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Old 05-29-2005, 12:01 AM
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Join Date: Oct 2003
Location: manila philippines
Posts: 15
Red face w140 aircon woe!!!

hey guys ,Need your professional advice, the w140 92 300 se im working on goes like this., the ac blower motor suddenly dropped dead, i jumped the terminals of the blower motor and it worked so naturally i thought that the resistor was the problem, however i noticed that the compressor is not engaging is this also related to the resistor?? is there a relay that controls the compressor that might be the cause? (klima?) thanks man i would really aprreciate it., oh the refrigerant charge is normal no leaks i checked.
all the fuses are ok
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Old 05-29-2005, 01:35 AM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: SF Bay Area
Posts: 20,382
Here's my amateur advice -

There's a blower control unit located beside the blower. You could check the input and output but I can't advise how.

What resistor do you think is the problem? The blower control unit is a solid state device, not a bank of resistors like in older/other cars. The compressor gets 12V or nothing. There's no resistor between the ACC controls and the compressor. I don't know if the system is smart enough to disengage the compressor if the blower isn't working. Typically the evaporator temp sensor does that job.

Does the compressor engage in defrost mode? If it does then the problem is in the cabin such as with the ACC controls or the cabin temp sensor.

I don't think the W140 uses a Klima relay as such that's common in the W126 and W124.

95 S420
87 300SDL
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Old 05-29-2005, 07:16 AM
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Join Date: Dec 2002
Location: Southern New Hampshire
Posts: 418
You'll need to do some circuit checking. If you've already been to the blower motor, you know how to get to the voltage regulator. There is no resistor in the blower motor circuit.
The regulator is screwed right to the top of the blower case; the output leads plug right onto the motor (blue/red leads). I've attached a pix. This is later version of the regulator; it has a larger heat sink.
This item has a high failure rate especially on the early W140's. They burn out due to heat buildup.
Check the voltage going into the regulator as you vary the blower control on the dash... I took some readings off of my '96 W140 a while back, I've attached those as well for achive purposes. They may help you toubleshoot your system. These readings are "no load" with the blower unplugged. Also, your dash mounted control unit is different, so its unlikely that voltages are identical, but you'll get the idea.
If you have voltage on the input side of the regulator, but none on the output, its the regulator that has died.
Lack of compressor engagement can be a number of different issues, but don't believe its related to your blower failure.
Hope this helps.
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w140 aircon woe!!!-blower.jpg  
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File Type: doc Measured Readings Blower.doc (20.0 KB, 158 views)

1996 S500 W140 Coupe
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Old 05-29-2005, 11:34 AM
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Join Date: Oct 2003
Location: manila philippines
Posts: 15
Red face RE: 140 aircon info

THanks guys i really appreciate it, i'll keep you posted on what happens, what you guys gave me was really helpfull
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Old 05-29-2005, 10:39 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Check the serpentine belt -- on the M103/104, if the compressor speed drops below engine rpm, the Klima will drop the AC, and on the W140, this may also drop the fan.

The speed control for the fan isn't a resistor, it's a switching unit that gets an analog (0-6V, I think) signal from the ACC pushbutton unit and uses that to determine the duty cycle of the swtiching transistor on the ground side of the fan motor. You need to check that the fan has full voltage (check the fuse for condition, it's an aluminum strip in a small box, I think, outside the fusebox) and that you have a signal on the control wire to the speed control box (yellow? Some one chime in here, I'm not sure!).

However, that particular climate control is vastly more complicated than the ones in the W124 chassis, and is usually non-repairable......

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 05-30-2005, 01:34 AM
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Join Date: Oct 2003
Location: manila philippines
Posts: 15
Thumbs up A/c now works!!!!

Thanks for the help guys, the problem turned out to be the blower regulator,
i had an inkling it was the part causing trouble, i just had apprehensions beacuse the compressor didnt work, but when i replaced the regulator it worked out ok., thanks again!!!
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