Aug 012008
 

Question:

2002 Pontiac Bonneville – manual a/c – compressor clutch does not engage – trying to troubleshoot – where should i start – refrigerant pressure is fine. Already Tried:
swapped a/c clutch relay w/another relay of same part number in fuse box.

Answer:

In the underhood fuse block, you have both the Clutch Relay, and the A/C Clutch Fuse. It is important to not only check the fuse…you also need to know that there is 12volts on both sides of the fuse. This can be checked with a 12 volt test lamp.

If the AC Clutch Fuse is bad, you may have a shorted AC Compressor Clutch Diode. Keep in mind, that the Powertrain Control Module controls the AC clutch engagement. The PCM looks at other inputs before it will allow clutch engagement. Therefore, you will need to verify that there are no trouble codes stored in the PCM that may cause the PCM not to allow AC clutch engagement. The AC refrigerant pressure sensor must also be functioning for compressor clutch engagement.

Below is a bit more information that is important.

The purpose of the air conditioning (A/C) system is to provide cool air and remove humidity from the interior of the vehicle. The A/C system is engaged when the A/C switch is pressed and the HVAC control module is in any mode except OFF. The A/C system will operate regardless of the temperature setting, as long as A/C line pressures are between 255-2968 kPa (35-430 psi) .

The HVAC control module receives power from the rear fuse block on the ignition 3 voltage circuit and battery positive voltage circuit. The ground circuits provide ground for the module.

When an A/C request is selected, the HVAC control module sends a class 2 message to the dash integration module (DIM). The signal is send through the class 2 serial data circuit. In order for the powertrain control module (PCM) to internally ground the A/C clutch relay control circuit, the DIM and PCM must communicate to each other over the class 2 serial data circuits. The PCM monitors A/C refrigerant line pressure and engine coolant temperature. This information is placed on the class 2 serial data circuit and is monitored by the DIM. The DIM looks at the A/C line pressure, engine coolant temperature and battery voltage along with the A/C request input from the HVAC control module. The DIM will request A/C operation from the PCM if these parameters are within normal operating limits and the HVAC control module is requesting A/C operation. The PCM turns ON the A/C compressor by providing a path to ground through the A/C clutch relay control circuit for the A/C compressor clutch relay. Power is provided to the A/C compressor clutch relay internally of the underhood fuse block. Once the relay closes its internal switch, power from the battery is provided to the A/C compressor clutch through the A/C compressor clutch supply voltage circuit. Whenever the compressor is turned OFF, the A/C compressor clutch diode prevents a voltage spike from burning up the compressor clutch coil. The ground circuit provides a pathway to ground for the compressor. The A/C clutch relay control circuit is grounded internally within the PCM. The A/C compressor clutch will be shut down under high electrical loads. The blower motor speeds will be affected first, then the compressor will be turned OFF, unless system is in defrost mode. The HVAC system will operate as if the OFF switch has been pressed. When this happens, the driver will still have the ability to override this mode.

The PCM will engage the A/C compressor clutch any time the engine speed is below 5000 RPM and the A/C is requested unless any of the following conditions exist:

  • Throttle angle is at 100 percent (WOT).
  • Vehicle launch, acceleration from a stop.
  • Idle quality
  • The A/C refrigerant pressure sensor is more than 2968 kPa (430 psi) or less than 255 kPa (35 psi) .
  • Engine speed is more than 5500 RPM .
  • Engine coolant temperature (ECT) is more than 121°C (250°F) .
  • Transmission shift
  • Engine torque load

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