AC compressor - PeachParts Mercedes-Benz Forum

mhin1956 · #1 07-14-2016, 12:44 PM

Ok, this is a new one for me. My AC has been "adequate" on the 300sdl for years, until my compressor went. New compressor installed and working almost too good. Blower speed works as it should with temp wheel, but temperature of air out of vents is COLD. I thought it used to be somewhat tempered? Also have noticed compressor does not cut off except when in econ mode. Is is supposed to run all the time, or cycle on/off as needed.

Thanks for any input!
Mark
87 300sdl 399k and climbing- all orig!

leathermang · #2 07-14-2016, 01:20 PM

Probably not going to get much sympathy around here for ' too cold ' AC complaint ... you are in the one percent I suspect...
If you peg your temperature choosing knob or wheel... then most of our units provide about %20 fresh air mixed with the cold ac air...
As you move away from the pegged blue line spot... then that amount increases....
Most people need the humidity control which the cold air provides by dropping the moisture out of the bottom of the evaporator core..
There is a slight possibility that your TXvalve is adjusted improperly.... IF you find that the evaporator ices up you might check that.. but as the last thing for sure...
Make sure the pigtail from the Txvalve is against the part of the evaporator it should be...and that until there it is insulated.. I think it came with some tar looking stuff covering it...
You might have some kind of variable compressor which would cause it to look like it is always on.. I do not have FSM for your car...

BillGrissom · #3 07-14-2016, 04:17 PM

In my 1984 & 85 300D, I am pretty sure the EC button is the only one where the AC compressor will not run, but check your manual. In the winter, AC is needed for defrost (as most newish cars).

Hit Man X · #4 07-14-2016, 10:35 PM

Quote:

Originally Posted by BillGrissom

In my 1984 & 85 300D, I am pretty sure the EC button is the only one where the AC compressor will not run, but check your manual. In the winter, AC is needed for defrost (as most newish cars).

This is exactly how the '86-91 W126s operate also.

It is also the middle of the summer, chances are the compressor will not cycle at this point in time.

leathermang · #5 07-14-2016, 10:43 PM

Quote:

Originally Posted by Hit Man X

....It is also the middle of the summer, chances are the compressor will not cycle at this point in time.

Particularly if the condensor AND the evaporator are not 100% clean... and few evaporators have been cleaned in decades .... IF EVER...

SD Blue · #6 07-15-2016, 02:15 PM

My original thought was, is he bragging or complaining? ;-)

Is the temp wheel not having any effect? It may be a problem with the Pushbutton Controller.

BillGrissom · #7 07-18-2016, 03:55 PM

Probably the push-button box and/or TemperatureRegular box (above glove-box liner). In my 1985, if I set the thumbwheel at anything other than the "min" click, it currently blows hot (didn't used to). I recall the click position forces 12 V to the mono-valve to keep hot coolant flow off. I have spares for both boxes, but happy w/ max cooling now since 100 F outside lately. I should stop waving at all the poor souls driving w/ their windows down, many in fairly new vehicles.

gskurka · #8 07-18-2016, 09:30 PM

What did you use as a replacement for the compressor?

mhin1956 · #9 07-19-2016, 02:54 PM

Ok, compressor is rebuilt by reputable shop; original brand and spec. Thumbwheel does work- controls blower speed well, but can't tell that the actual temperature is "tempered" as I thought it used to be. It still feels very cool (not bragging!!) but airflow reduces. Just don't want to lose a compressor if it's not supposed to run all the time. So far have been in traffic showing 112 on temp outside, stuck in place from traffic. Temp on engine climbs just under 100, never higher, and a/c from vents regulates airflow, but at low blower speed seems to still be quite cool. Maybe my car is related to "Christine" and it just keeps getting better with age?

Thanks for the input!

Demothen · #10 07-19-2016, 02:57 PM

I think there is a temperature sensor under the dashboard near the evaporator and expansion valve that might be disconnected or failing. I think this is the feedback necessary to regulate the compressor cycling. There's also a low pressure switch at the receiver drier, but that's more oriented towards keeping the compressor from running when there's inadequate charge, so that is less likely to be your problem.

No guarantees, but it's a place to start looking.