Hello-
I am having problems with my charging system. My battery idiot ylight turns on on the dash after the truck sits over night. The batteries will be below 12 volts, but the truck fires over fine. The batteries are 2 year-old Interstate 875 CCA's replaced at the same time. The alternator is an aftermarket that replaced a prior reman that went bad. I have had the batteries and alternator tested and they tested out fine. When I start the truck, the battery light goes out after 5-10 minutes depending on how long the truck has been sitting since it ran last. I have no accessory lights or stuff that I'm leaving on. Only recent changes were a trailer brake controller by Prodigy, but it has not been plugged into the wire lead, only leaving the wires to the harness and the controller sits in the glove box until I need it. Just cleaned my posts and cable ends when I had the batteries tested and there was minimal corrosion. Any thoughts or ideas? Seems to me that the batteries are not holding a charge or something is using a little power when the truck is off, slowly draining the batteries. Any thoughts or input would be greatly appreciated!!
Thanks guys!

 

I had a similar issue on my 03 excursion....ultimately it was the alternator, it tested fine on the truck then I pulled it off and the tested it inside and it tested bad...I would recommend testing alternator inside on the bench as the outside test for me was inaccurate....I also had a rebuilt alt installed at the time

 

Mine was tested inside on a bench at Schucks, if that helps. Still considering going to an alternator shop and having them test it to get a second opinion. Thanks. Any other ideas?

 

I had a similar problem to yours. The alternator checked good with standard tests. Have you tried to put a really good load on the alternator and then check it? That's how my mechanic found out the alternator was bad. On another car I had, the door switch was bad and that left one of the lower inside lights on, which I couldn't see in the daytime.

 

Have batts seperatley load tested, while disconnected from the truck. you may have a cell going bad. after running the truck for a while is the alternator way hot? (like could fry an egg, which is a sign of overworking the alt) or is it normal temp (can touch it, but not for too long). OR
The two wire connector on top of the alt. is for your idiot light, make sure the connector is tight to the alt.. sometimes a little dielectric grease will help the situation. Replacement plugs are sold at various auto part stores.

 

batts were load tested removed from the truck. I will try some dielecric grease for the contacts as suggested. So far I have .15 volts drop in the batteries over 3 hours. 12.85 down to 12.69 volts. Haven't noticed the alternator being hot, but will keep a better eye on it. That's what happened to the last one. It was fried and smelled bad!

 

Any starting issues? Sounds like either a bad battery or weak alternator. Start it up, turn on your lights and fans and check you battery voltage then.

 

starts no problem after sitting over night. I'll put a load on it and see what happens.

 

See below excel spreadsheet for load test. See if it says anything to you. Hi-idle seemed to make a difference with all accessories running. Before Hi-idle, the volts were declining steadily.

Any ideas?

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All those numbers look good enough to keep your light off. Was your Batt. light on during your test?

 

No, but I drove my truck home this morning. Light should be on tomorrow morning after the batteries bleed down a bit more. ???????????

 

How long after you start the engine before the light goes out? Have you checked the wires that plug in the top of the alt for corrosion?

 

I'm wondering if the alternator is going out. The voltage should be going up not going down as the load increases. When you put it in high idle the rpm's of the alternator increased thus increasing the output. It should level off at certain point and not go down as it did in the last 3 minutes of the test.

Have you tried disconnecting the batteries over night? Test the voltage right agter disconnecting and right before reconnecting in the am. There could be1 bad cell that is causing the others to drain down and "messing" with the voltage regulator in the alternator.

 

One thing that jumps out at me is the volts dropping from 12.8 to 12.2 in a few hours. That much drop should take close to a month with two big honk'in truck batteries. I would unhook the negative lead on both batteries, then stick an amp meter inline with either one of the two and see what you have. If you're showing over say .20 then I would start pulling fuses one at a time until it drops out, then go from there.

 

Thought you all might appreciate the update. AS you remember, I recently added a trailer brake system to my truck, but have left the wiring pigtail plugged in and the trailer brake unit unplugged. Per the manufacturer, I ran a test that checked the milli-amps with the truck off. With the wiring loom disconnected, it drew 8 milli-amps. With the loom connected to the truck but not the controller, it jumped to 28 milli-amps. After connecting the controller, it only increased to 29-30 milli-amps. So, tekonsha is going to warranty the wiring loom and replace it (suspected faulty). I will check the new one when it arrives with the same tests to confirm the fix. Thanks for the help. Oh, by the way, I also removed the wiring loom from the truck and let it sit for 2 days, and after that, the battery light did NOT come on as it had before. Enough proof for me to pin the wiring loom as the culprit.

Thanks again!