I've been chasing this problem on & off now for a few days. My fan runs as it should in Lo position when ignition switch is on and most of the time (not always) increases speed in position 2 & 3 as it should. Now when I go to high I can only get the fan to run in high speed when I set it in between position 3 and Hi. So I thought it was the switch and got a new one. Well guess what? Does the same thing with the new one! The resistor coils look good, undamaged so I guess all that's left is the relay. I checked voltage per a previous post/reply from Willcox but not sure what readings should be.
Dark Blue wire between resistor and relay: 4.2V?
Purple wire: 11.2V
tan and light blue wires 7.8v?
I did clean all relay and resistor connectors but wouldn't doubt possibly an issue with the old wiring.
Do local GM dealers or auto parts stores carry these relays and if so what's the p/n? Can someone with experience here please chime in?

 

With your switch in high position check voltage at the orange at the high speed relay.

 

12v

 

I'm not sure what I posted before, but here is a schematic from the web site:

I like this one because it has numbers So, here we go! The resistor show above as AC Resistor is what controls the motor speed on everything but high. You first need to verify that each of the three wires 50, 63 and 72 have 12 volts on them before the resistor when the switch is in the following positions.

Brown number 50 is low speed. So when your switch in the heater control is on low speed this wire will have 12 volts. (before it enters the resistor).

Tan number 63 is the number 1 speed. So when you switch to the first click on the blower switch in the dash this wire should have 12 volts on it. (before it enters the resistor).

Light Blue 72 is the number 2 speed. so when you switch to the second click on the blower switch in the dash this wire should have 12 volts on it.

Now... keep in mind that all lower end speeds go through the resistor. And that high is direct.

Once you have tested the three wires before the resistor... go back to the dark blue wire and see if the output voltage changes. When you test the voltage on the dark blue wire it should increase the higher speed you put the motor on until you reach high.

On high speed... you should have voltage present on the orange wire.

Post back...

Willcox

 

Now if you have 12v on the red at the relay and the black wire is in fact grounded,then the relay is most likely bad unless you have some corroded connections.

When the orange has 12 that tells the relay to close and connect the red wire to the purple.

 

Well, when I first touched the meter lead to the brown wire there was no reading then I wiggled it and the fan blew like I never heard it before! Apparently it was loose in the connector. I was just about to report back that all my voltages were 8V and less when voila! I pushed the probe into the brown wire connector and everything started working! You and Roger have been a great help to me since I purchased my 79' some three weeks ago. It seems like I'm on here every other day with questions. You both deserve a cold beer! thank you again....
Now to tackle the vents operation so I'll be back.

 

Well.. I'm drinking Rogers beer and taking tomorrow off.. he will just have to drive down here to collect..

Thank you for the kind words and I'm glad you figured this out.

Willcox