The Heater Relay-I Don't Know How It Works-Or Doesn't

[CORVAIRWILD]

In my 45 years of repairing GM vehicles, I never understood how the heater fan relay works. So today I have to repair a 1984 K5, and the heater was only working on high-speed. So I took the resistor out of the plenum, polished up the contacts and put it back together, same. Then I changed the relay for some random used one I found lying around, and then I had only intermediate speeds, but no high-speed. So I put the original relay back, and then back to high-speed only. So what the heck is going on? I've never known how this relay business worked, but today is the day to find out.

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[1987 GMC Jimmy]

This is a good “for dummies” audio visual explanation. The only issue I have is that he confuses flow thru ventilation with Astro Ventilation. All of these old vehicles had flow thru ventilation which used air velocity and strategically placed vents to move air through the cab no matter what worked, and I’m sure cars and trucks today use some variant of that system. Astro Ventilation is where the motor runs super low and slow all the time, but that was discontinued at some point probably around the late 70s. Otherwise, the information’s good. This guy taught me much of what I know about E4ME Quadrajets.

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[CORVAIRWILD]

I think I have a 10% better understanding, I'll poke around tomorrow, I'll pop open the relay cover and see what it looks like inside. I'm sure I have a few more relays somewhere

 

[PrairieDrifter]

I found this wiring diagram helpful. It was from another post but I screenshot it and studied it for awhile. This is for 80’s a/c setups I believe. It said c50 in the post but all of the wires I have in my 84 suburban matched up with the diagram so..

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[PrairieDrifter]

The red wire to the relay comes from the junction block by the brake booster. Which runs your high speed, and I believe that activates when the orange wire sends a signal when you flip the switch, which sends full voltage through the relay. Purple is to the blower motor, and the blower motor needs to be grounded of course.

Then the dark blue wire sends variable voltage through the relay depending what the position of the switch is other than high. Which gives you your other three speeds, then the relay has a ground as well.

Are you using regular relays or replacement blower motor relays? It could be possible the selector lever has gone out.

 

[CORVAIRWILD]

Are you using regular relays or replacement blower motor relays? It could be possible the selector lever has gone out.[/QUOTE]

I put in an exact replacement relay, and I got 3 intermediate or "resistor" speeds, but no "hi". The original only gave me hi speed. So logic would indicate the relays are both defective. I'm sure I have a few more laying around... Somewhere...

 

[PrairieDrifter]

Are you using regular relays or replacement blower motor relays? It could be possible the selector lever has gone out.

I put in an exact replacement relay, and I got 3 intermediate or "resistor" speeds, but no "hi". The original only gave me hi speed. So logic would indicate the relays are both defective. I'm sure I have a few more laying around... Somewhere...[/QUOTE]

That’s what it seems like to me as you know that both circuits of the system work.

 

[CORVAIRWILD]

Yeah, pretty interesting that one does hi speed, the other resisted speeds. I didn't go to my shop yesterday, I had other things to do around town, but I'll be going there today and I'll try to remember to look for a relay. I'll also have to print the wiring diagram as I look at it on my device, it's just too small to comprehend