A little background. This truck is a one owner and has been maintained very well by me. No rust, very good condition overall and garage kept until about five years ago.
The problem, sorry but this will be a bit lengthy but the following information is relevant.
Back a couple months ago, I went to the garage to get the truck and run some errands. I was puzzled when I opened the door to the garage and heard the radio playing. I thought "oh great the battery will be dead).
I started the truck and backed out of the garage and turned the key to the off position. The truck was still running and the headlights were still on. To make a long story short, I chased wires and voltage for a couple hours. I also pulled the power center and looked for corrosion underneath which is pretty common. I saw no problems and re installed it.
I "pecked" around with a screwdriver handle in the power center and could make it malfunction so I pulled all the relays and reseated them and the problem went away. Not sure if it would return or not but so far it has not.
About two weeks ago, I noticed my cruise control no longer worked. I hoped it was a fuse but it was not. I kept driving it until I had time to work on it. Last week I happened to be at the rear of the truck while it was idling and I noticed the hi mount brake light was on but dim.
A couple years back, I replaced the stock hi mount unit with an after market LED unit which has been working fine.
I worked on it today and the following is what I have found.
The hi mount brake light is on but dim (sometimes flickers a little but mainly lit about half of normal) anytime the key is on. It doesn't matter if the headlights are on or not.
All three brake lights work as they should
I am assuming the cruise not working is because of this issue. I hope.
I have replaced the brake light switch.
I also replaced the entire power center under the hood with a brand new OEM unit ordered from GM. (It came with all fused and relays installed so everything there is new).
I did this just to eliminate that as a possible issue (and the problem with it not turning off a couple months ago).
I also unplugged the trailer harness under the dash in case it was a problem with the wiring but it was not.
It was suggested that I look at the multi function switch next but I am not sure the hi mount stop light runs through it or not.
My next step is to chase and clean all the engine bay grounds and remove the hi mount light and take a look.
It could be a defective hi mount light assy but there should be no power to the unit unless I push the brake pedal or light up the cargo lights. Somehow there is some voltage feedback getting to the lamp between the brake switch and the hi mount stop light.
It has been a real head scratcher so I am hoping to get some guidance here.
Thanks in advance for any assistance.
Wednesday, May 5th, 2021 AT 1:39 PM