2001 cummins wont hold a charge and i dont think its my alt.. anything else i should check??? batteries are good too....

 

If you pull the battery cable and use either a volt meeter or test light between the battery post and the cable with everything turned off it will show if you have something draining the battery. Have you checked the voltage with the engine running? It should be between 14 and 15 1/2 volts. If it isn't the alternator is not putting out. I think the ECM has the regulator in it.

 

dont have no volt meter.... running 12.5 to 13 volts at idle... thanks though...

 

12 1/2 to 13 is too low to charge the battery properly. It would be worth getting a cheep meter. Sometimes Harbor Freight has digital meters for around $5. It's good to have one in the tool box. It sounds like either the voltage regulator or the alternator is bad.

 

ok well i'm taking the alternator to get it checked... how do i know if its the voltage regulator??

 

It is back to needing a voltmeter again. One of the small wires to the alternator will read 12v to ground. The other is a variable ground through the regulator. If you take a wire from this terminal on the alternator to ground you should see your charge voltage go up. If it does the alternator is good and the regulator is bad or you might have a broken wire in the harness.

 

oh well.. i'll get it...

 

It could also be bad connections between your batt cables and the battery. That's free and easy to check/fix.

 

are you SURE the batteries are good? Are you sure all the battery grounds are connected right? Sometimes the grounds LOOK good, but the wires under the sheild are corroded

 

Also check to be sure you don't have a grid heater relay sticking on draining the battery. The intake horn will real hot if it is staying on. It would drain my battery very rapidly when mine stuck and actually got the factory horn hot enough it discolored it.