Hi there..

My truck has been starting sluggishly lately. The batteries are 3 years old, and the voltage being displayed while running is 13.0 to 13.4 volts even at high RPM. Does this sound like my alternator is not putting out enough power. I've heard that around 15 volts is good. At idle I'm only getting 12.1 - 12.5 v

Also, is there a way to check drain on a battery when my truck is shut of with a voltage/clamp meter? If it's not my battery, I would have to assume something is draining power when it is parked overnight.

Thanks

 

Reading the voltage should be done with an accurate DVM, at the alternator post.

If those voltages are what you're seeing there, then yes, your alternator isn't putting out enough voltage, and likely very little current.

Rebuild or exchange time.

How many miles are on it?

Pop

 

The truck has 190,000 KM on it. I'm reading the voltage from the dash odometer display.

Any advatage to new over rebuilt?

 

It depends a lot on the rebuilder.

The thing that usually "wears out" are the brushes and commutator rings (they get a deep groove in them). A good rebuilder will not just "turn" the commutator, but will replace the entire rotor if the grooves are anything but superficial.

The brushes are an integral part of the regulator on these alternators, so that will be a "must", too.

Add to that front and rear bearings, and all moving parts will have been replaced. If all the diodes are good, then you have an alternator that is virtually new. The diodes don't wear out, and should not be a problem if properly checked by the rebuilder.

Pop

 

could also be a battery going bad i have seen this many times

 

Even if a battery is shot, a good alternator will attempt to charge it.

14.4 volts is the fixed set point in our trucks (gray color regulator), and what it should be attempting to put out.

Pop

 

but if the battery was going bad it would make it weaken the alternator like the symptoms he is describing could it not i am looking at the low voltage end more than the top end voltage

 

In this case, it isn't "the" battery, it's TWO batteries. This is a Powerstroke-powered truck.

There's a good way to resolve this:

Take BOTH batteries out and bring them up to full charge (if they will) on an external charger. Load test each of them.

Remove the alternator and have it bench tested.

Guess we'll know then for sure rather than speculating about it.

Given what he's described, I'd test the alternator first. That's easier than pulling the batteries, anyway.

Once again, a good DVM at the output terminal should tell us more.

Pop

 

I think I kind of agree with both of you. I think that in all but extreme cases of bad batteries, the alternator should be capable of putting out its full 14.4 volts at the alternator output terminal (as SpringerPop has pointed out).

I do believe a bad battery or batteries might affect that capability, but I think they would have to be REALLY bad, so bad that the truck probably wouldn't start anyway.

I would also add, bump the RPMs up a bit while making this measurement.

 

ok batteries but only one battery could be bad and cause this problem but if left unchecked it could take out the other battery as far as 2 in the psd most of the vehicles i work on have between 2 and 8 batteries and i have seen low voltage symptoms like his and had to track down which bat was bad and many times only 1 battery was bad

 

My 03 Diesel has a charge of 12.6 at idle and 13.6 at rpm. Aren't the oil burners charging lower because of the lower rpm?

 

A bad battery will cause Alternator output to vary greatly.I have actually seen them bad enough to shut alternator off completely.
Anything from 13.8 volts to 14.4 is acceptable.