LLY: Dual alternator issue/testing

[frankenstien]

I noticed the other night my dash lights dimming with my turn signal. only extras on the dash light circuit is 2 autometer gauges(led) and an inline dimmer for those 2 gauges.

i have 4 130w kc daylighters on a single switch, when i turn them on, you can see the volt gauge dip and my lights dim, it didn't used to do that. you can hear it in the lift pump as well when the voltage drops.

with a meter at the battery it drops a full volt.

so on to the testing. I have a factory dual alternator set up. one of my batteries is about 5 years old, the other is 4(someone stripped the connections) it runs about 14.5v at each battery at idle with the headlights on. I turn the truck off, unplug the drivers side alternator connector, start the truck, still 14.5v at both batteries and from alt + post to GND tab.

I turn the truck off again, plug back in the drivers side alternator, unplug the passenger side alternator, start the truck again, battery light stays on and dic says "battery not charging" still getting 14.5v at both batteries, tested from + post of alternator to GND tab, still 14.5. voltage appears to be moving around like it's charging.

i tested it this way because that's what i found on google, and it made sense to me i think it was off duramax forum. does this sound like a legit test? apart from taking off the alternators and batteries and having napa test them, anything else to look at? right now i am suspect of my driver side alternator

sorry for the novel, just trying to give as much info as possible

 

[2004LB7]

You should have your batteries load tested. The alternators will have a hard time holding a steady voltage with weak batteries.

Mine did the same thing for more than a year. Always cranked up and started fine. Replaced the batteries and now it hold a steady voltage even with some good loads on it

 

[JoshH]

The DIC message is because the ecm only looks at the passenger's side alternator to see if it is charting. If you look at the connector on the second alternator, you will see it only has one wire in it which is the wire that turns it on from the ECM. I agree with 2004LB7 about load testing your batteries. As old as they are, I wouldn't be surprised if they need to be replaced. If they test well, you may have to test the alternators to see what amperage they are putting out.

 

[frankenstien]

thanks! ill get em pulled and tested, hopefully tomorrow.

 

[frankenstien]

alright, took my batteries to napa, both failed, got new batteries, got em in and it seems to be doing pretty good so far, i still get a little dip when i turn the daylighters on, but google calc says its a 43 amp draw when i turn them on so its probably expected, and just got worse as my batteries did.

I may have them hook their hand held tester to check the altenators, if its possible.

thanks for the help, the one battery was a napa and was about 3.5 years old, so it got pro rated at least, i just didnt think they would be that bad cause the truck started ok, but now with the new ones, it cranks much faster

 

[JoshH]

Glad to hear you got it sorted out. :thumb: