Starting fresh with A/C system. Pressures too high - PeachParts Mercedes-Benz Forum

Diseasel300 · #1 08-17-2017, 12:13 PM

Nobody has mentioned airflow across the condenser at any point in this discussion. At 2000 RPM without the car moving, the pressures will be elevated due to lack of airflow across the condenser. For any meaningful measurement, you should have a fairly strong fan blowing across the condenser when doing your measuring. At 105˚F, the pressures are going to be high anyway. They'll be higher than when the car is moving, even with a strong fan blowing across the condenser.

jcyuhn · #2 08-17-2017, 02:36 PM

Quote:

Originally Posted by lorainfurniture

Pump down for about one hour. I have always used a micron gauge. I can usually pump down and get a somewhat stable 1000 micron reading. Then you know definitively you are tight and dry.

This is the right way to do things. That said, a minute quantity of moisture is not going to noticeably affect performance of the system. I would be skeptical that moisture is in any way responsible for the high head pressure/low performance of your system.

Quote:

Originally Posted by funola

124 is a newer model than 123 with a "better" compressor. Discharge air that is a 30F drop from ambient does not sound right. I have a stock system with R12 (with R4 compressor) in my 85 300D and I can get discharge air with a 40 F drop from ambient at idle. At cruise should get even better than that.

I am well aware of 123 vs. 124, have owned and fixed both! Different Denso compressors are used in different 124 models as well. I saw quite different performance in an early diesel (10p15 compressor, R12, single electric condenser fan and low idle RPM) vs later gasoline model (10p17 compressor, R134a, dual electric condenser fans, somewhat higher idle speed).

To be clear, the 30F drop is the temperature delta of the air into the evap vs. air out of the evap in a single pass with fan on maximum speed. I'm not talking about discharge air temp relative to ambient temp after the fan has slowed down - that measurement really doesn't prove anything because there are too many variables.

My '84 300D would discharge ~48F air on a humid 100F+ day in Dallas - at highway speed, after the cabin had cooled down a bit, and with the 100% recirc mod. Should have more completely specified the parameters around my statement, I suppose.

leathermang · #3 09-14-2017, 04:55 PM

Quote:

Originally Posted by Diseasel300

Nobody has mentioned airflow across the condenser at any point in this discussion. At 2000 RPM without the car moving, the pressures will be elevated due to lack of airflow across the condenser. For any meaningful measurement, you should have a fairly strong fan blowing across the condenser when doing your measuring. At 105˚F, the pressures are going to be high anyway. They'll be higher than when the car is moving, even with a strong fan blowing across the condenser.

TRUE.
Professional shops will usually have something like this on wheels so they can put it right up against the grill...

https://sc01.alicdn.com/kf/HTB1RQSjNpXXXXctXVXXq6xXFXXXi/3000-cfm-centrifugal-blower-fan-high-pressure.jpg