Why does the a/c compressor runs when the heater is on ?

the compressor should engage when running the A/C just like a home air condition...it should not run with the heater or defrost

First determine whether or not the compressor is running. The a/c line in the engine compartment coming out of the firewall shouldbe cold and maybe have frost on it if yhe compressor is running. If the compressor is not running, the system may need to be charged. If it is running, the problem is with the blend door inside the heater box in the car.
go to the last section at the linked page.
Do It Yourself Diagnosis and Repair

Your defroster will use the A/C compressor, but not the heater. If the compressor is locked up and you do not want to spend the money for a new compressor, you might look at just getting the clutch changed. This will still let the belt free wheel and the car run and drive, but the A/C will still not work.

On most years of the Ranger, it is normal for the compressor to run when the heater is on.
The problem is most likely that the heater blend door is opening when it should not. Check for vacuum leaks.

I'm not familiar with that code but, the A/C compresser is supposed to come on with the de-frost setting. Sounds like it could be a problem with the heater A/C control panel. As for the burning smell,it could be dust burning off the heater core, or possibly your blower motor trying to go out.

probably burnt the compressor out. for ac work it pays to go to a pro that does ac for a profession especially with Mercedes. one thing done wrong could backfire to mega bucks..

no it does not. The heater system works from a small radiator in side the cabin in a box with the blower motor. The compressor runs off the motor and is responsible for the cold air in the cabin. You may be confused because in climate control vehicles the compressor is running all the time and you vary the cold air by opening up the tap to the heater core in side thecar

I suspect that you have a climate control system in the car. These work by running the compressor and the heater togeather to obtain the optimum temp in the cab. The hotter you want the less the compressor works. The colder you want the less hot water allowed into the heater. At some point there will be a confilct in the setting thet has the compressor cutting in or out to meet that setting requirement. As for the motor cutting out check the idle speed sensor as it apparently not increasing rpm to compensate for the compressor running

your a/c compressor is puttin load on engine and a/c runs with heat also to dry air. have system ck for low 134 or problem a/c compressor .

Bad compressor this work also when heater is on. To check have someone turn on ac/heater make sure clutch on air comp is turning. If not comp clutch is frozen