A 2021+ Suburban issue, that's a big departure from the '98 you mentioned. Assuming it's one of the newer ones, I'd guess the 40 amp circuit is probably related to the ignition start system or maybe even the keyless entry.

 

It would only eliminate the wiring attached to the switch, but there could still be a short in the harness leading up to the connection for the switch.

According to the GM service manual, the only things powered by the IGN A fuse are the starter relay and ignition switch. Is it one of the other IGN fuses that’s blowing?

Have you tried disconnecting the brake controller?

 

The other 3 fuses I mentioned do get power somehow through the 40 amp Ignition fuse. My neighbor has a 98 Chevy truck. I verified that last night. I looked at the brake controller and it's wired to the brake lights and not the brake pedal switch.? I am basically treading water at this point! This may require a expert to figure out. I really do appreciate all your help, thank you!

 

Yes, those fuses receive power via the IGN A fuse, but only with the key in the ON/RUN position.

The brake controller is tapped into the brake switch circuit for a signal to apply the brakes when the pedal is pressed, but it will also have a direct battery feed to supply adequate voltage to the trailer brakes when needed. Whether or not it’s the IGN A circuit can only be confirmed with visual inspection and/or disconnection. I recommend disconnecting it as a way to be absolutely sure.

If the brake controller isn’t at fault, it’s likely the wiring harness that passes through the firewall has been chafed through.

Also, just FYI - I’ve been a practicing automotive technician for almost 10 years, and electrical problems are a large part of my daily work. I also have several customers with trucks of this era, so I’m more familiar with them than most.

That said, I actually haven’t had one in the shop for a while so I’m struggling to remember exactly how they’re set up.

By no means do I know it all, and it’s not easy making a diagnosis via message board, so I wouldn’t blame you for getting it to a shop in your area.

But as long as you’re asking my advice, I’ll guide you as best I can. 👍

 

So if it is chaffed going into the firewall, can you even get to that harness? To me it looks like the motor would have to be pulled? Tomorrow I will remove the brake controller and see what happens? This is one of those VERY FRUSTRATING problems! Thanks again for all your time and advice!!!

 

That’s why I said I’m struggling to remember how they’re set up. I can’t recall if it passes through a grommet in the firewall, or if it’s a bulk head connection.

You don’t have to pull the engine to get at it - removing the driver side fender liner should give you a good view.

 

OK will look at it tomorrow.

 

Update! I finally found the problem with my direct short! Was ready to have it towed to a shop. Kept looking around and followed the wiring harness coming from the firewall into the cab. The harness was literally laying against the emergency brake cable. I pushed the harness off of it and could see a chaffed wire. Checked my fuse box and the direct short was gone. Installed a new 40 amp fuse and it started right up! Problem now is how to fix that wire ? Not a lot of room to work in there. Thanks so much to Gumby 22 !! Appreciate all your help and advice! Big time helped me figure it out!

 

Any way you could get some shrink wrap insulation over the wire - assuming the wire itself is not damaged? Then secure it up off the stupid cable!
Great find, I have enjoyed your work!!
Red

 

Thank you! Will look at it closer in the morning. Appreciate your help too! This forum is great!