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

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85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

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.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

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?

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1994 E500 199/Gray 82k
84 300D (Salty) Orient Red/Palomino 141k
88 300CE (Ersatzhammer) 904/Java 163k -- Turbo Technics twin turbo kit, AMG Gen I body kit, Sportline steering box and steering wheel, Sportline/Eibach /Bilstein Sport/500E suspension, Quaife LSD in 210 mm diff case, Silver Arrow brakes.
88 300CE Brabus 3.6 199 on Black 44k

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

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.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

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.