Engine/Transmission (1994 - 1998) Dead batteries

[jsjaastad]

1995 4x4 106,000 miles 5spd

Replaced both batteries about one month ago. After truck sits for about ten to fourteen days both batteries are stone dead. Did not have this problem with old batteries. Charged both and both load tested okay. Alternator appears to be charging okay. Cannot see a drain when disconnecting the ground cables or see any light on after dark. Is it possible for the alternator to short out and drain both batteries and yet still charge? Thanks for any assistance, Tom/Idaho

 

[fest3er]

1995 4x4 106,000 miles 5spd

Replaced both batteries about one month ago. After truck sits for about ten to fourteen days both batteries are stone dead. Did not have this problem with old batteries. Charged both and both load tested okay. Alternator appears to be charging okay. Cannot see a drain when disconnecting the ground cables or see any light on after dark. Is it possible for the alternator to short out and drain both batteries and yet still charge? Thanks for any assistance, Tom/Idaho

Yes.



An alternator is usually a 3-phase generator. Each phase is run through a diode to keep the voltage positive (and to avoid draining the battery). If a diode fails, it will allow current to flow the wrong way. When the engine's running, the alternator produces enough voltage for the current to flow the right way.



An oscilloscope'd probably immediately show the bad diode: (Peak, peak, flat; peak, peak, flat; ad infinitum). I had a car once where a diode was failing, but only in extreme heat, like driving across DFW in the summer. Once it cooled down, the diode would work again and the battery would charge.

 

[rjrkihap]

fest3er



Excuse my ignorance, but can this drain happen if the negative cables are disconnected?



Thanks



Ron

 

[JMetzger]

Above might be what your problem is but I once had a glove box light stay on after the door closed. You can test it by pushing on the light switch to see it it turns off. Mine had a bad switch.

 

[fest3er]

fest3er



Excuse my ignorance, but can this drain happen if the negative cables are disconnected?



Thanks



Ron

Only if one of the batteries has an internal short. Current cannot flow if the circuit is open.



Technically, all batteries have a certain internal resistance that causes them to self-discharge over time. When the internal resistance drops (approaches a short), the discharge is more rapid. One bad battery can drag both down when their connected in parallel.



If you suspect the alternator, disconnect the 140A fuse in the PDC. If that stops the discharge, you'll have a good idea where to start looking for the problem.

 

[hsmith]

With the negative cables disconnected there can be no external drain. Load test them individually after they have sat overnight disconnected. One will probably show lower than the other. Internal battery problem then.

 

[PBJ2500CTD]

jsjaastad,

When you stated that you could not see a drain when you disconnected the negative battery cables did you mean that you did not see a spark when disconnecting the cables, or that you disconnected the cables and still experienced a discharge after a period of time?
If the former is true you could have a slight draw that may not be sufficient to cause a spark. The best way to check for a load is with an ammeter, then pull each fuse to isolate the circuit with the draw. You will have some small current draw in the circuits that maintain memories such as the radio.
If the later is true a battery may be bad - but you had them load tested and they passed...
My guess, if it isn't a battery, it may be a light (glovebox, underhood, etc) or a bad diode in the alternator. Start with fest3er's suggestion and pull the 140A fuse in the PDC.

 

[Wingate]

You won't see a spark with some of the convience lights... I recently had to put my Vm inline on my father -n-laws truck to find a battery drain that would kill the new batteries in about 5-6 days. Turns out, it was a courtesy lamp on the inside of the drivers side rear door. When the Door was closed, the the light was up against the side of the seat, so you could not see it glowing. The VM tells no lies. I disconnected one battery, then put my VM inline on the other battery and started pulling fuses... Once you get it narrowed to a specific circuit, then the real hunt begins... . And don't forget the under the hood light.



Or, you could have just bought a bad battery!!

 

[jsjaastad]

Fest3er: I took the alternator to the repair doctor and he confirmed it had a bad diode. It is now fixed, tested and back on my truck.

Thank you and thanks to all who responded. Tom/Idaho

 

[Matt42]

1995 4x4 106,000 miles 5spd

Replaced both batteries about one month ago. After truck sits for about ten to fourteen days both batteries are stone dead. Did not have this problem with old batteries. Charged both and both load tested okay. Alternator appears to be charging okay. Cannot see a drain when disconnecting the ground cables or see any light on after dark. Is it possible for the alternator to short out and drain both batteries and yet still charge? Thanks for any assistance, Tom/Idaho

What brand are the batteries? Mostly curious, but I have had recent very bad experiences with one of the better brands.