnight. I added a switch on the negative battery post as a temp fix. Then recently I traced it to the alternator. I disconnected the exciter wire. That fixed the problem. When I turn the truck on, the alternator does not kick on until I rev the engine. I fixed the problem, but don't really understand how that fixed the problem. And will running it with that wire capped off have any negative effects?

#1 is the Field Exciter wire. This wire receives voltage when the ignition is switched ON. It's the White 47Ω resistance wire to the ignition switch or Brown non resistance wire to the BATTERY warning lamp in the base model instrument cluster.
#2 is the Voltage Regulator Remote Sense wire. It's red and usually runs to the BAT terminal on the starter but could be wired to the J-Studs on the firewall depending on the year. There's a fusible link at the end of the wire away from the alternator.
The Charge wire is on the BAT terminal on the alternator and usually runs to the J-studs on the firewall. Again. There's a fusible link at the end of the wire away from the alternator.

The field wire should be an open circuit with the ignition switch in the OFF position. Either the regulator is bad or the ignition switch is bad. Unplug the alternator and check for voltage on the Field wire with a meter. If there's battery voltage on that wire with the ignition OFF the ignition switch is bad... if there's no voltage the regulator is screwed up.