Heat switch
 

I lost low and medium speeds on my blower motor on my heater on my 79. I replaced the resister on the heater box under the hood. No change. Think the heater switch is bad. Is there any way to test this. Looked on line. Only show you how to replace it. Need some help. Thanks
Chris

Re: Heat switch
 

Low & Med positions feed 12v to terminals on the blower resistor.

Your blower resistor will have four terminals or three terminals depending on how many medium positions you've got on the switch.

High triggers a relay that switches battery voltage to the blower motor on AC trucks.

On the regular heat-only trucks the high position goes to the motor output terminal on the blower resistor.

Re: Heat switch
 

Thought i would try a new relay. Blows the fuse and does not work at all. Are the top three of these relays the same?

https://www.oreillyauto.com/shop/b/l...cDDoDppaQoHpow

Re: Heat switch
 

I pulled the switch last night and checked that seems to work, any other thoughts? could I have bought a bad relay?

Re: Heat switch
 

I recently replaced the resistor and I pulled that and checked it is all good

Re: Heat switch
 

That SPST Micro ISO 280 relay in the O'Riley link is a modern GM part. My Impala & GMT800 use a bunch of them.
The blower relay on the side of the big Harrison Frigidaire AC box in your 1979 CK is a proprietary GM AC Delco 1581743. https://www.rockauto.com/en/moreinfo...t=3088&jsn=651
It's electrically the same as a 1" cube 40A Bosch SPDT Mini ISO relays.
If the AC Delco part suddenly became NLA you can move the Packard 56 terminals from the Proprietary GM plug to a five terminal Bosch ISO relay socket.

Download the 1979 wiring diagrams and look at the RPO C60 wiring detail.

Re: Heat switch
 

The test is for continuity based on position. Power goes in the same terminal for all positions and out one of the others based on position.

Re: Heat switch
 

I pulled the relay apart and cleaned the contacts. All good now.