Need help finding a short--non Ford--long

dennis112 said:

Hi guys,
If I place a voltmeter between the battery's positive post and the positive lead, I read 13 volts. [/b][/i]

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Check voltages with circuits intact and Currents by breaking them.


So what that means is you want to check the current between the post and the lead with the engine off. There may be a small enough draw to not light up your light but enough to bring the battery down after a few days.

dennis112 said:

Hi guys,

--So why does the truck drain the battery?

If I place a voltmeter between the battery's positive post and the positive lead, I read 13 volts. [/b][/i]

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By placing one lead of the voltmeter on the positive cable (assuming it's disconnected from the battery) and the other lead to the positive post of the battery, you've basically just put a jumper between the two. If you are reading a voltage of 13 volts, then that tells me there's a solid negative ground on the on the positive battery cable. I guess the next step is to disconnect the negative cable from the battery, place both leads of the voltmeter on the negative and positive battery cables (removed from the battery) and measure the ohms resistance. You may see a high resistance due to radios, lights, etc, but if you see a solid resistance reading (less numerically) then there's your culprit. A solid short is draining your battery. Good luck.

Maybe a shorted diode, or do you have an alarm?

If you don't have enough current draw to light a test light bulb when you checked it (that would fire off at less than 0.50A), then either it is intermittent, or you missed something. Or something is getting left on. I've seen rec's do this when a diode shorts and the outside temps drop. Rare, but does happen. Just for fun I stuck a rectifier with a bad diode in the alt of one of my cars. Ran through the whole summer, and didn't "act" up until winter when it got parked outside. However, when I checked it in the morning, there was a pretty substantial draw, and with the battery being cold, it was all ready behind the eight ball.
I'm not a fan of checking voltage. Check for current draw. If you are killing the battery over night, it's a big draw.

The other thing I've seen is car alarms causing a draw. They send enough current to drain the battery but not enough to make the horn sound loud enough to hear it. Usually, something sets the faulty alarm off several times, or it's one of the old type that don't automatically reset.


I had a car brought in for a drain. When the lady let it sit for a week or more, the battery would die. The short story was, there was nothing wrong with the car. She would pull into the garage which was well lit with the door open. Get out and not close the car door all the way, leaving the interior lights on. Looking through the windshield the car looked dark because of the well lit garage. She swore up and down it wasn't her, until her husband lead her into the garage at night... three nights in a row!

I bought him a bottle of Crown Royal, and adjusted the door striker for free.

Good luck,

Yes diodes are what is in the "rectifier"

The name Alternator comes from the fact it generates Alternating current. The way we rectify this to Direct Current is by using blocking diodes that only let the current flow in one direction (Think Flux capacitor from "Back to the Future" ). Viola, DC from an AC source. When we eliminated the external regulator, we moved the "Logic" circuit to the inside of the alternator.

If you have a diode go short under load, then remove the battery cable the diode will go open before you can get your meter leads hooked up. So it appears there is no short.
Try disconnecting the alternator over night, and see if that doesn't help.

While I'm not a fan of changing parts, if your alternator is that old the brushes and bearings themselves could be worn enough to warrant it. I would suggest you find the short first, before replacing the Alternator. I would also suggest replacing the regulator and alternator as a pair. Chances are the regulator is just as old.

Also, Most of the old alarms were wired in direct to the battery, and placed under the hood. Just in case you've got one of those.

In your truck with all things working properly, you should see aproximately 50mA to 250mA draw, depending on options. The Voltage regulator alone will be about 50mA.

Good luck,

Congratulations!

Great find!
Sounds like you've found your "smoking gun".
A real pleasure to see you work through the issue instead of throwing parts at it. Wish more people would do that.