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Registered
1994 500sl
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24 Posts
Discussion Starter #1
I replaced my ac compressor and the dryer today .
The old compressor locked up.
Vacuumed the system for about 3 hours.
Added Freon with the motor off the pressure was about 70lbs
this should be enough pressure for the compressor to engage
to finish filling the system.
I turned the ignition on to the first position and pressed the reset button
on the climate control unit i could hear it click and it reset.
My friend stared the car and turned on the ac.
The compressor engages turns a few times and then disengages.
We did this a few times same result.
So bypassed the pressure switch on the dryer
same result the compressor engages a few turns and then disengages .
I don't know what else to do.
I think it has something to do with the speed sensor or maybe
I did not reset it correctly.
New compressor new dryer .
Any one have any ideas?
Thanks
John
 

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Registered
1992 500sl
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7 Posts
Had a very simular issue with mine after replacing the compressor and dryer. But mine would not engage at all, ended up being the mas control module...basically a monster relay.
 

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Registered
1999 ML320, 1998 SL500
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948 Posts
I had a similar problem with just a simple refill. Somewhere there is my long story. My EC light was on and the compressor wouldn't run. The EC light won't go out if the compressor is running. I eventually found a way to clear the conflict.
 

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Outstanding Contributor
2003 SL500, 2003 SL55, and four R129s
Joined
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4,432 Posts
I think it has something to do with the speed sensor...
I think so too. Read codes for the Base Module, since that is what monitors the compressor speed signal.
 

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Registered
1994 500sl
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24 Posts
Discussion Starter #5
I don't understand if the ac isn't running how can it give me the codes .I don't even know how to check them much less how to interpret them. Is there any other sensors that would keep the compressor from turning.
What would you think would be the most likely problem.
At this point I don't mind replacing parts .
Thanks
John
 

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Registered
1992 Mercedes 300SL R129. 94 S320 Mercedes w140
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91 Posts
My opinion: owning a r129 without the appropriate code reader isn’t a good idea. If your car has the older style pin arrangement, not the odd 2 slot, then the reader isn’t expensive. Having said that, the ac/heater control usually can be set into diagnostic mode to give the codes. Then, either look them up in the internet or ask here. Then the diagnostics will be a lot easier and much cheaper than throwing parts at the problem
 

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Registered
1999 ML320, 1998 SL500
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948 Posts
I believe there is a break between 1994 and my 1998 but there is a way, on my model,, to get readouts directly from the climate control display. Maybe that is also possible for 1994 model year. I do have a dongle and appropriate software on a laptop but the data I needed was apparently only available from the control panel.
 

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Registered
1992 Mercedes 300SL R129. 94 S320 Mercedes w140
Joined
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91 Posts
I don’t want to be too pushy but the information you have is way off. See this post


both my 92 and 94 have the climate control diagnostics
 

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Registered
1999 ML320, 1998 SL500
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948 Posts
Yes, similar to what I was able to do with my '98 to clear the EC light so I could get the compressor to run when I was doing a refill. The controls are different but the method looks familiar.
 
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