'72 Factory AC - No 'hi speed' on fan
 

The heat/AC fan on my '72 C10 only blows low and medium. After searching this site, i found a couple threads that said to replace the relay under the glove box. I did that today ($19 from O'Reillys) and still no Hi speed on the fan. When I move the switch from Med to high, I do hear a click in the relay. I believe that the resistor controls fan speed on low & medium, and is bypassed completely for high speed. Not sure where to go from here.

Re: '72 Factory AC - No 'hi speed' on fan
 

The 12 gauge orange wire that powers the fan through the relay plugs into the CIG position on the fuse panel. Check that it's still plugged in. Measure voltage with a multimeter at the heavier-gauge orange wire at the relay.

Diagram without anti-dieseling relay
http://67-72chevytrucks.com/vboard/a...1&d=1484370971

Diagram with anti-dieseling relay
https://i.imgur.com/nW0bYIa.png

Re: '72 Factory AC - No 'hi speed' on fan
 

[EDIT] Dang it !!!! djmlambert, you beat me to it and posted while I was still typing..:lol::lol::lol:. The blower relay controls only the hi speed. The relay has a separate "hot wire" coming from the fuse box that is hot all the time. The relay receives a hi speed signal from the blower speed control switch through an 18 ga. orange wire. The hot wire for the relay is a 12 ga. also orange wire.. The relay will "click" when it receives the signal through the 18 ga. orange wire, but if the 12 ga. orange wire has no voltage on it, the blower will not work.. Check for voltage on the 12 gauge orange wire at the relay..

A complete AC electrical system wiring diagram can be found in the Heater/air conditioner section of the GM service manual for these trucks..

Re: '72 Factory AC - No 'hi speed' on fan
 

I had remembered the high speed on A/C tied in as mentioned but was not sure as it has been a while. So other than connections, the fuse for the cigarette lighter is the first thing I would check. I always keep to KISS it first. "Keep It Simple Stupid".