On the HVAC control panel, find the line at the rear of the unit with the controlling signal that operates the heat exchanger solenoid vavle. This means removing the unit from the dash and likely disassembling parts of the dash. Once you've found the "control wire" or signal wire that controls the motor to the solenoid operated valve, hook a voltmeter to it and watch the temperature reading display as you change the temperature up and down on the temp up & down buttons to see iff the voltmeter voltage changes as you raise and lower the temp. If the control line gives a dead signal, there's your problem; maybe go to a wrecking yard and buy another one (HVAC Control Unit) or buy a new one. If you are handy enough to disassemble the potentiometer, perhaps you can just replace that part if there are manufacturer part numbers on it. Maybe there's just a broken solder connection that needs resoldering or something simple of that nature.
See of you can obtain wiring diagrams off the Internet also to make your hunt easier to find the signal wire.
It appears you have tested the solenoid operated heating valve and if it is running satisfactory then it might be OK to keep, but since you have the whole dash apart, I would replace the heat exchanger valve, the controller and other bits prone to wear and tear.
Last edited by lucky_doggg7; 10-16-2012 at 11:47 PM.