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Old 08-17-2001, 01:14 PM
Posts: n/a
Unhappy W126 1990 300SE A/C Problem

Suddenly my A/C has low cooling output at below 1000 RPM
Pressures are 250 to 275 on High and 50 on low at idle going to 35 at 2500 RPM Outlet Temps hover at 60 degrees after a while.
No bubbles in sight glass. Need advice. Have had one suggestion that a clogged or blocked expansion valve could be the problem. Would like to hear what a replaced expansion valve should run. Would like to hear of ways to "clean out" a 10 year old system that work and are not harmful.
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Old 08-17-2001, 09:26 PM
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Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
This is a tough one. Usually this is an indication of a compressor that has a bad reed valve. Or, it could be a hundred other things.

I would get a professional opinion.
Donnie Drummonds
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Old 08-17-2001, 11:35 PM
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Join Date: Jun 1999
Location: Los Angeles, Calif, USA
Posts: 521
Normally, when the reel valves fail, you get high side low and low side high. It is because the valves are not holding pressure.

If the expansion valve is clogged, you get low side low, sometimes below zero. Because the compressor is sucking the freon and the valve is not releasing enough freon to fill in the suction.

Based on your gauge readings but without physically putting my hands on the system to feel the temperature, here is what I think (in order of likelihood):
1. freon slightly low,
2. not enough air to cool the condenser, or
3. an open expansion valve.

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Old 08-17-2001, 11:49 PM
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Join Date: Apr 2001
Location: Beech Island, S.C.
Posts: 468
The pressures sound okay to me so it should be cooling. My suggestion is to feel the low pressure line (the larger of the two) and if it is cold then the compressor is work properly. If the compressor is doing it's job check the monovalve (heater valve electronic). If it's bad it will let hot coolant circulate through the system and keep the A/C from cooling as it should.
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Old 08-18-2001, 10:44 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Well, I tend to disagree with everyone but Donnie, although I think David is right about the reed valves causings low high sides along with high low sides. I do not believe that a low charge could cause these conditions in any senario.

I believe overfilling, over oiling, residual flush could cause this or a STUCK OPEN expansion valve. The pressures are not OK. They totally reflect the temperature problem that is the symptom.

Low side pressures on a R12 system should drop below 30psi and the system should cycle when achieving its goal. The temp of the evap will be almost a direct conversion from the pressure into deg F. In other words something like this (I do not have the chart handy) 30psi = 35deg F, 40psi = 45deg F, 50psi = 55 deg F.

I would replace the exp valve and drier and evaluate the amount of oil that is released on evacuation. I would also see how much oil was in the drier and/or cut it open for a view of the inside of the system.

Absolute temperatures or pressures are hard to specify as this is a dynamic system very much dependant on the ambient temperature. If the day was 100 degrees I would accept a duct temp of 60 degrees and the resulting pressures at 50mph and hiblow. At idle they would go to 70 deg and probably 60psi.

Actually I would accept those numbers for a 134 conversion but it would be very borderline for a R12 system. A real good system would knock 50 degrees off ambient at 50mph hiblow. Most new 134 systems easily do this.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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