Ac 123 - PeachParts Mercedes-Benz Forum

mdlwolf · #1 05-20-2008, 02:03 PM

Last year I had the AC repaired in my 83' 300D. The repair included a new compressor, drier, and expansion valve. It performed marginally and after several returns to the shop it still wasn't cooling properly. After the first return I asked the service manager what the problem was and he replied, "Who the hell charged this thing, it was way overcharged. Works fine now." Work fine it did not so I figured for $1000 I can overcharge the thing myself and bought a set of gauges and a vacuum pump and began reading about automotive AC on the internet. Here's what I did to get my AC working. The best I have been able to get is 42 out the vents on full fan speed and 36 at low fan speed.

When I checked the pressures, high idle of 1500 with aux fan jumped on, I was getting a low side pressure around 18-19 that would quickly drop when the rpm increased and compressor seemed to be cutting out at 17 psi. I was getting 46 out the vents. The system seemed undercharged so I added 134 slowly and was able to get the low side to not dip to the cut out point at rpm's between 1200-2000. But I now had a high side pressure of 300 that seemed to be cutting the compressor out when the high side would hit about 310. It was 100 degrees ambient in from of the condenser at this time and I was now getting 44 out the vents but the compressor was cutting out when I revved the engine due to what seemed to be excessive high side pressure. It seemed I either had to settle for too low of a low side or too high of a high side, but I was getting better vent temps with the higher high side. If I could only get the compressor to not cut out with the increased high side pressures. Then I remembered that my Ford truck has an adjustable pressure switch on the accumulator which changed the low side cut out. I wondered if the Merc had one but upon initial inspection it didn't seem so. I cleaned the pressure switch with parts cleaner and in the very center I found a small allen screw. One click counter-clockwise and the compressor was not cutting out. High side pressures were up to 340 at about 2000 rpm. I watched the gauges for awhile and noticed that something else was cycling the compressor. After some research I discovered the evaporator temp switch under the passenger kick panel. In the center of that there is a small flat head screw. By turning it ever so slightly in both directions I could here the compressor cycling. One slight bump clockwise and I was able to get the vent temps to 42 at high idle with full fan speed. Road testing I got cold so turned the fan to low and I got 36 out the vents.

That's the best I think I'll be able to get out of this vehicles AC. I've asked around about the high side pressures and had people tell me that the it's too high. I've had other people tell me that it's fine because once the vehicle is moving the high side will drop as the air cools the condenser. As a test I sprayed water on the condenser and watched the high side drop dramatically.

So what do the AC experts think? Can the high side run this high at idle and be o.k.?

carnut · #2 05-21-2008, 06:20 PM

Any A/C repair manual or A/C theory of operation manual will have a chart showing ambient temp, relative humidity and normal high pressure readings for 134a. A/C operation and pressure readings are relatively the same regardless of what car its on. Most manuals say to have a fan blowing onto the condenser while recharging to simulate car in motion. If 310 is too high, then the pressure switches in line or at the rear of the compressor will either pop out or electrically disconnect the compressor. So, because you mention nothing regarding these things happening, I would say its working normally. The minor adjustments you did, I believe, corrected the different pressures related to R12 VS 134A effeciency.

dieseldiehard · #3 05-21-2008, 06:32 PM

without a parallel flow condenser you'll not get anywhere close to the same temps that R12 gave. Even with the paralle lflow it might not be the same but should be OK. It sounds like you have studied and come up w/ a way to dial it in, at least its working reliably, right?

pmckechnie · #4 05-21-2008, 07:51 PM

The high side pressure seem a little high. The fact that spraying water on the condenser made the pressure drop makes me think you aren't moving enough air through it. Maybe a good external cleaning of the condenser and radiator will help. Remove the upper clips that hold the radiator in and lean it back and look between the cond and rad and make sure there in nothing between them. Spray the condenser and radiator with a strong cleaner (greased lighting, etc), let it soak for a few min and rinse with a water hose. This may sound silly but make sure the aux fan is running the correct way. It should blow toward the rear of the car. I saw a problem of a fan running backward and the a/c high pressure would run wild. It didn't seem to bother the engine but sure drove the A/C crazy. I use Freeze-12 in both my cars with high side running about 200 lbs on a 80 deg day. Low side around 28. at 1500 RPM. Booth cool good.
Stay Cool
Paul

mdlwolf · #5 05-22-2008, 05:29 PM

I just installed a new radiator so will recheck the pressures. Overall the system performs well enough. I would really like to get a good a/c book.

