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Old 09-21-2002, 03:11 PM
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Join Date: Jul 2002
Location: Stony Brook, NY
Posts: 160
Talking ONE last question and my A/C problem is solved !!!

With the help of those who have responded to my Klima question, I've now found the cause of my problem. Now I need to impose upon you all for one last question. I'm not sure what the item is called, but I am speaking of the piece that has the sight glass on top of it to view the freon. My car is an 88 300SE.

This item has 2 sensors attached to it. The one on top activates the aux fan when jumped. The bottom one is part number 124 820 59 10 and FastLane says it's the "A/C Pressure Switch". When I unplug the 2 wires running to it and jump them with a piece of wire my climate control system works PERFECTLY!! The a/c compressor engages and disengages depending upon what button is pressed and what temp is selected. I checked this and removed the jumper wire so as not to mess anything up. I figure if MB has this switch in for some safety reason I didn't want the compressor engaged if MB wanted it DISengaged.

So, my last question is, what does the A/C Pressure Switch monitor before allowing the compressor to engage? Is it the pressure of the refrigerant (134a in my case as it has been converted) as I'm guessing? I dropped the car off at my local shop yesterday but they didn't have time to check the a/c system but will Monday. My guess is I'm either low on refrigerant pressure or the switch itself is bad.

I'd love nothing more than to be able to let them know what the problem is and save myself some money in having them charge me for hours of diagnostic work.
Ron Brooks
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Old 09-21-2002, 04:51 PM
jbaj007's Avatar
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Join Date: Jun 2001
Location: Santa Monica, CA
Posts: 2,053
Your assumption of what the pressure switch does is correct. While the switch itself can be bad, most often it's just doing it's job by shutting off the compressor when pressure is low to prevent compressor burn out due to low circulating refrigerant (and therefore oil). With 134a you just charge it up (with gauges )to see if it engages the compressor at the proper pressure. If it does, you then "seek the leak" that led to the low pressure situation. If charge pressure is correct and it still doesn't engage the compressor.....usually bad switch. Evacuate refrigerant and replace switch.

The sight glass "piece" is the receiver/dryer. Sight glass charging is of no use with a 134a conversion, so use gauges for sure.
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)
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Old 09-21-2002, 04:55 PM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,784
< >>

That is the switch that we told you to jumper is low freon compressor saftey ..
If compressor clutch came on, the sw is bad or the freon level is low..

I suspect low freon..
They will know that right away..
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Old 09-21-2002, 05:24 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Be very carefull if you replace the switch. The number you gave is a replacement number for an early number that had its function changed in about 1981. Many suppliers just use the early switch and change the number.

The difference in the switches is that the new number you gave is a low/high pressure switch not just a low pressure switch. With R12 it wasn't as necessary but with a conversion it is very important that the compressor be shut off at the high pressure setting (somewhere between 3-400psi. If you have any refrigerant the switch should make contact untill pressures get high.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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