Here's one for the A/C experts. I've converted my '81 300D over to R134. Did
a switch of the drier, drew system down to about 30", etc. Everything seemed
to function fine for several months. However, now when the engine gets past
idle, the cold air from the vents begin turning warm until it seems to blow
I slow the car down to idle and it begins blowing cold again. I've check to
make certain the compressor is running at all times (it does) and the low
pressure side (pipe running across top/front of engine) remains cold. Low
pressure side is at a rather steady 40 psi.
I changed the climate control module thinking perhaps the problem was there
but it has the same conditions. Vacuum is good but I have not checked to
make certain all actuators are without leaks but that doesn't seem to be a
problem since it shuts off immediately when the key is turned off. Any
yep - it's vacuum - Your low side should be down to 30. You have 3
choices - 1 - Take a pair of vice grips
and clam your heater hose where it will not mix - will bet your defroster
vent never cuts off. Chase the vacuum leak and possibly replace the blue and
white diaphragm in back of the evaporator in the dash (the one you can't get
to) 3. Take it over to Jose Duran at Duran Euro Motors on Shady Trail 956
221-1779. Glad that is not mine - I have a 300SD that
has the same ailment.
This sounds like a plausable explaination. Does the vacuum actuator
malfunction in a fashion that allows hot water to circulate through the
heater core or does it do something else to allow hot air into the vent
Hold on! I am lost on what you are saying... When you converted, you drew a
vacuum of 30 PSI... then you filled up the system with R134a... letting the
vacuum suck the R134a into the system... right?
There must be absolutely no air in the system... very critical.
Then with the windows closed and car running... climate control to the
max... idle for 5 minutes or so... what is the pressure reading? It should
be 34 PSI... not 40PSI (that's a bit too high).
If it is 40 PSI, then shut off the engine and let some pressure out.
Again... at idle or any engine RPM... the low side pressure should be
ideally at 34 PSI... if you go lower then your evaporator will freeze up.
If when your engine is at higher RPM... and your low side pressure is way
lower than the set 34 PSI... then you probably got a expansion valve
problem... not metering your freon flow correctly.
No, I drew a vacuum of 30 inches of mercury, not PSI. I then filled the
system with R134A. To reinterate my first post, the system worked fine for
several months, then the current problem started. As a result my assumption
is that something has "given up the ghost" somewhere in the system. As
stated before the low side is a bit high at 40 psi but that is not a real
concern for me since I usually pump my systems a bit higher than normal,
always without a problem. I find that the R134 does not have the cooling
capacity of the old R12, hence the hedge on coolant in the system. I have
four vehicles and have R134 in all but one (my trusty '81 240D) which still
has R12 and cools like crazy.
Again, my problem is that everything works fine at idle, cools great (for
R134A, that is). However, when engine speed is increased, whether on the
road or sitting still, the system begins emitting air that is not nearly
cool enough. Then, when I return to idle, it begins cooling again! The
compressor runs the entire time with the temp switch turned to "min" and the
A/C pipe running across the top of the engine (in which the fill valve is
located) is always very cold. Low side pressure always remains at about 40
So you are saying the low side pressure remains at 40 PSI regardless of
engine RPM? When you measure at any RPM... please hold the RPM for a minute
or so and take the reading. Refrigerant does not work in the principle the
more freon you have the better it cools... it actually works the other way
around. R134A and R12 is not that far off from each other in cooling
capacity although they do need bigger condenser to do the job...
Release some freon... reduce it to 30 to 34 PSI... The lower the better...
but I'd keep it in the range of 30 to 34 PSI.
There is something wrong with your system... what is the high side reading?
If you can take a reading...
Mmm... there is another possibility... that when you accelerate and your
fresh air entrance flaps opens up letting too much hot air inside... vacuum
canister may be shot... and change that yellow checkvalve... one in... two
out... yellow round looks sort of like a trumpet and grey at the two output
That was a thought I had as well. I'm going to release some 134 down to say
32 and see what happens. If I get the same results I'm going to have to
begin looking for vacuum a vacuum actuator problem. If and/or when I reach
that point I'll probably be back in touch. Thanks Tiger. You're a great
asset to this newsgroup.
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