This is an advanced how-to guide that describes how to adjust the Freon charge in the AC system for best performance. Low Side Temperature Sensor readings, BD28 in the on-board diagnostics, are used to fine tune the AC system. BCM data point BD28 is a measurement of the Freon temperature entering the evaporator in degrees Celsius.
Before using this guide you might want to check your AC sensors to make sure they are working properly and check the adjustment of the blend door rod. Both of those items can effect the ability of your AC systems to cool properly even if the Freon charge has been fine tuned as described below.
You MUST be close to having a correct refrigerant charge to be able to tune the system using the low side temperature sensor reading BD28. If the Freon charge is way too low, the refrigerant tends to phase change from a liquid to a gas even before it reaches the orifice. This is all due to pressure & temp in the closed system. In a properly charged system, the refrigerant is a liquid until the orifice (or expansion valve for some systems) at which time the restriction creates a low pressure situation which forces the refrigerant to rapidly expand and in so doing dropping in temperature.
A perfectly optimized charge level on our Reattas is the point where the compressor cycles the minimum (preferably no cycling) and the low temp sensor (BD28) hangs around 0 to -1 degrees Celsius. GM intended for the compressor to cycle from an economy perspective, but constant cycling is hard on the compressor and less than ideal for max cooling, especially when running R-134a..
If the system is constantly cycling the compressor, the average temp at the evaporator is running several degrees above ideal. The logic is to have a slightly overcharged, yes overcharged, system which does NOT allow the pressure on the low pressure side of the orifice to drop as low preventing the low temp (BD28) from reaching -2*C, allowing the compressor to remain engaged and resulting in a lower average evaporator temperature.
If at highway speeds, with the A/C set to MAX COOL, you find that the low temp (BD28) drops to -2, then cycles up to 9 then back to -2 and keeps repeating that indicates that the charge is just a TINY bit low for optimal operation, however it is NOT undercharged technically speaking. The easiest way to reach optimal is to charge it to the point where the compressor cycles, then add in a bit more and test drive. If the cycling is still occurring, add in a bit more and retest. You will likely overshoot the ideal point, but it is easier to release a tiny bit from the manifold than to add a similar amount from the can. It takes several attempts to find that sweet spot, but again, if at highway speeds the low temp reading (BD28) is 0 to -1, you are THERE! If you find that the temp is remaining above the 0 to -1 with NO compressor cycling, then you have a tad too MUCH refrigerant in the system. Remember that the goal is to be JUST enough overcharged to keep sufficient low PRESSURE on the suction side of the system to prevent the refrigerant from dropping to the -2 trip point that forces the compressor to cycle.
One last thing, as the interior of the car drops temperature and the Reatta begins to mix in a bit of warm air to maintain the climate control setting, the compressor WILL begin to cycle regardless. The goal is prevent compressor cycling while maintaining the evaporator between 0 and -1 until such time as the interior reaches the climate control set temp. In hot humid summer weather, the compressor may NEVER cycle when optimally charged.
FYI, the low temp sensor is IMMEDIATELY after the orifice, so you can get an good idea if the low temp sensor is reading close by using an infrared thermometer (gun type) focused on the joint between the tube after the orifice and the low temp sensor. The infrared reading will be a little higher as the actual temp sensor is IN the stream of the expanding refrigerant vs the outside of the tube where the infrared is getting its reading.
Hope this clarifies things a little.