If you've ever wondered what pressures are "normal" for your W124 with factory R-134a air conditioning, here is a view of the gauges during normal operation. This was taken immediately after an empty system was vacuumed for 30+ minutes and recharged to factory spec. Engine was at 650rpm idle, ambient temp was 85°F, and humidity was around 25-30%. In higher ambient temps and/or higher humidity, both low side and high side pressures will be proportionally higher. Vent temps with the fan on max were roughly 42-44°F.
The compressor cycles on & off based on the evaporator temperature sensor. When the compressor is OFF, the low-side pressure increases and high-side pressure decreases. When the compressor is ON, the low-side pressure decreases and high-side pressure increases. If you listen closely, you can hear the twin electric fans turn on & off based on refrigerant pressure.
The red switch located at the receive/dryer should turn the fans on low speed at 16 bar (232 psi), and turn the fans off at 12 bar (174 psi). Either the switch on my car is slightly off, or the gauges are slightly off, or a combination of both. The fans are turning on slightly early but turning off a bit late, which is fine. At higher ambient temps the fans may run continuously.
0:05 - Fans turn off @ 140 psi / 10 bar
0:19 - Fans turn on @ 225psi / 15.5 bar
0:38 - Fans turn off @ 140 psi / 10 bar
0:51 - Fans turn on @ 225psi / 15.5 bar
1:02 - Fans turn off @ 140 psi / 10 bar
The compressor cycles on & off based on the evaporator temperature sensor. When the compressor is OFF, the low-side pressure increases and high-side pressure decreases. When the compressor is ON, the low-side pressure decreases and high-side pressure increases. If you listen closely, you can hear the twin electric fans turn on & off based on refrigerant pressure.
The red switch located at the receive/dryer should turn the fans on low speed at 16 bar (232 psi), and turn the fans off at 12 bar (174 psi). Either the switch on my car is slightly off, or the gauges are slightly off, or a combination of both. The fans are turning on slightly early but turning off a bit late, which is fine. At higher ambient temps the fans may run continuously.
0:05 - Fans turn off @ 140 psi / 10 bar
0:19 - Fans turn on @ 225psi / 15.5 bar
0:38 - Fans turn off @ 140 psi / 10 bar
0:51 - Fans turn on @ 225psi / 15.5 bar
1:02 - Fans turn off @ 140 psi / 10 bar