Whiskeydan · #6 05-22-2008, 07:10 PM

Useful info:

Air conditioning and refrigeration is ALL about HEAT and the intensity of it! Air Condition is just a means of moving heat from one place to another. The amount of heat concentrated within a certain volume with determine it's temperature. If you remove heat from that same certain volume, it's temperature will fall (become cooler). The Automotive Air conditioning system is a mechanical means of acomplishing this, using compression to reduce a certain volume to a smaller size. That smaller volume still contains the same amount of heat as before, but now the heat is more concentrated within that smaller volume, and therefore it's temperature is increased. The heat is letterally "squeezed" out of the refrigerant gas. Then the hot high pressure refrigerant gas (could be approximately 180F to 200F degrees)is moved to an area where it's heat can be transfered to the outside air (condenser), it's temperature is therefore reduced, and still being under high pressure, it's characteristics cause it to condense to a warm high pressure liquid (could be approximately 130F to 150F degrees). The high pressure liquid refrigerant then moves to the refrigerant control, which is a calibrated designed restriction, (orifice tube or TX valve) that divides the high pressure side from the low pressure side, acting like a small garden hose nozzel, it allows a small stream of refrigerant to spray into the low side. The liquid refrigerant, now being under low pressure boils and ABSORBS HEAT from in the evaporator (could be approximately 30F to 35F degrees). The low pressure refrigerant gas and mist flow from the evaporator out the evaporator outlet pipe (could be approximately 45F to 50F degrees)to the accumulator, which provides a place for the excess refrigerant mist to turn to gas, and also has a bag of dryer chemical in it, to trap any moisture that may get into the system. Then the low pressure refrigerant gas is sucked from the accumulator, through the suction line, back to the suction port of the compressor (could be approximately 55F to 65F degrees), where the process begins again. At each portion of the Air Conditioning circuit, the temperature and pressure will indicate how the refrigerant within it at that point is behaving, and has a direct bearing on the air conditioning performance. As an example on the High Side, at the condenser, if the refrigerant is behaving properly, the refrigerant gas will enter at a high temperature, and the refrigerant liquid will exit at a lower temperature (because it has transferred heat to the outside air). The difference is the temperature drop across the condenser (could be approximately 30F to 40F degree drop at idle or 40F to 60F degree drop at 1500 RPM, and the high pressure gauge reading will corrospond to a value on the temp/press chart for the lowest temperature on the high side, meaning the high pressure reading on a R134a system should be 212 PSI if the refigerant is exiting the condenser at 135F degrees. If you don't have enough temperature drop across the condenser and high side pressure is too high, look at the possibility of an overcharge, air in the system, condenser, air flow and fans for a problem.
As an example on the Low Side, at the evaporatorer, if the refrigerant is behaving properly, the refrigerant liquid will enter and boil at a low temperature (could be approximately 35F degrees), and the refrigerant gas and mist will exit at the evaporator outlet pipe at a higher temperature (could be approximatly 45F to 50F degrees, because it has picked up heat from within the vehicle). Then the refrigerant gas and mist pick up more heat and are completely vaporized back unto a gas as they are sucked through the accumulator and through the suction line back to the compressor suction port. The temperature at the compressor suction port should be higher (approximately 55F to 65F degrees, because the refigerant gas/mist has picked up more heat and completely vaporized since leaving the evaporator). There should be approximately a 10F to 20F degree diffence between the evaporator outlet pipe and the line at the compressor suction port at idle, or 2F to 10F degree drop at 1500 RPM, and the low pressure gauge reading will corrospond to a value on the temp/press chart, meaning the high pressure reading on and R134a system should be 30 PSI if the refigerant is boiling in the evaporator at 35F degrees. If you don't have enough or any temperature drop from the evaporator outlet pipe to the line at the compressor suction port and the low pressure reading is high, look at the possibility of an overcharge. The correct charge will cause the line at the compressor suction port to be slightly warmer than the evaporator outlet, ensuring that liquid refrigerant will not be sucked back into the compressor. An overcharge will cause the system to not cool as well, pressures will be higher, and liquid refrigerant being sucked back into the compressor, shortening it's life. If you have very low pressure on the low side, the evaporator outlet pipe is much warmer than it should be, check for a low charge or restriction at the orifice tube or TX valve.

