Air conditioning refrigerant pressures? - PeachParts Mercedes-Benz Forum

Ron in SC · #1 08-12-2004, 09:34 PM

What should the high side and low side pressures be when checking the air conditioning of my 99 ML 320?
Following may be needed to ascertain this information:
Ambient Temp measured 2" from the front of the condenser is 95-100 degrees F.
Relative humidity is 60-70%.

I just installed a new compressor, expansion valve and receiver/dryer in my 92 300E. I now get 39 degrees at the vents with outside ambient at 80 degrees while driving. When I charged the system at idle ( ambient 95-100 degrees F) my high pressure side was 245 psi and the low pressure side was 39 psi. vent temp. in the car at idle was 55 degrees F. on the highest setting. I put in very close to 2.1 lbs. of R134a (vehicle had been converted from R12). The system held 2.4 lbs. of R12.

With the ML being manufactured in June of 1999 I think it would be a good idea to check the A/C system even though it still does get cold. I have not checked yet but I don’t think it gets nearly as cold as the 300E that I just finished up with.

Ockman · #2 08-13-2004, 06:22 AM

Just had my 98 ML 320 a/c looked at. The first thing you should check is the aux cooling fans. Do they operate when the a/c button is depressed? If not, check fuse #44. It's a 30 amp fuse that is known to blow often. MBUSA has a recall on the aux fan harness and should replace the harness and install a 40 amp fuse at no charge. The operating pressure was about 250/high and 35low with the outside temp at 84. The vent temp at idle was 41 (fan at position 2) at which point the a/c cycled to off and then back on.
Good luck.

P.S. More info on the #44 Fuse problem can be obtained using the search function.

Ron in SC · #3 08-13-2004, 11:21 AM

Ockman,

Thanks for posting regarding the pressures. That’s what I wanted to know. It sounds like they are within the same ranges as my 300E.

My ML’s A\C has always worked great. I just thought with it being 5 years old that it might be a good idea to check it. The vent temp while driving is 42 degrees F with the ambient being 75 degrees F. with the fan set on 2. With the fan on high the temp is not as low. Right now the ambient is unseasonably low due to the weather pattern we are in right now. I would think with the ambient so low that the vent temperature should be less. I see you are in Key West so hopefully you will not be impacted to severely by the storm. I’ve lived in Charleston, SC most of my life so I know how the hurricane situations can be. Good luck.

As far as the fans and harness go they have never been a problem. I had been running the 40 amp fuse for years with incident. A month or so ago I did the harness update just to be safe. In fact I posted the instructions on how to do the upgrade, with photographs, on this site. I was of course assisted by Gilly in doing the harness change.

Ockman · #4 08-14-2004, 10:37 AM

My aux fan fuse (30 amp) has blown once. I replaced it with another 30 amp fuse and will monitor it closely. If it repeats, I may try the 40 amp fuse and then do the harness change. The MB dealer is not very helpful since I purchased it new from another dealer. What did the harness cost? Seems like a fairly easy DITY job.

My 99 E300 has a totally different setup, a variable compressor. It also has a self diagnosis capability that I wish the ML had. Allows you to read the high pressure side and evap temp, etc from your instrument panel. Great info for DITY guys like you and I.

Good luck with the 134a conversion on the E300. Living in FL, I haven't had much luck with conversions. They work well until you do some city driving and then they really can't keep up with the FL heat. Did both to my Saab and a 83 300D I owned and went back to R12. Luckily, I have a 609 certification and did it myself. The R12 is about $25/lb these days if you shop at ebay.

Good luck and thanks for the excellent photos on the harness change.

Ron in SC · #5 08-14-2004, 03:43 PM

The harness cost me $32.98 which includes tax. It's part # 163-540-00-35

Robert W. Roe · #6 07-19-2005, 01:21 AM

Quote:

Originally Posted by Ron in SC

...
I just installed a new compressor, expansion valve and receiver/dryer in my 92 300E. I now get 39 degrees at the vents with outside ambient at 80 degrees while driving. When I charged the system at idle ( ambient 95-100 degrees F) my high pressure side was 245 psi and the low pressure side was 39 psi. vent temp. in the car at idle was 55 degrees F. on the highest setting. I put in very close to 2.1 lbs. of R134a (vehicle had been converted from R12). The system held 2.4 lbs. of R12.
...

