Run self test on heater control head. With the key press off and floor buttons at the same time for 2seconds then release and press auto button for a second.this will put system into self test.if no code all the blend door actuators are ok and I would think vacuum leak from main vacuum, black line going through fire
SOURCE: Just got heater core replaced
It sounds like when they replaced your heater core, that they didn't properly hook up the blend door contros at the control panel. I would take it back to the repair shop that did the work and have them go back over things again just to make sure.
SOURCE: 2000 Mercury Grand Marquis will only blow hot air when ac is on
needs re-gassing because one line should be hot (feed) and the return cold , This is a specialist job do not attempt to do it yourself because its extremely dangerous with very high pressures involved
SOURCE: this car has the automatic temperature control system
The hot/cold switch is probably bad.you have a blend door box that opens to let the cold air in and closes to let the hot air in.From what you are saying this part is no responding to the switch movement.Make sure power is at the switch and power should go through the switch when put on cold air this moves the blend door.Some times the blend door goes bad and has to be replaced its under the dash on the right side.
SOURCE: 2001 Mercury Grand Marquis LS.
This car has a "vacuum module located behind the control panel.Make sure all the vacuum lines are connected.
SOURCE: I can turn the ac
It sounds like thermostat (in coolant system) needs to be replaced. They only control you have for temps are the dials on dash, but if coolant thermostat is sticking you're limited to what it outputs. Replace coolant thermostat and you should be "good to go"
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Maybe I'm not explaining problem well enough. I can control temp with auto control. I can also see change when push buttons to go to defrost, floor or mix. it just does not switch to selected button. Even though control panel shows change, will only blow out of front vents, with a little coming out defrost. For what ever reason it will not switch from blowing out vent. To me, the heater/ac controller is working, however wherever the signal is sent to does not respond to command. Do not know if that is a relay or electric switch of some kind. My hobby is a small collection of 60's muscle cars.new car stuff is not my thing. I'm thankful for any help offered. Marty
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