Refer to Engine Control Schematic
A/C Controls
.
A/C Operation Description
The engine control module (ECM) controls the A/C clutch relay. ECM A/C operation is controlled by two input circuits, A/C request and A/C load, and one output circuit, the A/C compressor relay control.
The heater and A/C control sends an A/C request signal, approximately battery voltage, whenever the operator depresses the A/C selector switch on the control panel. When the ECM receives this signal, the ECM will turn the A/C clutch relay ON unless one of the following conditions is observed:
| • | The engine is not running. |
| • | The ignition voltage is less than 9 volts. |
| • | A wide open throttle (WOT) condition is present. |
The ECM will NOT detect the A/C request signal if the A/C system pressure is too high or too low. The A/C request circuit is in series with a portion of the A/C compressor refrigerant pressure switch. The pressure activated switch contacts will open if the A/C pressure is not correct for proper A/C operation. Refer to the electrical schematic diagrams for the specific operating pressures. The heater and A/C control monitors the A/C request circuit for shorts. A shorted condition will cause a heating, ventilation and air conditioning (HVAC) diagnostic trouble code (DTC) to set.
The ECM also monitors the A/C load switch circuit. This circuit is designed to increase the engine speed by about 50 RPM whenever a high temperature and pressure condition causes the A/C load switch contacts to close.
The ECM controls clutch operation by supplying a ground path for the A/C compressor relay. The A/C compressor relay control circuit is monitored for opens and shorts. An open or shorted condition will cause an ECM DTC to set. ECM control of the A/C clutch relay can be tested using a scan tool.
Circuit Description
The A/C compressor relay switch contact is supplied battery power from fuse 33. When the ECM commands the A/C clutch relay ON, the relay switch contacts close allowing current to flow to the A/C compressor clutch coil and also to the auxiliary coolant fan relay, causing the auxiliary coolant fan to operate. The current flows through the A/C compressor clutch coil to the secondary cooling fan temperature switch. The current continues through the secondary cooling fan temperature switch to ground. The temperature activated contacts of the secondary cooling fan temperature switch will open, turning the A/C clutch OFF, if coolant temperature is too high for proper A/C system operation.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
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You must perform the A/C Request Circuit Diagnosis prior to diagnosing the A/C compressor clutch circuit. This procedure contains important steps on checking the systems for diagnostic trouble codes.
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The engine coolant must be below the specified temperature in order to properly test the HVAC compressor clutch circuit. When the temperature threshold is exceeded, the secondary cooling fan temperature switch contacts will open the ground path to the A/C clutch coil.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
Did you perform the A/C Request Circuit Diagnosis? | -- | Go to Step 2 | ||
Important: Engine coolant must be below the temperature specified before proceeding. Does the A/C compressor clutch engage and disengage when commanded ON and OFF? | 100°C (212°F) | Go to Step 3 | Go to Step 4 | |
3 | ECM operation of the A/C compressor clutch is OK at this time. Is a concern about A/C operation still present? | -- | Go to Diagnostic Starting Point in HVAC | System OK |
4 | Does the A/C compressor clutch remain engaged all the time? | -- | Go to Step 10 | Go to Step 5 |
5 |
Does the auxiliary coolant fan operate when the A/C relay is commanded ON? | -- | Go to Step 8 | Go to Step 6 |
6 |
Does the test lamp illuminate? | -- | Go to Step 7 | Go to Step 12 |
7 |
Does the A/C compressor clutch engage while the relay terminals are jumpered? | -- | Go to Step 14 | Go to Step 13 |
8 |
Does the test lamp illuminate? | -- | Go to Step 9 | Go to Step 13 |
9 |
Does the test lamp illuminate? | -- | Go to Step 18 | Go to Step 11 |
10 | Remove the A/C relay. Is the A/C compressor clutch still engaged? | -- | Go to Step 15 | Go to Step 14 |
11 |
Was a problem found and repaired? | -- | Go to Step 19 | Go to Step 16 |
12 |
Is the action complete? | -- | Go to Step 19 | -- |
13 | Repair the open A/C compressor clutch power circuit between the relay and the A/C compressor clutch coil connector. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 19 | -- |
14 | Replace the A/C compressor relay. Is the action complete? | -- | Go to Step 19 | -- |
15 | Repair the A/C compressor clutch power circuit shorted to B+. Refer to Wiring Repairs in Wiring Systems. Is the action complete? | -- | Go to Step 19 | -- |
16 |
Was a problem found and repaired? | -- | Go to Step 19 | Go to Step 17 |
17 | Replace the secondary cooling fan temperature switch. Refer to Engine Cooling Fan Control Switch Replacement in Engine Cooling. Is the action complete? | -- | Go to Step 19 | -- |
18 | Replace the compressor clutch coil. Refer to Air Conditioning Clutch Coil Removal and Air Conditioning Clutch Coil Installation in Heating, Ventilation, and Air Conditioning. Did you complete the replacement? | -- | Go to Step 19 | -- |
19 |
Does the A/C clutch engage? | -- | System OK |
