Confirming -Bad Alternator -bad diode?

Tech11

Tech11

Well-Known Member
Joined
Dec 7, 2007
Location
Greensboro
I've had a electrical gremlin in my jeep for some time now. I would let the jeep sit for a few days and it would be dead. I couldn't figure it out. I thought maybe it was my gps I wired up, so I disconnected it, same problem. I disconnected my winch, thinking maybe the wireless controller was doing something fishy, or I had some sort of other short there. Same problem.

I had the battery tested and it is 100% (no dead cells). The store said to take my alternator off and they could test it. I live in an apartment and they frown on parking lot mechanics, so for now that wasn't an option.

I was going to disconnect the negative cable and come back in a few days, hook it back up and if the jeep started it'd have to be the alternator (wouldn't it?). But I remembered hearing about testing the alternator for AC and that would show it had bad diode(s) doing something and causing an electrical drain.

While running the volt meter showed 12.9-13.9 v and it was all over the place, up and down.

Turned the jeep off and took the negative cable off the battery, I hooked up my volt meter,pos to alt pos, and neg to a good ground, set it to AC volts, and it was all crazy showing

O.L,12,14,20,07,O.L,06,14,12,O.L,19,06 etc

just all over the place. Isn't O.L overload? Doesn't this mean my alternator is fubared, or did I do something wrong?
 
Sounds like you might have a drain on the battery if it is going dead after sitting for a while, with the key off, use a testlight and clip the wire lead to the negative battery cable with it off of the battery, then touch the tip of the testlight to the neg batt post. If you have any glow at all then you have a drain on youre battery, probably form an aftermarket accessorie. I have seen alternators draw with key off.
 
cars are 12 volt DC. don't ever test a charging system in a/c.

as already said sounds like you have a parasatic drain on the batt.

disc the negative batt cable. set the multimeter to dc amperage, move the cable on the meter to amps plug.

hook the red lead to the negative cable and the black lead to the negative batt post.

turn key in vehicle to "on" position. then turn it off and monitor the amps. should drop below 0.50 amps on jeeps.

if it doesn't drop below that start unplugging fuses until it does. then you've found the problem.
 
i went through this with my grand cherokee. turned out the rear glass switch was broken. this made the body control module think the glass was ajar. the bcm wouldn't go into sleep mode at that point and it would drain my battery
 
I remember hearing this from days ago and found it on another forum
Just an fyi, a bad diode in the alternator can drain the battery even when the engine is not running. You can check the amount of A/C voltage leaving the alternator by putting your multimeter leads on the battery (Positive lead on +, and Negative lead on -), with you multimeter set on A/C voltage you should read no greater than .5VAC, if you get more, your rectifier bridge on your alternator is bad and needs to be replaced.
Click to expand...

Thats what I was going on. - I was going to upgrade to a 160 amp alternator (from a Durango)
But NOW I know how to look for parasitic drains.:beer:
 
Easiest way to check to see if it's a bad diode is to take the belt off. If it's a bad diode the alternator will spin on it's own.
 
A bad diode should cause a current leak and trip the ALT light in the dash, too.

The quote Tech11 posted sounds like it should work, but I've never tried it myself. The idea there is that you'd see the leaky voltage coming backwards (A/C) out of the rectifier.

I can't tell what you were describing when you said you took the negative cable off the battery... but don't do that while it's running. If you didn't have a bad alternator before, running the engine without a battery for reference voltage is a good way to ruin one.
 
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