heater controler problems

Okay here is the issue. 92 chevy with AC and the 3spd digital temperature controler. when the truck is running the screen flakes out and the controler will not allow blower speed and heat. What is crazy is it works fine with the key on engine off. replaced the controler with another used one and that did nothing. I am going to exchange for another used part this week and retry. However anyone else seen this problem and have an idea? thanks

I should add the lights on the controler flash/blink. so they cycle through all the positoin settings and then independently the blower speed does the same.

no ideas??

Sounds to me like there is a problem with the ground. It can have poor contact because of corrosion or fatigue, or a metal connector may have become loose or oxidized due to thermal cycling...I've seen them become brittle because the connection heats up as the resistance increases due to oxidation.

Also check the power wire from the fuse block to the A/C head conx, and even the fuse itself. Wiggling the wiring harness will sometimes cause the symptom to occur...disconnecting/reconnecting it a few times will usually scrape away the oxidation and reestablish contact.

the ground makes sense. do you know where this one is located? I will also check the power in also.

Usually bolted to a crossbrace behind the dash, but could be routed in the wiring harness to a common ground.

okay here is what we have found. put in a second controller resulting in the same blinking. noticed that when the truck is running and atempting to use the defrost the a/c clutch cycles on and off in 3-5 second intervals. should cycle but not that much. So disconnected the power to the a/clutch and now the controller no longer blinks and works well. I would assume the temp controller tells the clutch to cycle. correct? so is this a bad/crossed/shorted wire somewhere?

okay here is what we have found. put in a second controller resulting in the same blinking. noticed that when the truck is running and atempting to use the defrost the a/c clutch cycles on and off in 3-5 second intervals. should cycle but not that much. So disconnected the power to the a/clutch and now the controller no longer blinks and works well. I would assume the temp controller tells the clutch to cycle. correct? so is this a bad/crossed/shorted wire somewhere?

You need to check the ac charge and pressure readings.
Low charge will trip the controller.
It has to be recharged and controller reset to work right.