98 C280 AC blowing hot air

[clankford]

Oh, mighty MB gurus!

My 1998 C280 is still having AC issues. I finally have some time to work on it today. I cleared the diagnostic codes and drove it for a couple of days to see what would come up. No new error codes have appeared. With the car parked, outside temperature measuring 83F and the inside set to 72F, I ran the dash AC diagnostics. The following are the readings with engine off and then with the engine on. Any thoughts on this one?

Also, does anyone have access to material that shows the correct amount of R134a that should be in this system?

ITEM - OFF - ON
1 - 110 - 109
2 - 92F - 93F
3 - 100 - 101
5 - 98F - 101
6 - 143 - 152
7 - 1'5 - 4'5
8 - 96F - 96F
9 - 50F - 50F
10 - 465 - 465
20 - 00 - 00
21 - 000 - 070
22 - 000 - 000
23 - 000 - 000
24 - 11.4 - 13.7
40 - 02 - 02
41 - 03 - 03
42 - 18 - 18
43 - 6-C = 6-C
50 - 100 - 102
51 - 72F - 72F
52 - 50F - 50F
54 OFF - OFF

[Matt L]

If #7 is your refrigerant pressure in bar, you're very low on refrigerant.

[clankford]

What should the high side and low side be with the engine running and AC on?

Originally Posted by Matt L

If #7 is your refrigerant pressure in bar, you're very low on refrigerant.

[Matt L]

Originally Posted by clankford

What should the high side and low side be with the engine running and AC on?

That depends on the temperature and humidity. There are charts in the FSM that you use for this, however they are for confirmation of proper performance and not to judge the refrigerant level. The best thing to do is to have the system evacuated and recharged by weight, then check the pressures.

[clankford]

I'll drag my scale out and get started. Currently it is showing 5.1bar with motor off and 7.5bar with engine on and AC on. On the low side, I am showing about 36lbs with the engine on and AC on. With engine off, it is show about 75 lbs. Looks like the compressor is kicking on, but just not cooling.

Originally Posted by Matt L

That depends on the temperature and humidity. There are charts in the FSM that you use for this, however they are for confirmation of proper performance and not to judge the refrigerant level. The best thing to do is to have the system evacuated and recharged by weight, then check the pressures.

[Matt L]

I thought that you listed 1.5 bar above, which would prevent compressor engagement.

7.5 bar probably isn't enough pressure to do much of anything, and this could be the problem. Be sure to verify the reading on the pushbutton unit with a gauge on the high-side service port.

The resting pressure means very little, unless it is very low. The pressure that you will see depends on the refrigerant temperature alone, as long as there is any liquid in the system.

[clankford]

Matt:

My mistake, the ON reading should be 4.5bar.

At an outside temp of 90F, with the engine on and temp set to 72F, the low side measures 36lbs and the high side measures 100lbs. Starting to sound like a bad compressor, low side too high and high side too low??

The dash diagnostic at 1200 rpm shows (for 07) 4.5 bar?

kid

Originally Posted by Matt L

I thought that you listed 1.5 bar above, which would prevent compressor engagement.

7.5 bar probably isn't enough pressure to do much of anything, and this could be the problem. Be sure to verify the reading on the pushbutton unit with a gauge on the high-side service port.

The resting pressure means very little, unless it is very low. The pressure that you will see depends on the refrigerant temperature alone, as long as there is any liquid in the system.

[clankford]

Matt, do you know the correct amount of R134a that should be in this system?

Originally Posted by Matt L

The best thing to do is to have the system evacuated and recharged by weight, then check the pressures.

[Matt L]

I do not know how much refrigerant you need. There should be a decal under the hood.

36 PSI isn't so bad for the low-side. Mine runs as high as 2.2 bar on the chart (again, depending on temperature).

Note that you have a variable-displacement compressor, as do most newer cars. The displacement varies depending upon the inlet pressure, which is why it's not taking your low-side line into a vacuum. I would bet that unless it is leaking, your compressor is fine.

[clankford]

The decal says 1.9lbs, seems like a lot, but that's what the label says....

Thanks so much for all your help, Matt. I will evacuate it and recharge tomorrow and go from there.

kid

Originally Posted by Matt L

I do not know how much refrigerant you need. There should be a decal under the hood.

[Matt L]

If you still see a low high-side with the proper refrigerant level, along with a (perhaps slightly) elevated low-side, be sure to check the TXV before you condemn the compressor.

But hopefully just getting it filled will do the trick. We're about out of AC season here, but I imagine that you still want yours for a while.

[clankford]

Yes, we are almost into fall, but then it becomes a NEW problem. Without the AC, your car will fog up inside because it depends on the AC to dehydrate the air inside the car. Thanks again for all your time today, Matt!

kid

Originally Posted by Matt L

We're about out of AC season here, but I imagine that you still want yours for a while.

[clankford]

As for the TXV, I know that a "partially" plugged TXV can cause this condition. Is that what you are referring to? Can I find some info on this model's TXV here or somewhere you know of? Such as, where it is located and what type of TXV does it incorporate?

thanks Matt!

Originally Posted by Matt L

If you still see a low high-side with the proper refrigerant level, along with a (perhaps slightly) elevated low-side, be sure to check the TXV before you condemn the compressor.

[Matt L]

I'm not familiar with the layout of your system, but it should be at the evaporator inlet, against the firewall. It's not easy to access on the W210, as the wiper mechanism and water tray must come out.

If the TXV is at fault, it would be because it is stuck open. It should be open at room temperature, but it should close if you put it in the freezer. I replaced mine when I had the system open because it was cheap. I did not test the new part before putting it in.