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

[dennis112]

Hi guys,

I have a 68' GMC that wants to drain the battery over a day or 2 period. The alternator charges at 14V. The battery will hold a charge for weeks and start the truck if the cables are removed from it. I have a load tester and the battery checks fine. I've replaced the regulator with a new GM one.

So the truck charges the battery and the battery holds a good charge--So why does the truck drain the battery?

OK, so here is the technical stuff. If I remove the positive cable, there is no spark. I can place an electrical ice pick between the battery's positive post and the positive lead and the light will NOT light.

If I place a voltmeter between the battery's positive post and the positive lead, I read 13 volts. If I disconnect the regulator, it reads closer to 12 volts. Same thing happens if I remove the bulk head connector that enters the cab--this effectively isolates the problem to the engine compartment. I have cut all the tape off the wiring harnesses and have separated all the wires and yet the problem exist.

I thought that I should not read any voltage at all when all draining devices (radio, interior lights) are deleted from the equation. It seems to come down to the alternator creating some type of drain. All wires are fresh to the alternator.

So, I guess my main question is--Can an alternator (type with external regulator) drain a battery on its own with the key off?

 

[67coupe351w]

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

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.

 

[2nd Mustang]

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]

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.

 

[66Runt]

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,

 

[dennis112]

Do the old external regulator alternators even have diodes??? I know they commonly went out in "one wire" alternators. Do all alternators have diodes? I suppose that could be one of the differences between a generator and an alternator.

With the bulk head connector removed from the cab, all power is isolated to the engine compartment. Nothing gets to the fuse box nor the ignition switch. All cab connection will be dead.

I will try the MA test--it makes sense that there should be no draw.

I agree that it could be something that is intermittent. I've found the battery dead overnight sometimes and have hooked my ice pic on the positive lead/post and got no draw. No matter what, the battery will drain in a couple of days if it is hooked up. If the cables are removed, it will be fine for weeks. This problem thoroughly sucks. LOL

I need to figure this thing out as this truck used to be able to set 3-4 months through out the winter and then start up with no help. Now I need to remove a battery cable to do it.

I've ordered an alternator--I can't believe that what was once the most common alternator in the world needs to be ordered from, literally, 1000 miles away. Sigh. That's progress I guess. . . . . . At least it will be only $31 when it gets here, at my local "good", non-chain auto parts store. Worth the piece of mind.

Thank you for any and all help.

 

[66Runt]

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,

 

[dennis112]

Here is an update. After leaving the battery cables hooked up overnight with the bulkhead connector unhooked, I went out to check on the truck again this morning. Yesterday I had 12.76 volts at the battery after driving it around-after setting all night it was 11.8 volts.

I reached up to remove a plug from the back of the alternator and noticed that the alternator was warm to the touch, even though the truck had sat undisturbed all night in a 50 degree garage. Bingo I now believe that the alternator is definitely bad, especially since I had already replaced the voltage regulator with the same results. I also went ahead and verified all connections (again) with an ohmmeter. A check with the MA meter and know for sure that the alternator/regulator are creating a draw (without being influenced from the switch/fuses.)

It is a relief to know that a logical diagnosis has been made. Now it is just a matter of replacing the alternator and retesting the system. Hopefully the new alternator will arrive from Georgia tomorrow as expected. I'll update you with my findings.

Thank you for your help guys, and especially 66Runt.

 

[66Runt]

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.

 

[dennis112]

Final update. I replaced the alternator,reinstalled all the wiring as original and let the truck set overnight. Went out the next day and felt the alternator-cold. I tested the battery before starting-12.5 volts! It started and ran fine all day.

The alternator was the problem all along. . . . .

Note to others--although I was working on a GMC, the diagnosis and cures could very well apply to our Ford products too since they are of similiar construction having external regulators.