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i have a 2017 GMC W/ Fisher “V”. The truck was ordered with plow prep & duel batteries. Both plow & truck were purchased new & are on their second season.

Today, I was out cleaning up after yesterday’s event. I put the plow into scoop & my dash and radio went dead. It seemed to be a intermittent issue that continued to happen, but resolved itself fairly quickly each time it did happen. The only thing that continues is the radio will not allow me to reset the presets.

I called the plow dealer. They told me they think it is related to a ongoing
alternator/voltage issue experienced by many GMC & Ford trucks. Apparently, the trucks is supposed to come with a wiring harness from the factory, loose in a bag. I never saw this harness! So they say the dealer will wire this harness to the truck, then they, the plow dealer will install a third plug direct from alternator to plow motor.

Has anybody any experience with this. Nobody I talk to has ever heard of this harness/third plug. Frankly, I’m terrified to have the truck dealer do anything. They always manage to screw something up!
 

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It's a known issue. Here is the GM bulletin regarding it.

https://www.gmupfitter.com/files/media/photo/695/UI Bulletin 124b.pdf

Basically, when the plow is operated, the alternator ramps up the voltage to compensate. When the plow stops, the need for the excess voltage disappears, but the alternator doesn't respond fast enough causing the voltage to spike to everything else. The "Everything else" shuts down briefly to protect itself.

There is a GM supplied harness that connects to the alternator and needs to connect to the plow solenoid also. Since the plow solenoid on your plow is on the plow itself, there is an additional harness that comes from Fisher that makes that connection. The part # for that kit is 78490

Incidentally, the GM harness is free, so far as I know, but I have only seen it included loose starting with the 2018 model year
 
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