Crazy A/C - PeachParts Mercedes-Benz Forum

Sopicki · #1 07-18-2007, 09:09 AM

Hi folks,

I know that there are a lot of threads on A/C problems, but I just can't seem to find one that matches my situation and I'm getting frustrated. The a/c works, compressor comes on and at times I get air from my vents as low as 50 degrees. However after 20 min or so it starts to warm up and eventually the vent air goes to around 85 degrees. There is air being drawn into the temp. sensor vent. The a/c has been converted to R134 also. This winter it demonstrated a similar problem, but it would go colder instead of warmer.

Any thoughts??

Help!

Thanks,

Bob Kopicki

brewtoo · #2 07-18-2007, 09:30 AM

Perhaps the evaporator is freezing. Is the compressor still running after the temp has risen to 85?

Sopicki · #3 07-18-2007, 10:45 AM

It never occured to me that the evaporator could be freezing since the air never really got that cold. I'm about to take the car out so I'll check if it is running when it goes hot.

I did do some reading in the MB climate control manual and discovered that my auxillary fan should be running any time the A/C is on. There is a high pressure switch (S32) that senses 20bar at the dryer and is suppossed to turn the aux fan on. Well, my aux fan has only ever turned on when the engine gets hot. I still don't know much about this system, but I suspect that the opposite may be happening, the R134 is getting too hot and something is shutting the system down to protect it. Just a hunch.

Anyway, I jumpered the wires at S32 and now the fan runs anytime the ignition is in run. I ordered a replacement switch - but the parts supplier called it a temp switch??? I'm sure it's the right switch since there are only 2 at the dryer and the other one is completely different.

Thanks again,

Bob

zcc · #4 07-18-2007, 11:15 AM

How about the R134a level?

saumil · #5 07-18-2007, 02:19 PM

Do you get sufficient quantity of air thru the vents ? The fact that the problem also occurs with heat tells me there is a clog in the air flow somewhere. What car is this ? Does this car have a cabin air filter ?

Sopicki · #6 07-19-2007, 12:13 AM

R134 level is up to full, but I have had to top off a couple times this summer. I put dye in it last month and found a leak at the top of my expansion valve (as a side note I've never seen an expansion valve leak - it's not leaking from a seal but from the seam between the diaphram and housing of the valve!). The valve is also on order.

The car is a 1987 300e. There is no cabin filter and there does seem to be sufficient airflow from the vents.

I had a very similar problem with my '85 XJ-6. Its cabin temp sensor is right under the dash. It was drawing air from the vents into the temp sensor, so when it was pumping cold air into the vents, it would think the cabin was cold and start pumping heat. It would yo yo continuously like this and took me a couple years to figure out what was going on. I'm wondering exactly where the cabin temp sensor is in my MB. Is the sensor in the ceiling where the little air grill is, or is the actual sensor located behind the glovebox where the fan is? If it's behind the glovebox then there are probably lots of places where I could be leaking air from places other than the cabin. On the other hand, if the sensor is located in the ceiling, then I've got bigger problems.

Thanks all for your help.

Bob Kopicki

brewtoo · #7 07-19-2007, 09:31 AM

I believe the part behind the glovebox is only the fan. I doubt they would measure the air temp after it passed thru that long black hose.

gmercoleza · #8 07-19-2007, 09:49 AM

Quote:

Originally Posted by Sopicki

...I get air from my vents as low as 50 degrees. However after 20 min or so it starts to warm up and eventually the vent air goes to around 85 degrees.

When this occurs, is the compressor still engaged? Is the suction hose cold and sweaty?

Quote:

Originally Posted by Sopicki

...This winter it demonstrated a similar problem, but it would go colder instead of warmer.

This tends to make me believe it is either A) a monovalve or B) vacuum actuator issue - or both.

marvinstockman · #9 07-19-2007, 08:16 PM

Check the air gap on the clutch. If it's too large, it won't pull in when the system gets hot. It recently happened to me.

saumil · #10 07-19-2007, 11:27 PM

Here is the manual page.

Sopicki · #11 07-21-2007, 11:41 AM

Hi all,
Thanks for your input. While checking out the A/C system I discovered in addition to my expansion valve leaking, I found dye coming out of the seams of my compressor and high pressure hose to the condenser. So now the car is inop sitting in my garage waiting on a new compressor, hoses, expansion valve, dryer and a compete set of o-rings. $950 in just parts.

After this work is done I'll get back to the forum and then we'll work through whatever issues remain.

Thanks,

Bob Kopicki