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Car Forum / Mercedes-Benz Cars / September 2004A/C problem on 300D
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 ambient. 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 ideas? 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. > 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 [quoted text clipped - 12 lines] > problem since it shuts off immediately when the key is turned off. Any > ideas? 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 system? > yep - it's vacuum - Your low side should be down to 30. You have 3 > choices - 1 - Take a pair of vice grips [quoted text clipped - 26 lines] > > problem since it shuts off immediately when the key is turned off. Any > > ideas? Did you eventually solve the problem? > This sounds like a plausable explaination. Does the vacuum actuator > malfunction in a fashion that allows hot water to circulate through the [quoted text clipped - 36 lines] > > > problem since it shuts off immediately when the key is turned off. Any > > > ideas? Not an expert by any stretch just wondering if your Climate Control is set to "min" when the temperature change occurs? Yes, always at lowest temp setting of "min". > Not an expert by any stretch just wondering if your Climate Control is set to > "min" when the temperature change occurs? 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 psi. > 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 [quoted text clipped - 14 lines] > lower than the set 34 PSI... then you probably got a expansion valve > problem... not metering your freon flow correctly. 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 side. 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. > 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 [quoted text clipped - 14 lines] > out... yellow round looks sort of like a trumpet and grey at the two output > side. |
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