2007 GMC Envoy Electrical Gremlins!

[Adamj214]

Hello GM Nation!
This is my first post here and I did search before posting for my issue. I could not find anyone here or on google with exact symptoms.
Little back ground info, I am an ASE certified tech with 5+ years experience on cars and diesels.

The issue I am having is with the key off and out of the truck on this 2007 envoy you can turn the blower motor to all speeds and off completely. It has MANUAL hvac control. I have seen the TSB for the automatic hvac to replace the LPM (or blower motor resistor). But even with this being an manual control I tested and replaced with no luck. Before I get into this anymore do these manual control units go bad? or is it something in the harness back feeding power to the blower motor and controls. With the key out of the truck the blower can be turned to any speed and is the only thing that works with the key out of the truck.
Thanks and I hope we can figure this out!


PS I am also a Ford guy.......

 

[The_Roadie]

Welcome! You found the right place where folks have access to the schematics and an enthusiasm to help other members. And I especially love an electrical troubleshooting challenge. (An electronic engineer starting my 40th year in the business next week!)

The manual control module almost never goes bad, and in this system the fan control signals don't go through the electronic part of the module at all. I'm sure what's happened is that the RUN bus coming out of the ignition switch is hot all the time or got shorted to the battery B+ bus elsewhere. You're right, in my 9 years on three forums and hundreds of thousands of posts, nobody has had exactly your symptoms (trust me, I'd remember).

If all speeds run, check the HVAC fuse #44 in the rear fuse block. It should be hot only when the key is in and the switch is in the RUN mode. If it's hot all the time, then trace backwards to the ignition switch under the steering wheel. The electrical part is separate from the key cylinder. Here is the wiring at the switch:

RED on the ignition switch is 12V, fused by underhood fuse #34, and feeds the following three circuits:

White is hot in ACCY, RUN, START
Orange is hot in RUN
Yellow is hot in START

RED/WHITE is another 12V, fused by underhood fuse #36, and feeds two circuits:

Brown is hot in ACCY, RUN
Pink is hot in RUN, START

I enjoy working with folks who I don't even need to ask if they own a voltmeter and know how to use it.