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Old 02-18-2022, 06:24 PM   #1
Junior Member
Brand: Still Looking
State: Alabama
Posts: 1
THOR #26021
Battery Light / Parasitic Draw From Alternator

Hello -
I own a repair shop and someone brought in a 2015 Thor that had a bad alternator. The customer stated that the alternator was recently replaced at another shop so I figured she ended up with a faulty alternator and this would be an easy fix...

Two months later I'm still trying to figure this thing out. Sorry for the wall of text but I want to provide as much info as I can.

This is on a Ford E-350 chassis with the 6.8L V10. We replaced the alternator with a 120 amp alternator as was recommended based on the VIN number. After replacing the alternator the battery light would still come on in less than a minute after cranking the motor.

I figured that maybe the alternator needs to be the 225 amp variation since this is an RV with an entirely separate house electrical system that is also powered by this alternator while the vehicle is running - so we sent the 120 amp back and installed a 225 amp alternator.

Unfortunately the battery light still comes on with the 225 amp alternator - and now the alternator created a parasitic drain on the battery with the ignition off and the key out. The battery would be completely drained in less that 12 hours. I confirmed this with an amp clamp - and you could see the parasitic drain stop as soon as I unplugged the alternator from the harness.

I thought maybe we just had bad luck and got a faulty 225 amp alternator - replaced it with another one - and we have the same parasitic drain that goes away as soon as you unplug the alternator from the harness.

I noticed that the 225 amp alternators seem to have a clutch while the 120 amp alternators do not - could this be a clue as to why the larger alternators create a problem? The wiring diagram does show 2 options based on if the vehicle has a 120 amp or a 225 amp. But regardless - the battery light still doesn't go off with either alternator.

I have replaced the chassis battery with a new one as well - so the battery is good.

The only "solution" I've found is that I can unplug the alternator from the wire harness and the light goes off - while still charging the battery as confirmed by the amp clamp - but I obviously don't think that is a good idea.

Any help would be appreciated. Thanks in advance.

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Old 02-20-2022, 03:40 AM   #2
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16ACE27's Avatar
Brand: Thor Motor Coach
Model: ACE 27.1
State: Florida
Posts: 15,218
THOR #7035
I assume you have an RV repair shop and not an automotive repair shop?

Fords have some complicated wiring in their ignition/battery light/charging control circuit that has nothing to do with the RV side of the vehicle. Could be a loose connection, a rodent eaten connection, a poor ground, etc. I would say it probably is not an alternator issue.
Ted & Melinda
2016 ACE 27.1
2020 Chevy Colorado Z71 Trail Runner Toad
2024 Chevrolet Trax 2RS - Soon 2B BU TOAD
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Old 02-20-2022, 11:59 AM   #3
Senior Member
Brand: Thor Motor Coach
Model: ACE 33.1
State: Massachusetts
Posts: 115
THOR #26005
From what i was told the RV has a draw period do to microwave clock if it has a wired smoke/carbon detector tv’s etc. This will take from the house battery or batteries. Can you post the actual model of the RV
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