1990 Mercury Sable LS

[lastalgonquin]

Hello, I have the above car with a problem with the heating and defrosting. If you turn the ignition key on but do not start it the heating and defrosting comes on. But the moment you start the car the heating and defrosting stops. The garage I took it to determined that when the car gives 12 volts the heater and defrost work and the fan works fine. But the moment the alternator gives 14 volts the heater and defrost stop. On rare occasions it has suddenly come on while waiting at a drive thru and then stopped. There must be a control unit that will not take 14 volts now. This is the electronic digital dash board and climate control. No levers to open a door to allow heat or defrost to work. Sorry it does not have the regular way to get heat and defrost. I can feel a little heat up by the defrost but its not enough to work on a cold day or a rainy day. Anyone know about where a control unit might be found or have an idea to fix it. Thanks