QuestionQUESTION: Hi Roland,
I have a 1999 300M Last weekend my A/C stop being cold. Replaces Freon and clutch relay. Tested power to compressor, there was not power. We bumped the compressor and it turned on. What would stop power to the compressor?
Lonnie
ANSWER: Hi Lonnie,
Of course there is no 'power to the compressor' because it is driven by the belt. The only electrical power involved is to activate the clutch which then connects the belt pulley to the compressor. The large middle pin of the ac clutch relay's socket is connected directly to the clutch hy a dark blue/black wire and so if the system has enough refrigerant there should be 12V showing on that wire at the clutch plug when you request AC. To complete the circuit and activate the clutch the black wire at the clutch plug has to be grounded to the rear of the engine block on the left side of the motor. My suspicion would be that there is a flakey connection at the clutch plug proper, or an intermitent wire between the relay and the clutch or between the clutch and ground.
Roland
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QUESTION: Hi Roland,
Thank you for responding. I did not check the ground because I already know that I am not getting any power to the a/c clutch. What would stop power going to the a/c clutch. I already tried replacing the clutch relay.
Thank You
Lonnie
ANSWER: Hi Lonnie,
The compressor clutch will only activate if there is more that the minimum amount of pressure in the AC system from the refrigerant present inside. If it drops below that threshold then a sensor notices the pressure is low and will then prevent the computer from energizing the clutch. You may be just below the minimum amount (thus it came 'on' by chance when you bumped it). So I would consider getting a can of R-134 and an adapter hose to add that to the system. Or have it done at an AC shop.
The only other electrical explanation would be if fuse B (30 amp) in the powerbox under the hood were blown so check that to see if it is OK, first.
Roland
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QUESTION: Hi Roland
I had the freon removed and replaced they made sure that it was the right pressure. The compressor is still not turning on. I check all the fuses in the car and in the box under the hood. I am not sure wich one is the one that you said but all the fuses there good.We used a test light on the plug to the clutch and there is no power. We tried putting electric direct to the compressor again and it turns on. Is there any sensors or switches that we need to check.
Thank You
Lonnie
AnswerHi Lonnie,
There is a refrigerant pressure sensor and its wiring to the computer, and also the wiring between the computer and the clutch relay and the relay to the clutch should be tested. You could verify that there is 12V on the rearmost pin of the clutch relay socket when you remove the relay. Then put the relay back. Then you could start the engine and let it idle, then push the button for AC while a helper checks to feel a click of the relay or not. If it clicks then check the wire between the middle large pin of the relay socket and the clutch for continuity. If the relay doesn't click then measure the voltage on the dark blue wire of the three pin plug of the AC pressure transducer which is located in one of the AC refrigerant lines in the right front corner of the engine compartment, very near to the compressor. Use a fine pin to penetrate the insulation and reach the conductor of that wire. Tell me the voltage that it shows. It has to be between 0.5 and 4.6V for the computer to tell the relay to close.
Roland
PS The B fuse is the one in the rear row, one position from the inboard end of the box.
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