Battery draws down after 24 hours

The_Family_Tahoe said:

but it seems like 12.0 would be enough after one day of sitting to crank the starter.

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….so did it?

Just Nobody said:

This has happened three times to me.

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How do I test for shorted diodes?

Caman96 said:

….so did it?

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I didn't try. I should have tried to crank it, but instead I put the charger back on to top off the battery for a load test.

Sounds like a sulfated battery, 12v will be slow to crank but still might not have enough amps to maintain a crank

a fresh battery sitting idle should read 12.6v... 2.1 per cell

RichLo said:

a fresh battery sitting idle should read 12.6v... 2.1 per

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I tried to start it, but now the vats is preventing it from running. I'll have to figure out how to solve the vats then return to the voltage issue

The_Family_Tahoe said:

I tried to start it, but now the vats is preventing it from running. I'll have to figure out how to solve the vats then return to the voltage issue

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Only sure way to get past VATS/Passlock is to have it deleted in the PCM. Been there done that on my Burb.... Rawhide the crew cab is a '97 so he doesn't have that BS. Does have a magic button, that activates when someone plays around with the battery. Then you have to perform the Special Starting Procedure, which I've had a bit of practice with....

The_Family_Tahoe said:

How do I test for shorted diodes?

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Test, with a voltmeter, from that big power wire terminal on back of alienator to the alternator body. There should not be continuity.

Start with a load test on the battery. A bad cell in the battery can do weird things- had one recently that showed 12.4 volts- until you cranked it and would drop to about 6 volts. It would not stay running if you jumped it even though it should because the alternator was charging.

Cool trick- use a quality test light instead of the ammeter hooked between the disconnected cable and the battery. Will light initially due to computer and other parasitic loads- jump across the test light for 30 seconds with it still Hooked up between the battery and the cable- (this gives enough current flow and time for everything to power down. When you remove the jumper after 30 seconds there should be no light. If the light comes back on- you have a draw and you need to pull fuses until the light goes out.

If you can’t isolate it pulling fuses - other weird things that can cause a draw on these trucks that I’ve seen-

1- as stated before diodes in alternator- disconnect it and if see if the draw goes away

2- ABS pump motor shorted - disconnect 2 wire plug for abs motor and see if draw goes away

3- body control module- has relays in it that can fail and cause things to stay on- had this issue on my 99 Denali - and door locks could be affected by this as they run through the body control module- radio as well