Thread: Headlights draining battery. HELP!!

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Have a 1987 S-10 with small block 350. Truck runs great. My alternator went out. I replaced it with brand new one. Everything seemed fine. That night I turned on the headlights. The battery was dead when I got home. Bought new battery, replaced alternator with another new one. The truck still runs great but the headlights still drain the battery. Have checked the wiring and see no visible problems. What else could this be??

What is the output voltage and amps of your alternator? Just because it's new doesn't mean it's working right!

Where did you buy the alternator from? Do they service / test them? There are a few things that could be causing you a problem... Simplest things would be to check all the battery connections at both ends of the cables!

Do you have a volt meter? Even a in-expensive one will be helpful!

What's the odds it came from O'Reilly's?

Welcome to CHR! No intention to offend with a basic question, but have you cleaned the battery cable connecting surfaces? I've seen several alternators and batteries replaced when all that was needed was to clean the oxidation from the terminals for a good, sound electrical connection. Lights pulling down the battery says that either the alternator internal regulator is bad, the alternator itself is bad, or the path for the power to get to the battery is blocked or high resistance. A good auto parts guy should be able to pinpoint this problem in about five minutes with some test gear once all connections are clean and tight.

Sometimes there is a fuseable link wire between the alternator and the battery, this may be bad and could be the cause of the trouble.

by no means an expert but it sure sounds like you have a dead short that is related to the headlights or headlight switch - - - one question, if I may, does the battery stay charged when you DON'T use the headlights?

My money is on a bad fuseable link (or equal) between the alternator and the battery.

IMHO, the fuseable link wire probably went out when the alternator failed. The motor will run (for a while) off the battery without the charging circuit working, but turn on the head lights and it is too large of a load without the charging circuit, which kills the battery. If the head light circuit was a dead short it would blow a fuse, stop the motor when the lights were turned on, or worse start a fire in the wiring.

A fuseable link would explain two replacement alternators and a new battery not solving the problem. Just my 2 cents worth.

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If I remember the alt lead goes to the starter on that truck. it could be a bad connection at the starter. I don't recall a fuseable link in that circuit but sure sounds like it.

a cheap or damaged cable does the same as corroded terminals
I always recommend buying the cheapest, smallest diameter, ALUMINUM cable from KMart or equivalent. It will assure a problem.

Make sure the regulator is wired to the vehicle. I've seen a few "Pro Built" street rods that the alternator had battery + to but the regulator wasn't hooked up. This might be a good time for you to purchase a decent volt meter from Sears or any where. It beats not having one and guessing.