What are the correct low/high A.C. pressure radings for a 190e with R12??? - PeachParts Mercedes-Benz Forum

Tinker · #1 07-19-2003, 06:13 PM

Hi all.

Anyone know what the correct low/high side air conditioning readings should be for a 190e with R12?

Larry Bible indicated (thank you Larry) my current readings are close to spec. But I am curious what the exact specs are?

I only have the factory A.C. shop book for a W124, and my specs are way off from those. Per this spec book, all reading should be at 2,000 rpm.

Thanks.

Tinker

psfred · #2 07-19-2003, 07:27 PM

Hard to tell right off the bat, but with the windows open, blower on high, engine at 2000 rpm, you should have about 35 psi on the low side and somewhere between 250 and 350 psi on the high side, depending on ambient temp and whether or not the aux fan is running.

Low side temps will drop much lower than that when the expansion valve closes, though, but shouldn't rise much higher.

For R12, the pressure and the temp in degrees F is the same on the low side, you never want it below 32 psi with the expansion valve open or the evaporator will freeze. It will drop to 15" of vac sometimes, with the expansion valve closed, before the evaporator temp switch cuts the compressor off.

pressures over 40 indicate a failure of the expansion valve or air in the system, usually.

Very low low side pressure (15" vac all the time, suction line not cold) indicates low charge. High side will also be well under 200 psi in this case, also. If the high side is 350 to 450, the expansion valve is plugged or stuck shut.

I didn't look these up, so someone with better information can correct me if I'm wrong!

Peter

Peter

Tinker · #3 07-20-2003, 06:27 PM

Thanks Peter.

I am only seeing low side pressure in the low 20's and high side of around 230 psi @ 85 F & 2500 rpm. This was after I added 12oz of R12. I wouldn't think the auxillary fan would be kicking on at these low pressures. I thought it would be closer to 250psi (??) but its comes on. Problem is my vent temps are only in the mid-low 60's, so I am thinking I am still a bit low on charge???

Thanks again.

Tinker

psfred · #4 07-20-2003, 08:49 PM

The low side pressure can be from a clogged expansion valve too, but I'd say you are low on charge.

However, if you have a leak, I'd get the R12 recovered and fix the leak rather than just add more, you really need to evacuate the system if it has been leaking, since air will be diffusing in while the freon is leaking out.

The switch for the aux fan can be either pressure or temp -- don't know which one yours is. I don't get aux fan operation on the 300TE until 300 psi or so, usually.

Peter

Tinker · #5 07-21-2003, 06:14 PM

Peter,

I think it has a slow leak.

I have owned the car for four years and never really used the A.C. It had a few electrical issues that I resolved and the A.C. started to work, but only blew cool. I checked the pressures before I added the 12 oz freon and it was reading 10 psi low side and 180 psi high side @ 2500 rpm and 80 F. Figured if it held a charge for over four years, I'd give it a try without pulling it apart and spending some $$$$. Some lose is expected over four+ years?

Thanks again.

Tinker

psfred · #6 07-21-2003, 06:41 PM

If it loses pressure, it has a leak, although a small one.

Most likely you have leaky o-rings or a compressor seal, these are the normal problems. The expansion valve will also leak, but that is usually somewhat faster.

Peter

csnow · #7 07-23-2003, 11:15 AM

I'm not sure I would characterize needing some refrigerant every 4 years as a "leak". May not even be detectable. Auto AC systems are inherently leaky to some extent. Ok, this is not ideal, but is it really at the point of repair? Do a cost/benefit analysis.

If it were me, I would topoff the system and add some UV dye.
That way you would likely be able to see where the [very] slow leak is over time.

You can get a really nice Tracerline UV dye kit from thetoolwarehouse.net for around $55.

Best of luck.