Fan switch not working at all.

[howzzzit]

It's been a few years since I used the fan switch on my 72 c10. Used to have to run through the speeds quickly up and down with the lever and then it would work. Recently nothing with the switch so I installed a new one. Old one looked like it had old baked grease inside. Figured problem solved. Not so fast as the new one doesn't work at all. No speeds. Fuses good. Help!

[dmjlambert]

Is that a truck with heater, or A/C? I would say check with a multimeter or lighted test probe at the fusebox, at the switch, at the high speed relay if you have one, at the resistors, and at the motor, and check that the motor is grounded and spins freely. You can also connect a jumper from motor to battery to see if the motor works. Check the condition of the plug and wiring at the plug that connects to the switch, that is where it is common to fail and melt the connector on these trucks.

[howzzzit]

With ac. I'll check the connector for voltage? Visually it looked ok. If the relay is bad will it kill all the speeds?

[dmjlambert]

On with-A/C it will kill the relay on high speed (lever all the way down) only. Check at both sides of fuse terminals for voltage, don't touch the probe to the fuse barrel ends, but rather check on the terminal that grips the fuse. Of OK move down the line and check for voltage at the switch.

[dmjlambert]

Wiring is in the service manual. For 72 that would be page 1A-41. If you don't have a paper manual, PDF is here https://67-72chevytrucks.com/vboard/....php?p=5781194

[howzzzit]

Thanks. I'll get to work.

[howzzzit]

DMJLambert I did as you suggested. The power from cig was hot. Key on and no voltage at the a/c or heater sockets. Proceeded to try and remove the plug from the back of the new switch without lowering the a/c assembly. Only able to move out a little. Pushed it back on and blower now works. Since switch is new I am assuming the plug may be a little fubar. I'll see if it will run another 51 years. Thanks for your help and suggestions.

[dmjlambert]

It might have resistance in the connection which could come back to haunt you. It is common on these trucks for the connector and some of the wiring near the connector to melt at some point. I suggest work the connector off of the switch and perhaps squirt something in there like corrosion remover or WD40 or something like that to clean it up, and then put it back on. Also check that none of the terminals are working their way out of the back of the connector, that they are fully seated. It might make it last 52 more years.

[howzzzit]

Yeh. I'll let my great grandson deal with it.