A good way to check all the line temps is with an infrared thermometer. It's the best thing since sliced bread, and I would'nt be caught dead without mine!

Here's some info and guidelines for using it:

Using the Infrared Thermometer:
If you have an infrared Thermometer, you can easily measure the temps of the lines and get the charge just right. It works best on Dark Surfaces, so before starting, neatly spray paint the surfaces you intend to measure with some Flat Black Spray Paint and let dry. Wet surfaces should be wiped dry before measuring. Hold the thermometer close to and pointed at the object to wish to measure temperature. Since there is so much hot metal in the engine compartment and hot air blowing in the engine compartment, these will affect the thermometer to make reading inaccurate. To minimize these effects, take your readings quickly to keep the thermometer from getting hot, allow time for it to cool between each use, blow cool air on it between each use. Use the thermometer only when the thermometer is cool and stable. You must also use a heat shield around the component you are measuring, to keep stray heat away from the thermometer. Using a piece of light colored cardboard is a good heat shield against engine heat, exhaust manifold heat, and radiator heat. Place the heat shield in place quickly and measure the temperature quickly, then remove the thermometer and heat shield quickly, so that the heat shield itself does not become hot. Allow the heat shield to cool between uses.

These are APPROXIMATE TEMPERATURE numbers and will vary according to application, make and model, but are still good guidelines:
FOR AUTO RADIATORS:
At IDLE - Inlet Tank temp should range from 155 to 185 degrees, depending on thermostat and cooling system condition.
Outlet Tank temp should range from 130 to 170 degrees.
IMPORTANT-the DIFFERENCE between the inlet tank temp and outlet tank temp is the most important factor to determine how well the radiator is doing it’s job, which is TEMPERATURE DIFFERENTIAL ACROSS THE CORE.
With the A/C OFF, the temp diff should range from 20 to 35 degrees.
With the A/C ON, the temp diff should range from 10 to 25 degrees.
If there are problems with a too low temp diff and overheating, check temps across the core for evenness, check air flow through radiator, check fan & fan clutch and check coolant flow through core.

FOR AUTO AIR CONDITIONING R134a SYSTEMS:
At IDLE - Condenser inlet temp 170 to 180, Condenser Outlet temp 135 to 150 (usually a 30 to 40 degree drop in temperature across the Condenser), Evaporator Outlet temp 45 to 55, Compressor Suction temp 60 to 70 (usually a 15 to 20 degree drop in temperature from the evaporator outlet to compressor suction port). With the engine IDLING, you want to adjust the charge so the line at the Compressor suction port is about 10 to 20 degrees WARMER than the Evaporator Outlet pipe, and with the engine running at 1500 to 1800 rpm, you want the line at the Compressor suction port to be about 2 to 10 degrees WARMER than the Evaporator Outlet pipe.

At 1500 RPM - Condenser inlet temp 190 to 200, Condenser Outlet temp 135 to 155 (usually a 40 to 60 degree drop in temperature across the Condenser), Evaporator Outlet temp 45 to 55, Compressor Suction temp 55 to 60 (usually a 7 to 12 degree drop in temperature from the evaporator outlet to compressor suction port).

At 90 degrees ambient, for R134a systems, look for about 200- 220 Pressures on the high side and about 25-30 Pressures on the low side at idle, and at 1,500 RPM look for about 220- 230 Pressures on the high side and about 20-27 Pressures on the low side.