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post #1 of (permalink) Old 12-09-1999, 01:04 PM
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battery is draining while truck isn\'t in use....broken relay?

hi, sorry so long:
i first noticed that something wasn't right when my factory voltmeter was less than where it usually is. that lasted for about three days. then the truck wouldn't start. i jumped the truck and it started right up, but the voltmeter was still on the low side. i bought a new battery and the voltmeter was fine for the first two days. then it started showing less and less until a few days later the truck wouldn't start. my friend works at sears and he tested the battery and alternator. he said the battery was good and the alternator was bad. but why would a bad alternator drain the battery overnight. i know you're gonna say a bad alternator wouldn't charge the battery. but while driving, the voltmeter moves up in relation to rpm, so that makes me think the alternator is working. also, while the truck was on, we disconnected the negative cable and the truck continued to run. that makes me think that the alternator was putting out enough voltage to run the truck; hence i think the alternator is fine.
also, a friend of mine used a voltmeter or something to see if there was a drain on the battery will the motor was off. he said at first he saw a flight drain, but after about ten minutes, he saw no drain at all.
my other friend's bronco had a sticking fuel relay and the fuel pump stayed on. are there other relays that commonly stick on broncos?
i'm not good with electronics, so if you think you know what my problem is, please use laymans' terms.

1991 eddie bauer with 5.0.
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post #2 of (permalink) Old 12-09-1999, 02:08 PM
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Re: battery is draining while truck isn\'t in use....broken relay?

Hi, just a few comments. If one of the diodes in the alternator (the diodes convert multi-phase AC that the Alternator makes, into pulsating DC) shorts or goes to a low resistance both ways (still bad!), the alternator could still develop some power. Those diodes are always connected. When the engine is OFF, they can drain the battery down via the alternator output lead, through the bad diode, to ground through the guts of the alternator. I had this happen to me almost 20 years ago. Had to disconnect the battery Positive lead each time I parked until I could replace the alternator. Took the old one apart, and found one of the diodes shorted and overheated, un-welded itself, and tacked itself back on next to where it usually sat on the heatsink.

Back to that part above about "pulsating DC". Don't disconnect the battery while the engine is running. Without the battery there acting like a big capacitor to smooth those pulses out into nice DC, your electronics everywhere on the vehicle are getting slammed hard by high voltage pulses. Not good - can lead to real problems, expensive ones!

With the engine OFF, disconnecting one side of the battery, and putting an ammeter in series, can tell you about drain while OFF. For many vehicles now, they draw some, till the electronics goes to sleep after a while, then the drain really drops, but there always is SOME.

Without looking at a wiring diagram, I can't say for sure about a sticking relay, but you're probably right, in that is another drain that can happen.

If you suspect the Alternator, disconnecting the fat lead from the alternator to the battery, putting Ammeter in there in series (all with engine OFF), will tell whether the alternator output diodes are draining the battery. Should be zero drain there, if all is OK.

That's my 2 Lincoln's worth UncleDon

post #3 of (permalink) Old 12-10-1999, 12:17 AM
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Re: battery is draining while truck isn\'t in use....broken relay?

This exact thing just happened to my neighbors 79 Bronco. It had a 3 amp drain with the key off and all of th fuses removed. I disconnected the regulator and the drain stopped. He replaced the reg but it still had the drain. Replacing the alt cured the prob.

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