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Old 05-30-2003, 07:57 PM
BJ300SDL's Avatar
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Join Date: Apr 2002
Location: FT.Wayne,In
Posts: 134
Question A/c clutch cycling too much please help

On our 87' 300sdl our a/c wasn't cooling very well. I set the climate control knob to min. and set the rpm to 2000. The vent temp drops to about 60 deg. then the a/c clutch starts to cycle on and off.

On my service cd I read about the a/c clutch control module. I removed the module and found the two wires that can be shorted to engage the clutch all the time. When I did that the clutch stayed engaged and the vent temp. got down in the mid. 30's!

I've read posts about a evaporator temp sensor. I thought I found it in the air box above and to the right of the fuel pedal.
Pulled it out of the air box and ran it (still plugged in) and the clutch did not cycle. put it back in the air box started to cycle again @ 60 deg. vent temp.

Ordered a new sensor from the dealer. After the third switch they finally got the correct one?! All of them said evap temp. sensors on the package! He even showed me three of them on his MB computer screen for our car? Plugged in the new switch and no improvement?! All of these tests were done at 75 deg ambient temp., 50 % humididty.

What do I do now?

I hate to keep throwing parts at it!
87' 300sdl
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Old 05-31-2003, 07:53 AM
BJ300SDL's Avatar
Registered User
Join Date: Apr 2002
Location: FT.Wayne,In
Posts: 134
Doesn't anyone have any ideas?
87' 300sdl
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Old 05-31-2003, 09:55 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
One thing I have noticed is that of the 4 sensors involved 3 have the same characteristics - resistance wise. The later cars facilitate testing by allowing the sensors to be monitored on the A/C display.

On your car you must view your wiring diagram and see how the controller views the resistance of each sensor. Then test them for values at the controller. There could be further resistance in the circuit or you could have a bad sensor. We often use a sensor simulator (looks like various prescribed resistances in this situation). Just remember that since three should be the same that if you are testing after the car has sat for ten hours or so all the sensors will be at the same temp. As such three should have the same resistance and the fourth different.

And then comes the real bad idea. That being that something inside the evap causes the sensor to get the wrong conditions. I have seen sensor covered with leaves soaked in water that retain lower than expected temps on the sensor.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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