1996 Lazy Daze on a Chevy P30 chassis.I have an alternator that is not charging. The simple answer might be that it is the alternator but these Chevies have an "exciter" circuit that determines whether the alternator charges.I get 12.2 V on the battery with the engine on or off.I don't get 13v+ with the engine on anywhere, including the alternator. Here are the voltages I get with the engine on:Batt - terminal to terminal - 12.2Alt red terminal to batt negative - 12.2Isolator 1 - 12.2Isolator 2 - 12.2Isolator E - 11.7 (this is strange - lower than the others)Isolator A - 12.2Is it true that since I'm getting voltage readings my isolator is ok? Any suggestions on testing the alternator or anything else?Thank you.

Time to replace the alternator.12.2 is only around 60% charged.12V chart for BatteriesGood alternator with engine running should give you 13.8-13.9 with batteries that low of 12.2.

Make sure the alternator belt is not slipping... I'm assuming your information panel on your truck display is showing the ALTERNATOR FAULT symbol...If all of this is OK around your alternator and start battery then your problem is downleg from the alternator...I always get confused about which battery folks are talking about... You have a engine start battery under the hood and then a different battery solely for the house items... I'm never sure the term 'chassis battery' would be the engine 'start battery' system.In my case it is easy hehe... I have a truck with a start battery pulling a trailer with a battery bank. Roy Ken

If you are handy at all, those chev nators are easy to overhaul. And they usually just need a set of brushes and a front bearing,.....and a dab of grease in the rear needles.

I get 12.2 V on the battery with the engine on or off.I don't get 13v+ with the engine on anywhere, including the alternator. Here are the voltages I get with the engine on:Batt - terminal to terminal - 12.2Alt red terminal to batt negative - 12.2Isolator 1 - 12.2Isolator 2 - 12.2Isolator E - 11.7 (this is strange - lower than the others)Isolator A - 12.2Sounds like bad alternator, don't get caught up in the "exciter" ideaQuick testOn the isolator, Move the alternator wire on top of the chassis battery wire, usually bat 1, but verify this before movingStart the engine check for voltageIf you have voltage it's a bad isolatorTest twoNo voltage move the 'E' wire on top of the other twoIf still no voltage bad alternatorReplace wires to normal positions after testing, before putting away the tools

Yes it's the truck/starter battery. Is it possible that a load on a severely depleted house batter (two Trojan 6 volts in this case) would load down the alt so much? I have never seen below 13.x volts on a running vehicle but I don't want to replace the alt unless I'm sure it needs it.Given that I do see the voltage at the E leg of the isolator, I do suspect that it is indeed the alternator as I understand that is either "on" or "off". Is that correct?I was going to try a "hail Mary" of disconnecting the house batts and seeing how it runs with just the truck battery (what I always thought was called the chassis batter but who knew).Thanks again.

Alternator Not Charging Chassis Battery | Good Sam Community

15 Replies

matt7591
Explorer
Feb 27, 2017
Yes it's the truck/starter battery. Is it possible that a load on a severely depleted house batter (two Trojan 6 volts in this case) would load down the alt so much? I have never seen below 13.x volts on a running vehicle but I don't want to replace the alt unless I'm sure it needs it.

Given that I do see the voltage at the E leg of the isolator, I do suspect that it is indeed the alternator as I understand that is either "on" or "off". Is that correct?

I was going to try a "hail Mary" of disconnecting the house batts and seeing how it runs with just the truck battery (what I always thought was called the chassis batter but who knew).

Thanks again.
MrWizard
Moderator
Feb 27, 2017
I get 12.2 V on the battery with the engine on or off.

I don't get 13v+ with the engine on anywhere, including the alternator. Here are the voltages I get with the engine on:

Batt - terminal to terminal - 12.2
Alt red terminal to batt negative - 12.2
Isolator 1 - 12.2
Isolator 2 - 12.2
Isolator E - 11.7 (this is strange - lower than the others)
Isolator A - 12.2

Sounds like bad alternator, don't get caught up in the "exciter" idea

Quick test
On the isolator, Move the alternator wire on top of the chassis battery wire, usually bat 1, but verify this before moving

Start the engine check for voltage
If you have voltage it's a bad isolator
Test two
No voltage move the 'E' wire on top of the other two

If still no voltage bad alternator

Replace wires to normal positions after testing, before putting away the tools
RJsfishin
Explorer
Feb 27, 2017
If you are handy at all, those chev nators are easy to overhaul. And they usually just need a set of brushes and a front bearing,.....and a dab of grease in the rear needles.
RoyB
Explorer II
Feb 27, 2017
Make sure the alternator belt is not slipping...

I'm assuming your information panel on your truck display is showing the ALTERNATOR FAULT symbol...

If all of this is OK around your alternator and start battery then your problem is downleg from the alternator...

I always get confused about which battery folks are talking about... You have a engine start battery under the hood and then a different battery solely for the house items... I'm never sure the term 'chassis battery' would be the engine 'start battery' system.

In my case it is easy hehe... I have a truck with a start battery pulling a trailer with a battery bank.

Roy Ken
Triker33
Explorer
Feb 27, 2017
Time to replace the alternator.
12.2 is only around 60% charged.

12V chart for Batteries

Good alternator with engine running should give you 13.8-13.9 with batteries that low of 12.2.

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Alternator Not Charging Chassis Battery

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