Bad Alternator ?

[rkiceman]

Hi, this is my first posting. I hope i'm doing this right. I have a 2002 silverado HD2500 2WD with the Duramax Diesel. When I start the truck for the first time when it's cold, the Volt gauge reads down around 7 for about a minute. The lights are dim then after the minute it spikes back up to around 14 with the lights brightening then stays like that till the next cold start. Bad alternator or batteries ?

[brims]

Put a volt meter on the batteries before you start it to see what voltage the batteries are at, if you have 2, disconnect 1 and read them individually. If the batteries are low the alternator shouldn't wait to kick in. You can get your alternator tested at AutoZone, they can even load test your batteries for you.

[Diesail]

Sounds like batteries to me. It is normal for the alternator to be doing nothing at just after the start and then the ECM will ramp it up to the correct voltage. So when you are seeing 7 that is your battery voltage before the alternator start putting out.

[FarmerDave]

Might have a dead cell in one of your batteries. If you read less than 12.4V with the engine off, you will probably need to replace the battery(s). Should be around 13.6V when running.