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How to Track Down Cause of Compressor's Short Run TimePosted 7/23/2007
By Ken Hughes
Recently we received a call regarding a 1999 Cadillac Deville with an air conditioning (A/C) problem. The A/C compressor would come on for about 20 seconds and then disengage for nearly two minutes before cycling back on. We checked the A/C refrigerant charge and it was within factory specification.
Using the vehicle's onboard diagnostics, as well as two different brands of scan tools, we discovered that no codes were stored in either the climate control or the powertrain control module (PCM).
This A/C system uses the PCM to command the A/C compressor on or off, based on sensor and switch inputs to the HVAC programmer and the PCM. In this case, the A/C compressor was being turned off when the high side pressure was at 200 psi and the low side pressure was at 50 psi, starting from a rest pressure of 78 psi. The scan tool showed that when the A/C compressor was engaged, the A/C request and relay command were both on. When the A/C compressor shut off, the A/C relay command on the scan tool switched to off and the A/C request remained on. With no codes set in the HVAC or the PCM, and the A/C relay being commanded off, it would indicate that the PCM was turning the A/C compressor off most likely because of a bad sensor or switch input. Without the A/C programmer or PCM seeing an extreme sensor failure, no diagnostic trouble codes would be flagged.
The following inputs are used by the HVAC programmer for control of the A/C compressor:
The 1999 Deville's onboard diagnostic display has limited heating, ventilation, air conditioning (HVAC) sensor information, and a scan tool is needed to display this data. Not all scan tools will display the necessary information.
The scan tool used on this vehicle supplied the following information: Vehicle speed 0 with the vehicle stationary and the problem occurring. The ambient air temperature reading was 65 degrees Fahrenheit (the A/C compressor will not be commanded on if the ambient temperature is below approximately 40 degrees Fahrenheit). However, the scan tool would not supply any information on the high and low side refrigerant pressure switches or the high and low side temperature sensors.
The high and low side refrigerant pressure switches are simple on/off switches. Looking at the wiring diagram made it easy to determine if either switch was faulty. The low-pressure switch receives battery voltage from the cruise fuse on a pink wire at pin A. This switch is closed at normal system pressures (opens below 10 psi) passing voltage through to the high-pressure switch on a dark green wire. The high-pressure switch is normally closed (opens above 430 psi), passing voltage on a dark blue wire to the coil side of the A/C relay and to the HVAC programmer pin D15. The voltages at both the A/C relay and the HVAC programmer were battery voltage when the A/C compressor cycled off, indicating both switches were OK.
The high and low side temperature sensors are both two-wire sensors with five volts and ground applied to each sensor from the HVAC programmer. The resistances of the sensors change as the temperature changes, and this will vary the voltage of the circuit. The programmer will command the A/C compressor off if the high side temperature is too high or the low side temperature is too low. With no scan data available on these sensors, the voltage on the gray wire for the low side temperature sensor was checked next with the key on and the sensor unplugged. Five volts were measured, and a good ground was found on the black wire. The dark blue wire for the high-side temperature sensor showed the same five volts, and it also had a good ground on the black wire. Back probing the sensors with the connectors plugged in showed the gray wire had 2.52 volts and the dark blue wire had 4.01 volts. Since the Cadillac service information relies on scan tool data to diagnose sensor problems, and many times this scan data is unavailable, Identifix has composed the chart above that converts pressure to voltage for both the high and low side temperature sensors.
With the low side temperature sensor showing a value of 2.52 volts (indicating approximately 40 psi), when the actual gauge pressure was 78 psi, the skewed sensor would cause the A/C compressor to be turned off at a higher than normal temperature, causing poor cooling. The normal turn-on voltage for the low side temperature sensor is 2.4 volts and the turn-off voltage is 3.5 volts. The fix was to replace the low side temperature sensor.
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