E300D Mer. a/c compressor doesn't energize

You should have a fuse and relay in the underhood fuse box. If I remember right, the A/C fuse is at the very front of the box by itself and the relay is on the far left and the top relay towards the cowl

Check out your thermostat - it is wired in series with low pressure switch and if is open clutch will not activate and the compressor will not run.

I don't know if my air condition services were cheating me but I had twice problem with the compressor and after two different checkups both time they told me I had to install a new compressor. and that repairing it was not a good idea.

You need to ensure the engine is running when you are testing the AC or heating systems, as they rely upon a compressor to work the heat pump.

If your engine is on when you are testing this and no different temp air comes out, you should ensure your AC system is properly charged with coolant and that the compressor is engaging and functioning. If not, it will need to be replaced.

Sounds like a possible bad a/c compressor. Get a mechanic to check it.

Question. Is the flapping at a very fast pace that would indicate the speed of a fan blade hitting something? If so, you may be on to something there such as a misplaced gasket or something has dropped/fallen into the blower box via a vent.

If it is not that speed then see the following..

Hope I can give you some direction. Speaking from a generic sense all ac/heaters these days have a door or doors in the blower box that control air flow from the main part of the box to different positions when you ask for vent, heater, mix, defrost etc..

Most these days are operated with a small electric servo/motor and unfortunately the mostly have plastic gears that wear and can "thump-thump" as the motor cycles trying to open or close the flapper door.

Try to change the air direction from Max AC to regular ac to heater, defroster, bi-level etc all with the AC switch in the on position if you have a separate on/off switch.

Listen to the sound and see if it comes and goes with a different location of the selector switch.

One last question. Does it happen at idle or driving down the road at any particular speed?

Lets start with that and see what happens. This is interesting.

Good luck!

Have the dealer check to see if the AC compressor cycling switch is stuck.

First try replacing the Relay for the system. If that doesnt work check/replace the pressure and temp switch on the drier. The 124 body style have issues with A/C overall so dont be too frustrated. Also in the short term try turning the temp wheel to max cold (blue dot) and restarting the car. Sometimes it will force the relay and compressor to lock on while driving. Also be aware if your Aux Electric fans dont come on the system doesnt get cooled properly and will not stay on.
Good Luck.

What year is the vehicle ? The A/C button on the control panel is a direct input to the PCM /ECM - engine computer . The PCM/ECM energizes the compressor clutch relay to engage the compressor .

That fan comes on with the a/c compressor to assist cooling while a/c is in use....diagnostics must be performed with a/c on for that fan to be energized...start at the relay in the underhood fuse/relay box and check for 12 volts to fan when a/c compressor clutch is energized