George,
Hopefully I won’t upset the gracious host of this board, who is in the business of selling parts. I recommend that your friend do some diagnostic work before replacing $300 parts.
The cabin temperature is regulated by the climate control unit. (CCU). It is located behind the glove box near the firewall. The temperature select wheel on the left side of the pushbutton unit (PBU) is a potentiometer that varies resistance to the CCU. The cabin temperature is sensed by a thermistor located under the small grill on the top center of the dash. It varies resistance to the CCU based upon its’ temperature.
I’d start by testing the thermistor. Remove the glove box. Reach in above the clock or center vent and pull the round unit out where it connects to the little grill thing. There’s a plug back where the glove box would be. Unplug it and take it out. The thermistor is a single resistor on a small circuit card. It should be clean and free of gunk. With a multimeter measure the resistance between the two wires at the plug. It should be about 2500 ohms at 65 (f). Warm it gently with a blow dryer, at 100(f) the resistance should drop to about 1100 ohms. An important part of this unit is the ‘snorkel tube’ that is connected between the plastic pipe that is beneath the grill and the return-air side of the fan. It pulls cabin air over the sensor at a rate proportional to the fan speed. This hose cracks and falls apart over time, leaving the sensor testing stagnant air beneath the dash. The failed area can be replaced with standard ½” i.d. vinyl tubing (home depot). If lucky the far end of the tube will be ok since you can’t get to it without major surgery.
If that checks out, I’d then test the potentiometer at the pushbutton unit (PBU). Pull it out and test the resistance between pins 9 and 10 on the left side plug. At coldest setting (not the MAX click) the resistance should be about 300 ohms. At highest heat (again short of MAX) it should be about 1400 ohms. This should change smoothly as you rotate the wheel. A good cleaning of the actual POT (metal contacts) is about all that could be needed here. If you go in here, make sure to align the little tit on the POT with the notch on the control wheel when reassembling. The PBU can be taken apart simply, just a bunch of plastic clips. The only part the PBU plays in temperature regulation is a place to plug in the POT. It doesn’t process any info. There is a circuit card in the PBU, but it deals mainly with fan speeds. The joints on this card do fail, but seem to affect the fan.
Don’t have a multimeter? Don’t know how to use one? Go to Radio Shack and buy one for about $12. Take that thermister and these numbers with you and ask the salesperson to show you how it works.
Beyond these two things I’d suspect the CCU (behind the glove box) instead of the PBU. The CCU is just a simple comparator circuit with switching on a single layer card. There is nothing high tech on the card. Most likely failure of the CCU is broken solder or burned resistor. A failure here could affect the mono-valve, the compressor or the general logic of the whole circuit. Any of the components on the card can be had for a few dollars and the layout is open enough that anyone should be able to handle the solder job.
The fan is another seemingly common problem. Before blaming the PBU check the voltage to the top of fuse #6 when the fan fails to blow. If there is no power there, then the ignition switch is probably bad.
The correct diagnosis is ‘could be anything’, but blind parts replacement can be an expensive way to narrow it down.
If a person is considering buying a replacement, why not have a shot a fixing it first and save those $$. Sooner or later you’ll need them for that ‘dealer only’ part.


Teach a man to fish, he’ll steal your rod.
Give a man a fish, he knows where to get free fish.