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Mercedes AC trouble-shooting help

MaximusMax on Sat June 26, 2010 4:13 PM User is offline

Year: 1980
Make: Mercedes
Model: 300SD
Engine Size: 3
Refrigerant Type: R-12
Ambient Temp: 100
Pressure Low: 35
Pressure High: 240
Country of Origin: United States

I have a Mercedes 300SD with a Seltec compressor, parallel flow condenser, and a fill of R-12 (filled using sight glass).

At 100 degrees ambient, the engine running about 1700 rpm, and the AC on Max cool, I am seeing around 35 psi on the low side and about 230-245 on the high side.

The lowest vent temp I can get is 62, and that is after a sustained trip on the highway.

The compressor, condenser, R/D, and hoses are all new. I have the heater core bypassed so I am not getting any extra heat. I have the fresh air bypassed so I am in continual recirc mode.

I suspect the problem is either the expansion valve (I did not replace it) or the condenser (the one I used was a couple of inches smaller in both length and width than the tube-and-fin it replaced).

I ran cold water from the hose over the condenser while it was running and that did not seem to have any impact on the vent temps, so in my mind I can safely rule out the condenser.

Although the pressures look ok to me, is there any chance it could be the expansion valve? Is there any way to test short of replacing? What are the chances I overfilled with R-12 and that is causing my issue?

MaximusMax on Sat June 26, 2010 5:04 PM User is offline

I taped the temp probe from my digital volt meter to the metal fitting on the suction line at the evap, just after the expansion valve. It measures low 60's - same as the air from the vents, so I think I can rule out a dirty evap.

I also forgot to mention that there are no bubbles in the AC sight glass.

bohica2xo on Sun June 27, 2010 3:33 AM User is offline

Could be the TXV is restricted. It is always the part you don't check that bites ya.

Overfilling with refrigerant would jack up the high side pressure, but it would take quite a bit. Did you fill it using the sightglass?

The ugly truth is that eurocars of that vintage did not have great A/C to start with. The evaporator & airbox in that car is on the small side, and will never give the "meatlocker" performance of a US car. They simply did not understand the market here in the US back then when it came to A/C.

You should be able to do better than 62f, but don't expect 38f @ 100 ambient.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

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