Problem with air conditioner on 300 ce
 

At times the air conditioner on my wife's 1991 300 CE will not work when the fan button is pushed. It will go like this for a while and then, without doing anything, the fan will come on and cold air comes from all the vents. Then, out of the blue, the cold air stops and it blows hot air. This happens erratically and without touching the controls. Can you please tell me what the problem may be?

Thanks a million,
Antonino

Two possibilities:

1. Ignition switch is worn. Try moving the key slightly CCW when the fan off, to see if it comes on.

2. Pushbutton unit has failed. Replace with used or rebuilt....

Three, three possibilities...

3. Aux water pump is tripping the PBU. Disconnect power to aux water pump.

Jim

Problem with air / heat on 300 ce
 

Dear Jim,

Thanks for your reply. Is the aux water pump you refer to one other than the main water pump and if so where is it located on this vehicle?

Aux water pump is mounted on the right front inner fender, with heater hoses attached to it.

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When it blows hot, you need to determine if the compressor clutch is engaged or not. If the compressor disengages, resolder or replace the Climate control push-button unit.

Is there an overheating fail-safe mechanism built into the aux water pump? In other words, if the engine reaches a certain temperature, does the circuit cut off the a/c to conserve temperature?

Well also ps2cho, if it disengages it could be a low freon condition or relay? yes?

problem with air on 300 CE
 

To all,

Thanks so much for you advice and responses. As soon as I can get the car away from my wife I will take a look at the possibilities you suggest and hope one of them does the trick.

Again, many thanks,
Antonino

The system uses the coolant temp sensor on the head to cut off the A/C when engine temperature reaches 115C/120C...can't remember which one.

115 is when it cuts off, you are dead on there ps2cho. 120 is overheat so they cut off just before, if the aux fans let it get that far there is something wrong is what MB figures I guess.