Battery Light

JoeyF1722 · October 31, 2022

Yesterday I installed a fisher plow harness into my 2020 GMC Sierra 2500HD, had to remove the passenger side battery to run the remote wire through the fire wall, once that was done, I reinstalled the battery and installed the power and ground wires form the plow harness. Now my battery light is on and is not charging. I double checked all the connections and checked all the fuses. I Ohm'd out the cable from the alternator to the fuse block on top of the battery and that that was good. I hooked up my scanner and there are no codes besides the one's from disconnecting the MAF sensor (which I have cleared) I'm at a loss has anyone come across this before? Any help would be great thanks!

Edited November 1, 2022 by JoeyF1722

sheath · October 31, 2022

Hi Joey,

I've not run into this particular issue, but I preach voltage drop for main power circuits instead of using the Ohms function. Take your meter set to DC voltage 20V scale, apply the positive meter lead to the input side of the fuse or wire, and the negative lead to the output (or load side) of the fuse/wire. Allowable voltage drop would be about 0.2V (what your meter reads) through any single connection point or switch. I normally don't get excited until I see a half volt drop, or greater than 1V across the total circuit.

If one of the main power fuses are open you'll see pretty close to battery voltage reading on your meter. You can perform the same check on the power circuit wiring.

Hopefully @newdude or @MTU Alum will see this and reply. They are both pretty sharp on GM product electrical (as well as pretty much every other technical thing :cheers: ).

JoeyF1722 · October 31, 2022

Thanks for the info Scott, I'll do a drop test latter today

JoeyF1722 · October 31, 2022

I forgot to mention that I did check the cable coming from the alternator for power while it was running and I was only getting 12.8V, I checked the plug on the alternator as well and it was good. looks like the ECU is not telling the alternator generate power, would it be possible that the ECU is latched up?

newdude · October 31, 2022

4 hours ago, JoeyF1722 said:

Yesterday I installed a fisher plow harness into my 2020 GMC Sierra 2500HD, had to remove the passenger side battery to run the remote wire through the fire wall, once that was done, I reinstalled the battery and installed the power and ground wires form the plow harness. Now my battery light is on and is not charging. I double checked all the connections and checked all the fuses. I Ohm'd out the cable from the alternator to the fuse block on top of the battery and that that was good. I hooked up my scanner and there are no codes besides the one's from disconnecting the MAF sensor (which I have cleared) I'm at a loss has anyone come across this before? Any help would be great thanks!

Does your truck have VYU plow prep from the factory?

Where exactly did you hook up the plow bits?

Did you install the GM plow load harness?

You could have a code you can't read also. If you have a simple handheld scanner, majority of those will only read engine codes (P codes). If the body control module is setting a code (B code) you need something that can read those.

Edited October 31, 2022 by newdude

JoeyF1722 · October 31, 2022

It does not have the VYU plow prep

I hooked the power and ground to the passenger battery. haven't hooked up the small wire to the fuse box yet

I did not install the GM plow load harness? wasn't provided one from the dealer, should I have been?

JoeyF1722 · October 31, 2022

The only part of the plow harness connected to the truck at the momnet is the light harness everything else is disconnected and it still doesnt charge

newdude · October 31, 2022

25 minutes ago, JoeyF1722 said:

It does not have the VYU plow prep

I hooked the power and ground to the passenger battery. haven't hooked up the small wire to the fuse box yet

I did not install the GM plow load harness? wasn't provided one from the dealer, should I have been?

22 minutes ago, JoeyF1722 said:

The only part of the plow harness connected to the truck at the moment is the light harness everything else is disconnected and it still doesnt charge

Since you don't have plow prep from the factory, no, the harness would not have been provided. I suggest you purchase one and install it however. Assuming your plow is a 2 connector, you will also need Western/Fisher p/n 78490 to add the third connector to both the GM harness and the plow side as the newer blades the solenoid is out on the plow, not under the hood. If you have a 3 wire plow already, the solenoid would be mounted under the hood and you can to direct to that.

What # headlamp harness kit do you have for the plow? 72199? How about the isolation module? Did you also get p/n 90730 fuse kit?

GM info on that harness below:

p/n 84731643

instructions - UI-Bulletin-161b.pdf (gmupfitter.com)

Edited October 31, 2022 by newdude

JoeyF1722 · October 31, 2022

I'll get that info for you later today when I get home, thanks

JoeyF1722 · November 2, 2022

Thanks everyone for the help, I removed the alternator and had it tested and it was bad. was it a coincidence I don't know, installed a new one and everything works like it should including the plow.