No high speed A/C

fortop · April 26, 2004

I have lost the high speed on the A/C in my 1983 C20. One weird symptom - when I had my foot on the brake the blower would run at medium speed when the fan lever was set at high speed. When I took my foot off the brake the blower would stop completely. Now the blower won't operate at all when the fan is set to high speed. I installed a new heater control about one year ago and it has worked fine until last month. The air is cold and the medium and low fan speeds work fine. Is this a vacuum problem? If so, what is the fix?

Thanks.

GM GIMP · April 26, 2004

I think those models have a separate relay for high speed.

It should be located near the passenger side firewall. You can unplug it

and and use a jumper wire in the plug. If the high speed works

just go buy a new relay and plug it in. Also the high speed

could have a separate fuse. My '76 Malibu had an inline fuse.

As far as the brake pedal low speed issue I can only say to

check all grounding. Bad ground connections will make automotive

electrical devices do strange things.

I hope this helps.

Good Luck.

citydriver · April 27, 2004

There is also a seperate 12gauge wire that feeds the blower high speed relay (usually a red wire) that has a plastic connector in-line...usually runs across the rt valve cover...sometimes the connector would melt inside and loosen the connection

fortop · April 28, 2004

I replaced the A/C blower relay - still no high speed A/C. There is a resistor in the A/C box which is supposed to be wired in series. Should I replace this also ($5)?

citydriver · April 29, 2004

I replaced the A/C blower relay - still no high speed A/C. There is a resistor in the A/C box which is supposed to be wired in series. Should I replace this also ($5)?

If I remember correctly the resistor is for the low and medium speeds...make sure that there is constant +12volts to 1 of the relay terminals(I cant remeber which 1 w/o a wiring diagram) it should be 1 of the big wires going to the relay,if not you should check the wire/connector I stated in a previous reply

fortop · May 1, 2004

Finally - found the problem. Lose nut at the fusible link on the driver's side firewall , next to the brake booster. After spending two days tearing apart the dash and tracing wires I finally figured out what you guys were trying to tell me. I somehow had the notion that the power started at the heater control switch, then went to the blower/A/C unit. Instead, power flows from the fusible link, to the blower relay, then back to the heater control, etc. Following the color coded wires form the heater control back to the engine compartment was the way I finally figured out how this thing works. I got sidetracked because I did not see an inline fuse or connector feeding the relay. I should have looked farther to the fusible link.

I tried in vain to find a wiring diagram for this truck. Does anybody know where to get a genuine Chevy Truck OEM shop manual for a 1983 C20? Aparently, even HELM does not carry them, and Haynes and Chilton are useless for this sort of thing. Ebay might have a used one (have not found one yet) but they go for about $50 well worn.

Thanks truck brothers.