Intermittnt 240D AC Mystery - PeachParts Mercedes-Benz Forum

Mark DiSilvestro · #1 08-12-2010, 04:08 PM

I have a 1982 240D. The AC ha a peculiar problem.
Sometimes, anywhere fom 5 minutes to a half-hour after the AC is switched on cooling is great. Then it gradually declines over 5 to 15 minutes, until there is none. If I park the car for about a half-hour, it may work fine again, perhaps for the rest of the day. Car has R12, a rebuilt compressor, and new rec-drier. Negligable freon loss since last summer when the work was done.
When cooling ceases, high and low pressure readings are near equal. I suspect a defective expansion-valve that is intemittently going to WOT, but a search here didn't find a similar condition.
What do you think?
Thanks in advance.

Happy Motoring, Mark

mbdoc · #2 08-12-2010, 04:45 PM

Could be the evaporator temp sensor "probe".

Does the compressor continue to operate when that happens?

Mark DiSilvestro · #3 08-12-2010, 05:23 PM

Quote:

Originally Posted by M.B.DOC

Could be the evaporator temp sensor "probe".

Does the compressor continue to operate when that happens?

Yes. I;ve observed the compressor-clutch continues to be engaged when this happens. And inside the car, I can feel the engine load gradually decreasig as the cooling declines (I can certainly feel the AC load on this 240D) There's not a sudden change in load, like what normally happens when the AC clutch cuts off.
There's no loss of vent airflow, like what happens if the evaporator fins ice-up. (I did experience that on a W115 220 many years ago, where the temp probe had been pulled out of the evaporator)
And I don't think the intenal expansion-valve port is icing shut, as I believe that would greatly increase the high-side pressure, greatly decrease the low-side pressure, and also increase the compessor-engine load.

Happy Motoring, Mark

david s poole · #4 08-12-2010, 06:16 PM

does the blower continue to operate so that you have air coming out of vents because if not your coil may be icing up.this could be caused by a dirty coil or a worn blower motor not spinning fast enough[not enough air across the coil].

david s poole · #5 08-12-2010, 06:22 PM

i didn't read your second post sorry.this would almost have to be an issue with the compressor valve plate[since you said that pressures were equal] this should be a gm round r4 compressor not known for being robust when used as a rebuilt.

Mark DiSilvestro · #6 08-12-2010, 08:30 PM

Quote:

Originally Posted by david s poole

i didn't read your second post sorry.this would almost have to be an issue with the compressor valve plate[since you said that pressures were equal] this should be a gm round r4 compressor not known for being robust when used as a rebuilt.

I'm aware of the R4's failings. If I could have easily substituted something better I would have. The repeating pattern of gradual loss of cooling, followed by normal operation the next time I use the AC, seems to indicate some sort of variable valve failure, other than the ones in the compressor, and one part that wasn't replaced is the expansion valve. Is this problem unlikely to be caused by a faulty expansion valve?
I'm not ready to condemn the R4 just yet, at least not until I'm certain it isn't he expansion valve. But if I have to get another compressor, I'm not wasting any more money on R4s. I'll try to rig up something decent, like a Sanden or Nippondenso.

Happy Motoring, Mark

DANSMB · #7 08-12-2010, 08:55 PM

It's the evaporator freezing up, turn the temp up or turn it to vent for 5 minutes for it to thaw.

Dan

Mark DiSilvestro · #8 08-12-2010, 09:29 PM

Quote:

Originally Posted by DANSMB

It's the evaporator freezing up, turn the temp up or turn it to vent for 5 minutes for it to thaw.

Dan

As I said in one of my posts, I had this happen in a '72 W115 220, where the evaprator would ice up, accompanied by blockage of airflow. Then it would dump about a quart or more of watr if I shut off the car and let it defrost for awhile.
On my 240D, there's no reduction in airflow through the evaporator or ducts, and no dumping of massive quantities of melt-water after shutoff. Just a repeating pattern of gradual loss of pressure differential in the system, sufficient to reduce, then eliminate cooling, until after the system is turned of for about 30 minutes.

Happy Motoring, Mark