WTF GM?!?!?

[longbedder]

So I hopped in the ol' Blazer to drive to work at dark-thirty, fired her up, and thick electrical-smelling smoke starts pouring out of the center HVAC register (only the center one). I shut the truck off, cussed, and swigged some coffee for a minute to try and let my brain slip into gear for the day.

The register is right above both the radio and the HVAC control unit, so I pulled the 3 fuses for:
-HTR A/C
-AUX HTR A/C
-RADIO

I threw a fire extinguisher in the back seat, fired the truck back up, and drove in to work without further incident...but my speedo now doesn't work and I get an SES light. I find a handy-dandy wiring diagram for a '91 and come to find out that GM supplied power for the DRAC from the HTR A/C circuit. WTF???

So now I gotta figure out what's on fire with power applied to it and replace it. Should it end up being the HVAC controls, does anyone have a good source? LMC comes to mind...not sure how good theirs would be.

Second question: in a Blazer with no rear/aux HVAC, what's actually powered by that circuit? I can't seem to find that answer in the wiring diagrams I have (GM X-9140).

 

[longbedder]

Well, I found the problem over lunch today...

 

[6872xtc]

So did the switch take a dump, or did something else kill it?

Does Rockauto have it?

 

[brans87]

Napa https://www.napaonline.com/napa/en/p/ECHHS297/ECHHS297_0238658690

Looked it up for 86 k1500 truck

 

[brans87]

Napa https://www.napaonline.com/napa/en/p/ECHHS297/ECHHS297_0238658690
https://www.napaonline.com/napa/en/p/ECHHS297/ECHHS297_0238658690
Looked up for 86 k1500

 

[longbedder]

I found the replacement part on Amazon: AC-Delco 15-7004

I'd never paid any attention to the HVAC controls before...didn't know what I was getting into, but it was very simple in the end. Pull dash bezel, unscrew HVAC control unit, disconnect harness, unscrew rotary switch, pay Amazon $18.

So did the switch take a dump, or did something else kill it?

I pulled the switch apart and it was full of old grease, dirt, corrosion, etc. I think it was just done after 25 years, but when I get the new one I'll run an amperage draw check on that circuit.

Still scratching my head on why GM would power the DRAC (and therefore speedometer) off this same circuit, and what the AUX HEAT A/C circuit may be powering.

 

[frankin5]

The rear heat and ac only have power if you have them. They have there own wiring looms that plug into the fuse box.

 

[longbedder]

The rear heat and ac only have power if you have them. They have there own wiring looms that plug into the fuse box.

While I understand that, what happens if there's no rear heat or AC? The circuit still has power to that fuse slot...does it just dead-end at the output side of the fuse block?

 

[Big Ray]

my 30 year old switches are still going strong...knock on wood!

 

[frankin5]

While I understand that, what happens if there's no rear heat or AC? The circuit still has power to that fuse slot...does it just dead-end at the output side of the fuse block?

power at the fuse panel is all you have. You could plug in anything and have power that is switched. The Blazers use the the same fuse panel and it was not an option to have rear heat and air. I added it to my Blazed from a Suburban.

 

[longbedder]

power at the fuse panel is all you have. You could plug in anything and have power that is switched. The Blazers use the the same fuse panel and it was not an option to have rear heat and air. I added it to my Blazed from a Suburban.

Cool - thanks!

 

[rayfin]

My 90 chevy 1500 I once pulled the fuse for the shitty anti-lock brakes and the speedometer quit...

 

[Joe In Montana]

I found the replacement part on Amazon: AC-Delco 15-7004

I'd never paid any attention to the HVAC controls before...didn't know what I was getting into, but it was very simple in the end. Pull dash bezel, unscrew HVAC control unit, disconnect harness, unscrew rotary switch, pay Amazon $18.


I pulled the switch apart and it was full of old grease, dirt, corrosion, etc. I think it was just done after 25 years, but when I get the new one I'll run an amperage draw check on that circuit.

Still scratching my head on why GM would power the DRAC (and therefore speedometer) off this same circuit, and what the AUX HEAT A/C circuit may be powering.

View attachment 218024

That "grease" is prolly dielectric and should be there ... at least some new grease on the Molex connector pins of the new replacement switch.

It not only keeps corrosion away: it is a thick viscous heat sink that's there to draw heat away from the contacts inside the Molex. Those contacts are just barely serviceable on a good new circuit, with very little allowance for an old blower motor which will require more current to make work.

Corrosion or weak tension on the connex creates resistance - resistance creates heat - which lowers available voltage - so the amps draw goes up - and that creates more heat - and something melts or burns when it finally gets hot enough to smoke.