A/C Clutch Control Circuit Diagnosis
A/C Clutch Control Circuit Diagnosis
Circuit Description
When air conditioning and blower tan are selected, and if the system has a sufficient refrigerant charge, a 12-volt signal is supplied to the A/C request input of the powertrain control module (PCM). The A/C request signal may be temporarily canceled during system operation by the electronic thermostat in the evaporator case. The electronic thermostat may intermittently remove the control circuit ground for the A/C thermostat relay to prevent the evaporator from forming ice. When the A/C request signal is received by the PCM, the PCM supplies a ground from the compressor clutch relay if the engine operating conditions are within acceptable ranges. With the A/C compressor relay energized, voltage is supplied to the compressor clutch coil.
The PCM will enable the compressor clutch to engage whenever A/C has been selected with the engine running, unless any of the following conditions are present:
- The throttle is greater than 90%.
- The ignition voltage is below 10.5 Volts.
- The engine speed is greater than 4500 RPM for 5 seconds or 5400 RPM.
- The engine coolant temperature (ECT) is greater than 125° C (257° F).
- The intake air temperature (IAT) is less than 5° C (41° F). The power steering pressure switch signals a high pressure condition.
Diagnostic Aids
To diagnose an intermittent fault, check for following conditions:
- Poor connection at the PCM-Inspect connections for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal-to-wire connection.
- Damaged harness-Inspect the wiring harness for damage. If the harness appears to Ok, observe the A/C clutch while moving connectors and wiring harnesses related to the A/C. A sudden clutch malfunction will indicate the source of the intermittent fault.
A/C Clutch Diagnosis
This chart should be used for diagnosing the electrical portion of the A/C compressor clutch circuit. A Tech 2 will be used in diagnosing the system. The Tech 2 has the ability to read the A/C request input to the PCM. The Tech 2 can display when the PCM has commanded the A/C clutch "ON." The Tech 2 should have the ability to override the A/C request signal and energize the A/C compressor relay.
Steps 1 - 3:
Steps 4 - 12:
Steps 13 - 24:
Steps 25 - 34:
Steps 35 - 44:
Steps 45 - 50:
Test Description
IMPORTANT: Do not engage the A/C compressor clutch with the engine running if an A/C mode is not selected at the A/C control switch.
The numbers below refer to the step numbers on the Diagnostic Chart:
3. This a test determine is the problem is with the refrigerant system. If the switch is open, A/C pressure gauges will be used to determine if the pressure switch is faulty or if the system is partially discharged or empty.
4. Although the normal complaint will be the A/C clutch failing to engage, it is possible for a short circuit to cause the clutch to run when A/C has not been selected. This step is a test for that condition.
7. There is an extremely low probability that both relays will fail at the same time, so the substitution process is one way to check the A/C Thermostat relay. Use a known good relay to do a substitution check.
9. The blower system furnishes a ground for the A/C control circuit, and it also shares a power source through the Heater and A/C Relay. The blower must be "ON" in order to test the A/C system.