A/C issue

good afternoon.

Do you mean it blows cold air in the heat setting?

I am wondering if have a problem with the heat mode.

when you think the AC is on, is the compressor running?

It could also be a mode door not switching either.

https://www.2carpros.com/articles/replace-blend-door-motor

https://www.2carpros.com/articles/car-heater-not-working

If you are simply seeing the AC compressor engaging when the heat is requested, that could be perfectly normal as many cars are wired that way because the AC removes moisture from the air.

I have driven it for eighteen years and this is the first time it has done it. The heater does not run with the AC setting or circulating the air in the cab. I started having trouble with the thermostat and that is when it started doing this. I have changed out the thermostat housing and the water temperature sensors, the heater water control valve and the climate control switch, and checked the air blend door and actuator and all this works fine. I checked the Freon and it runs about 35 lbs. Static pressure is about 90. I unplug the low side pressure sensor and the AC quits running with the heater.

I am afraid I have to disagree with you on when the compressor is programmed to come on.
The following is from the service manual.

System Operation
Vehicle Heating and Air Conditioning Description and Operation Components General Information System Operation
SYSTEM OPERATION








PANEL/FLOOR
When PANEL/FLOOR is selected:
^ The air inlet duct door is set at no vacuum, blocking the recirc passage and admitting outside air.
^ The heater water control valve is also at no vacuum, allowing hot coolant into the heater core.
^ The floor/panel air flow door is in the partial vacuum position, allowing airflow to both the A/C registers and the heater outlet floor duct.
^ The panel/defrost air flow door is at full vacuum, closing off airflow to the windshield defroster hose nozzle and directing airflow to the A/C registers. There is also some airflow to the side window demisters.
^ The A/C compressor will be enabled when PANEL/FLOOR is selected.
^ The blower motor is on.



FLOOR/DEFROST
When FLOOR/DEFROST is selected:
^ The air inlet duct door is in the no vacuum position blocking recirc air and admitting outside air.
^ The heater water control valve is also at no vacuum, allowing hot coolant into the heater core.
^ The floor/panel air flow door is in the partial vacuum position, allowing airflow to both the windshield defroster hose nozzle and the heater outlet floor duct.
^ The panel/defrost air flow door is in the no vacuum position, directing airflow to the windshield defroster hose nozzle. There is also some airflow to the side window demisters.
^ The A/C compressor will be enabled when FLOOR/DEFROST is selected to dehumidify the air and reduce windshield fogging.
^ The blower motor is on.

There was a lot of good information, a lot of it I already knew and a lot of it I had not thought about, but it still did not answer the question, "What would make the AC run maximum air with the heater setting?" Because there is no way the cab will heat up when the AC is running with the heater and it has never done that before. The AC works good by itself with everything set on AC. But the AC works with the heater too. The heater has always worked outstandingly since it was new until I started having problems with the thermostat. I replaced the thermostat and the housing, new sensors. The cooling system works fine since I did all this.
Thank you.

The compressor running does not change the air temperature. That is regulated by the blend air door after the air passes through the evaporator. This is the way it was engineered to operate.