2002 Chevy Duramax Diesel - Low Voltmeter Reading

How is the cranking speed? Sound normal, or weak? Original battery?

Have the battery checked, then put a new alternator on it.

If you supply a secondary 12V power source, while you do it, you won't have to reprogram the radio.

Lynlynch.......The cranking speed is fine. And I replaced the batteries since the problem started. I think the batteries were at the end of their normal usable life--a little over 3 years. The original Delco's didn't last as long as the current Walmart Everlast. I've thought about replacing the alternator but it is not a very cheap option to guess at. Can the typical car repair shop, or Chevy dealer, determine what the problem is without charging an arm and a leg for the diagnosis?

Thanks,

Don

Pull the starter and take it to any local auto parts place. They all have alternator testers.

First check the amp draw of the starter and clean the ground terminations.

So, is the symptom low voltage reading due to cranking, in which case the volt meter reading would gradually increase to 14V from 10V, or does the volt meter jump from 10V to 14V as if the alternator suddenly starts charging?

It sounds like the alternator is not putting out any current for those first 30 seconds, then starts working normally.

Some day, I suppose, it might just quit charging altogether. Then you'll know for sure.

It doesn't sound like something that will leave you stranded in the middle of the night.

I believe that the brushes in your alternator need replacing. If you do it now, and clean the brush slots carefully as well beforehand.

Also 1) note the orientation of the old brushes 2) insert the new brush using the same orientation.

Not all have an orientation. Check this out:-

I hope this simple picture helps.

If the brush replacement does not help, or the com rings are already damaged, then a replacement alternator is the best bet.....(this is assuming that you have an alternator with the electronics built in. If you have an alternator with a seperate controller, all bets are off (except for brush replacement of course!)

Andy, are the orientation marks the result of brush wear? And will the new brushes have the same orientation or have a flat end? Brush slots? Are those the slots/holes the brushes fit into? How should I clean the slots? Sorry for all the questions. I'm OK at changing parts but I'm not much of a mechanic.

Thanks,

Don

They are the result of normal wear.

Some manufacturers "pre-form" the brush end, some don't.

Use a vacuum cleaner and a piece of soft wood to make sure the slot is clean and the brushes can move freely (most important!).

Some manufacturers will not supply new brushes......I have taken bigger ones and cut them down to size, its not as good as original ones, but will usually give you another 20-50,000 miles......better than a kick in the teeth. They must fit well without "wobbling"....

Make sure the replacement is (as far as you can tell!) either the same/similar hardness or softer than the original brushes....

If you are lucky, yours are only dirty and sticking, but still long enough after careful cleaning of the slots......I usually find that alternators can go to around 200,000 miles or over 300,000 Kms on the same alternator......

By the way, the effect you see is when an alternator cannot supply the full load.....

If you take it off and go to a scrapyard (mark yours and let them know that it is your original first!!), and provided it is an alternator where the electronics is built in, look around on cars of the same manufacturer for a replacement with the same fitting/brackets, or even one with a bigger output would be great.....wiring size will not be a problem.....though many might think that it might be.....

I also tend to fit replacement batteries that physically fit in place of the original, but are heavier duty!!! But thats just me!!!

Andy and Rockbit, Thanks for the info. I think I'll check the brushes just to see how they are wearing.

Don