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Questions about new installation

grassvalley1 on Mon August 18, 2008 5:42 PM User is offlineView users profile

Year: 1971
Make: MB
Model: 280SE 3.5 coupe
Engine Size: 3.5
Refrigerant Type: R-12
Ambient Temp: N/A
Country of Origin: United States

Hello all,
A few questions I'm sure the forum can answer. I am adding A/C to my Mercedes coupe. The factory unit is an under dash unit utilizing a A6 compressor and a TXV. I have acquired all components to complete a factory installation-no easy task since there were only 4500 of these cars built in convertible and coupe form. I have a rebuilt compressor, new dryer and hoses. I plan on using R-12 as the original system did. I am satisfied with other posts I read concerning testing the TXV and flushing the reused evaporator and condenser. Both are used but show no signs of oil or past leakage and the condenser is well protected from road debris by the aux fan and grill. After reading about the function of a POA valve I realized my system does not have one. How does this impact the operation of the system? There is a large tank built into the compressor manifold suction line. Is this to hold liquid that may come out of the evaporator so I does not enter the compressor?

My other question may seen basic. It is based on my assumption that the compressor is a positive displacement pump and it's displacement is directly proportional to it's RPM. My system relys on a adjustable thermostat with the sensing bulb on the evaporator to control the temp of the system. The thermostat directly controls when the compressor clutch is engaged(through a relay) More modern systems maintain a constant evaporator temp but if I understand the operation of my system the evaporator temp will vary as the thermostat is adjusted. On my system there is no way to mix warmer air since it is a under dash unit. So when the thermostat is turned to a warmer temp will it just change the compressor on/off duty cycle so the compressor does not run as long/often? Would this cause the damp/musty smell from a warmer evaporator?
When the compressor is running fast as driving on the freeway what happens to the extra volume created by the fast running compressor? Does the duty cycle change to maintain correct pressure in the system?
Thanks for indulging me.
PS-I have no way of knowing the correct oil/refrigerant charge. Even the dealer can't tell me. I have contacted other owners of my car and there is no underhood decal with servicing information. I even contacted the Mercedes Classic Center and got no answer. I have read other posts that seen to explain a method of charging the system when the capacity is unknown. I'm sure the compressor oil charge is common knowledge since it is a widely used A6. Any input appreciated.

Tony H

Edited: Mon August 18, 2008 at 5:46 PM by grassvalley1

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