85 300d ac system pressures - PeachParts Mercedes-Benz Forum

pilot53 · #1 04-29-2021, 05:13 PM

I had to replace an o-ring on my ac system (klima sanden conversion kit), when i did I replaced the dryer then vaccumed the system for a few hours and let it sit overnight. no leaks. I added three cans of r12 (system has not been converted) and all was working. According to the factory sticker and the weight on the cans I needed a fourth to get to the 1.3kg. So I added the fourth and drove a bit and all was ok. Today the compressor stopped coming on this afternoon. i hooked my gauges up to it and Im seeing 130 psi on the low side and 110 on the high side. What do you think is wrong, is it overcharged somehow? Ambient temp under the hood must be at least 110F. This is all with the engine off since I cant get the compressor to turn on.

pilot53 · #2 04-29-2021, 05:27 PM

Update: I jumped the compressor on for a few seconds and both the high and the low side equalized at around 50-60 psi. The ac cold nice and cold for those few seconds so i think the compressor is ok. Does this seem like its over or undercharged? Howcan i test to see if the pressure switch is giving a high or low signal? it shows 2 ohms of resistance right now

engatwork · #3 04-29-2021, 05:49 PM

How hot is it outside? The pressures seem whacked out to me. If it is 90 dF outside and you are at idle you should see 200 to 250 on high side. It should not be that close to equal with compressor running. The low side should be around 35 or so.

Going on memory here so y'all correct me if I'm wrong.

pilot53 · #4 04-29-2021, 05:53 PM

its about 85 today, could it be because i only ran the system for about 5-10 seconds? I didnt want to damage the compressor if it was low or high. Not sure if I'm dealing with a charge issue or an electrical problem. it sucks that the factory manuals are not available online easily anymore...

Usaguy · #5 04-29-2021, 05:53 PM

There should be 2 switches - one for the fan and one for the pressure (its 2 function - it will open if the pressure is too low or if the pressure is too high)

The pressure switch is on the "high" side of the ac system and it's either open or closed so don't look at resistance but look at continuity.

If everything is ok it should be closed. If the pressure is too low or too high it should be in the open (no continuity).

110psi the switch should be closed.

It should open if it's below ~30psi and it should also open if it gets above ~300psi

pilot53 · #6 04-29-2021, 05:59 PM

Quote:

Originally Posted by christuna

There should be 2 switches - one for the fan and one for the pressure (its 2 function - it will open if the pressure is too low or if the pressure is too high)

The pressure switch is on the "high" side of the ac system and it's either open or closed so don't look at resistance but look at continuity.

If everything is ok it should be closed. If the pressure is too low or too high it should be in the open (no continuity).

110psi the switch should be closed.

It should open if it's below ~30psi and it should also open if it gets above ~300psi

Ok thanks that helps a bit, sounds like the freon charge is not the issue then since the switch has continuity. Jumping it didnt make any difference either.

Usaguy · #7 04-29-2021, 06:20 PM

Quote:

Originally Posted by pilot53

Ok thanks that helps a bit, sounds like the freon charge is not the issue then since the switch has continuity. Jumping it didnt make any difference either.

If the compressor is not coming on then the next thing to check would be the klima relay (ac clutch relay)

You can try and remove it and power the ac clutch directly to check if the clutch works.

The relay is next to the "blue ufo" on a 123 but I could be wrong.

Of course you will need a wiring diagram to know which pin does what.

Usaguy · #8 04-29-2021, 06:40 PM

Ok i forgot that you jumped the compressor already so here's what to do next:

With the klima relay removed check at the plug:

Signals from the pressure switch, evap sensor, ccu and engine overheat sensor (not sure if your car has it)

Then we will be able to say if the relay is faulty

pilot53 · #9 04-29-2021, 06:51 PM

Thanks for the quick replies Chris. I actually had a spare original klima relay, I swapped that in and all is working. Turns our theweird gauge readings are because I had the gauge valves set incorrectly

. I definately lost a bit of freon swapping the hoses on and off, now I have about 40 psi on the low side whit the ac running, do you have any idea what I should be seeing at around 85 degrees, lots of florida humidity today. I'm thinking I should give it a shot of freon (r12) to get it around 55 lbs?

Diseasel300 · #10 04-29-2021, 07:11 PM

Quote:

Originally Posted by pilot53

Thanks for the quick replies Chris. I actually had a spare original klima relay, I swapped that in and all is working. Turns our theweird gauge readings are because I had the gauge valves set incorrectly

. I definately lost a bit of freon swapping the hoses on and off, now I have about 40 psi on the low side whit the ac running, do you have any idea what I should be seeing at around 85 degrees, lots of florida humidity today. I'm thinking I should give it a shot of freon (r12) to get it around 55 lbs?

40PSI is roughly a 37˚ coil. 55 PSI is grossly overcharged (saturation temperature of ~52˚). Pressures are only relevant with full load on the system. Max fan speed and all the doors open on the car. At 85˚ ambient temp, you should have roughly 55˚ air blowing from your center vents. You're looking for a temperature split across the evaporator of 30˚ (85 - 30 = 55). Being slightly undercharged is preferable to being overcharged by any amount. Overcharging leads to liquid refrigerant circulating through the compressor and will quickly wipe your bearings out and score the cylinders.

pilot53 · #11 04-29-2021, 07:15 PM

Quote:

Originally Posted by Diseasel300

40PSI is roughly a 37˚ coil. 55 PSI is grossly overcharged (saturation temperature of ~52˚). Pressures are only relevant with full load on the system. Max fan speed and all the doors open on the car. At 85˚ ambient temp, you should have roughly 55˚ air blowing from your center vents. You're looking for a temperature split across the evaporator of 30˚ (85 - 30 = 55). Being slightly undercharged is preferable to being overcharged by any amount. Overcharging leads to liquid refrigerant circulating through the compressor and will quickly wipe your bearings out and score the cylinders.

Oh thank you, I'll leave it the way it is then!