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Old 02-11-2019, 12:59 AM   #1
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Problem with chassis system charging. Alternator keeps shutting off?

Hi everyone. This is not a question on my rig, but my cousins 2004 RoadTrek (2004 Chevy Express 3500, 10/03 Build date, 6.0-4L80, vin# 1GCHG39U441143376).

I recently helped him find a class b and everything was fine. He was coming up to my dads place when the dash battery gauge (talking about the Chevy van part, not house) dropped to 11-ish volts. The battery warning light came on, long story short, we replaced the alternator. Stated it back up, and it was fine.

10 minutes later.. He started it again and the battery was down to 11........ We put it in gear and drove it, then it jumped back up to 14+.

My question. Could the battery isolator/controller under the hood be causing this issue? I'm not sure if the ecm on this van regulates the alt input/sense voltage like many newer vehicles, so any input would be helpful.

Thanks in advance!!

If you get bored with my problem, and who wouldn't, enjoy The Supremes, You Can't Hurry Love:

https://youtu.be/YU8oNu3pdS0
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Old 02-11-2019, 05:08 AM   #2
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During the time while you were changing the alternator, did you recharge the battery ?

If it was low, and you didn't, it would be low after you did, and would take some time to come back up.

In most cases the alternator is connected to the chassis battery, just like a factory van.
The isolator connects the house battery to the chassis battery when the chassis battery is charging, but in many cases, only after the chassis is up to 13+ volts.

If all batteries were low, the chassis may have needed to recharge, then the house was tied in, causing a drop in voltage as it started charging.

Has it returned to acting normal after the last drive ?
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Old 02-12-2019, 05:55 PM   #3
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Twin Boat,

We did charge the chassis battery while changing the alternator. It was in the low 11's after running the engine with no alternator input for about 20 minutes, the chassis battery itself was just changed by the previous owner, I'm guessing the old battery died due to lack of use...

His house batteries, as we discovered, were complete garbage. Disconnecting them from the system made no difference, although I'm sure they were putting a real hurting on the old alternator.

The problem seems to be within the Chevy part, and not the house, as we seperated the two systems completely under the hood. I'm out of guesses at this point, we tried getting voltage input readings from the alt and it seemed fine. However, when he drove it later on the problem returned again, and we haven't been able to nail it down.

He's back out on the road and just risking it. The alternator doesn't seem to charge for approx 20 minutes at a time, then turns back on and everything is ok.

Without throwing tons of $$$ into useless parts, and having it in front of me, I'm out of clues. I honestly think the ECM, or whatever feeds the input voltage to the alternator, is starting to fail in this case.

No clue.
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Old 02-12-2019, 06:11 PM   #4
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You may just have a bad rebuild.
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Old 02-13-2019, 07:37 AM   #5
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It's also possible that the battery was bad from the start. I have gotten bad batteries off the shelf that would not accept a charge.
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Old 02-13-2019, 10:28 AM   #6
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Had a company Chevy 3500 van that had the same issue. Aftermarket alternators would not work. Had to use oem from Chevy dealer.
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Old 09-25-2019, 07:08 PM   #7
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Update! Roadtrek owners please read!

The problem was in the wiring leading to a small generic 12v relay mounted on the side of the underhood battery system combiner. The relay is mounted to the right side of the unit. It apparently is used to hijack the alternator 12+ feed to not only power to the relay coil and alternator 12+ feed, but also to provide the house/chassis combiner with a "turn on/engaged" signal to allow the charging process to occur.

The feed from the factory wire (and all of the feeds in/out of the relay) are nothing more than hacked-in black lead wire with cheap, open spade terminals mounted to the relay pins. Even though there was no rust on the terminals, corrosion still occured. So much corrosion that the connectons, although tight, would not send power to the coil.

The relay was replaced with a better unit.... We used the headlight relay and wiring pigtail for a 1997 Cad Eldorado. The Eldorado headlight relay is a completely sealed unit made by Hella. The connector has a full seal around the plug, eliminating the enviromental issue. There have been no more issues with charging since then. The RT runs and CHARGES!!!! YAY!
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