I just topped off my 1991 300TE that had been retrofitted to R134.
I now have 40 psi on the low side at 91F ambient temp at idle.
Our numbers sound close.
Just wanted to chime in here, because I'm not sure 40 psi is exactly right, but when I got to about 36 psi or so my aux fans came alive, and I have nice cold AC...

jcyuhn · #7 07-19-2005, 10:18 AM

You might be a touch overcharged. Rule of thumb for an R-134a conversion is 80% of the R-12 charge. You are a bit closer to 90%.

I ran R-134a for a while in my 124 diesel wagon after rebuilding the a/c system. (I replaced everything but the metal lines.) Was pleasantly surprised at how well it cooled, despite all the bad stories. I'm trying to recall how much refrigerant I used - think it was 2.5 cans - maybe 30 ounces?

MB says to check the high side by running the engine at 2000RPM for 10 minutes, fan speed on high, car in the shade. This will produce a higher number than you are seeing at idle. I don't have the shop manual handy, but if this test produces a number above 300PSI you are likely overcharged.

On the other hand, if the pressures are under control and it cools OK, then by definition it's charged right.

- JimY

Robert W. Roe · #8 07-19-2005, 08:27 PM

Quote:

Originally Posted by jcyuhn

You might be a touch overcharged. Rule of thumb for an R-134a conversion is 80% of the R-12 charge. You are a bit closer to 90%.

I ran R-134a for a while in my 124 diesel wagon after rebuilding the a/c system. (I replaced everything but the metal lines.) Was pleasantly surprised at how well it cooled, despite all the bad stories. I'm trying to recall how much refrigerant I used - think it was 2.5 cans - maybe 30 ounces?

MB says to check the high side by running the engine at 2000RPM for 10 minutes, fan speed on high, car in the shade. This will produce a higher number than you are seeing at idle. I don't have the shop manual handy, but if this test produces a number above 300PSI you are likely overcharged.

On the other hand, if the pressures are under control and it cools OK, then by definition it's charged right.

- JimY

Hmmm, I only have a low side guage at this point.
Now I'm wondering if I've overcharged....
Would this cause damage?
Should I try to lower the pressure somehow?
As you might tell, I am new at AC work, although it is a topic about which I'm willing to learn more.
I haven't checked the center vent temperature after a long drive yet, but after driving several miles at 87F ambient last night, the car cooled off nicely.
Now I'm worried that I overdid things.
Any thoughts appreciated.
Thanks, Bob

jcyuhn · #9 07-20-2005, 11:08 AM

Mildly overcharging the system will cause some additional stress on the components (from the greater high side pressure), reduce system performance (low side pressure is too high, which raises the temperature of the evaporator), and increased fuel consumption (compressor takes more energy to spin).

There's really no way to tell without high side pressure readings.

A few clues that you may be overcharged: The compressor should make essentially no noise at all. If it's knocking, banging, grinding, or otherwise sounding unhappy, you might be overcharged. The pressure switch on the dryer will cut out the compressor if high side pressure runs away - I think it shuts off the fun at 400PSI. That should prevent catastrophic failure, such as blowing out a high side hose.

But the only way to tell for certain is a set of guages with both high and low side readings. Otherwise, you're basically gambling.

- JimY

Robert W. Roe · #10 07-20-2005, 05:36 PM

Sounds like I need to get one of Harbor Freight's 49.99 manifold guages.

Now I don't want to use my AC until I get some good readings... and I need to find the high side fitting... the MB engineers who designed my 124 must have hidden it pretty well... I wonder where that puppy is?

jcyuhn · #11 07-20-2005, 06:31 PM

Hint: Look directly in front of the a/c compressor pulley, the hard metal line near the radiator. Tight quarters, but you'll find it there. I bought a nice 90-degree adaptor for my guage set from Grainger. Works great on a 124.

- JimY

Robert W. Roe · #12 07-20-2005, 07:29 PM

I did find that fitting after looking for a bit.
Thanks.
I did the low side test a bit more correctly this time, revving the engine to 1500 rpm, and I was a bit below 25 psi on the low side, maybe 20 psi.
I was getting 64 degrees out of the center vent. At idle I was only about 34 psi, so maybe I have a leak?
Revving and adding a bit at at time I finally got the low side pressure up to 26 psi and that was the end of the 12 oz can.
Center vent temps are now at 60 deg F.
Not great, but better than the 110 deg that it gets to sitting in the sun all afternoon.
Harbor Freight won't have my guages til Friday so I won't get to do much more for a couple of days...