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How to read pressures on gauges

rickp on Mon March 29, 2010 1:04 AM User is offline

Year: 1990
Make: Mercedes
Model: 300se
Refrigerant Type: r-12

It's been a while since I've used gauges on my 1990 Benz which has an r-12 charge. In observing the pressures, I see there is a high reading which drops to a low reading and back to high on both the high and low side gauges.

Could someone refresh my memory of which reading (high or low) on the gauges is the correct pressure to correlate with the ambient temperatur; in other words, as the gauges swing from high to low and back as the compressor cuts in and out, which is the proper reading?

Thanks a lot.

Cussboy on Mon March 29, 2010 9:47 AM User is offline

Read pressures while engine is at approximately 2000 rpm and AC compressor is engaged, after maybe 10 minutes of running the AC system. The high side pressure will be more temperature-related, could be almost 300 psi on a real hot day, and low side pressures typically run about 30 psi; consult a manual, or someone here will know for sure and post.

rickp on Mon March 29, 2010 10:28 AM User is offline

Thanks for the reply. It sounds like it should be when the compressor is engaged? When the auxiliary fans in front of the condenser goes off the high-side pressure drops which makes sense but I'm wondering if that reading is the one to note? The pressure on the high-side rises to some point and then drops when the fans go on.

I want to top off the R12 but don't want to overcharge.

k5guy on Tue April 06, 2010 6:59 PM User is offline

Yes. AC on full. Blower on max. I usually open a car window. Make the AC system run hard @2000 rpm. Then take the readings. You'll find the high side, when working properly should be about 2.2-2.5 x ambient. The low side will vary with the temperature of the evaporator on a typical system. Having said that, I'm no expert on Mercedes, so I am heavily generalizing.


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NickD on Wed April 07, 2010 7:52 AM User is offline

A good guide for R-12 on a 85*F day under the conditions previously listed, door open, blower at max, engine speed, 2,000 RPM, AC on is in the 25-30 psi range. If low side pressure is lower, can leave one vent door open with a thermometer struck inside that vent, and by slowly adding refrigerant, that vent temperature should decrease allowing time for the thermometer to equalize. Will reach a point where adding a tad more refrigerant will not decrease the vent temperature. Better shop manuals list a P-T charge for various low side pressures versus both ambient temperature and relative humidity. And depending on if you have a TXV or a CCOT system, low side pressures may go up to 65 psi at 95*F on CCOT, this shows instantly the advantages of a TXV system where the valve compensates.

Always have to keep an eye on the high side gauge, at 85*F don't expect more than 250 psi, with around 220 being the norm. But the first step is always cleaning up the system first, can always find debris in the condenser and the face of the evaporator by removing the blower motor first. Adding air filters was a good idea, also make sure the condenser fans are operating properly.

For high ambients, best to undercharge, for low ambients, best to undercharge, this is a question of judgment and skill to get it just right, that is why most manuals recommend measuring the amount of refrigerant you have in the system by removing it with a charging station, than adding or subjecting the correct amount of refrigerant to their recommended specification. But this also has holes in the theory because of production tolerances that affect the volume of the system resulting in a high range of acceptable pressures. With skill and patience you can fine tune a system for the best overall performance.

Really no way of knowing how much oil is in the system, especially on an older vehicle that has changed hands several times. Only way to know for sure, is to remove and drain the compressor and flush the rest of the system out. Not convenient.

Then there are leaks, always find leaks in MVAC systems, question is how much, if less than 2 ounces per year, normally let those go. Soft leaks like I like to call them are the first to check wherever rubber is involved like in the compressor seal, service ports, Rubber hoses, and the most detestable, where they use O-Rings in the connections. Rubber is the first to dry out and become brittle, these are never self healing and just become worse. Hard leaks like in the evaporator or condenser require replacement. Can also find O'rings in the compressor and the fittings to the inlet or outlet. Should outlaw rubber in these applications.

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