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"Air Conditioning Operator" Switch connections | |
The "Air Conditioning Operator" Switch supplies one switch to +battery input to the "Transmission/Chassis ECM". The ECM does not monitor the switch circuits that have one input for diagnostics.
During normal operation, the ECM internal pull down voltage will hold the switch input circuit low until the +battery signal from the "HVAC Fan Selector" switch is passed through the switch contacts. The ECM detects the high +battery signal in the circuit and determines that the switch contacts are closed.
Note: For a more detailed explanation of the operation of the air conditioning system, refer to Systems Operation, "Features" section of this manual.
If the switch circuit is OPEN or the switch contacts have failed in the OFF state, the ECM will not detect that the switch contacts are closed and will not activate the air conditioning system.
If the input circuit of the switch is shorted to frame ground or to another ground source, the "HVAC" fuse will open when the switch is moved to the AC ON position.
If the contacts of the switch fail in the AC ON state, the ECM will not detect the low in the circuit to indicate that the contacts are open. When this occurs, the air condition system will be active at all times.
Some conditions that can cause the AC operator switch input circuit to fail are listed.
- The HVAC fuse is open due to a short to ground in the switch input circuit or other factors.
- The switch has failed.
- The input circuit of the switch is open or the switch has a poor connection in the machine harness.
- The input circuit of the switch is shorted to another +battery source in the machine harness.
- The ECM has failed. This is not likely.
When a switch circuit is suspected of not operating correctly, perform the following troubleshooting procedure for the switch circuit.
Test Step 1. CHECK THE SWITCH
- Turn the disconnect switch and the key start switch to the OFF position.
- Disconnect the machine harness connector from the switch.
- At the "AC Operator Switch", use a multimeter to measure the resistance between the switch contact 2 and switch contact 3. Measure the resistance of the switch in the OFF state and in the ON state several times each.
Expected Result:
When the switch is in the AC OFF contact state, the resistance measurement is greater than 5000 ohms. When the switch is in the AC ON contact state, the resistance measurement is less than 5 ohms.
Results:
- OK - The switch operation checks correct in both of the switch states. The switch is working correctly. Proceed to Test Step 2.
- NOT OK - The switch operation is not correct. The resistance measurement of the switch is not changing when the state of the switch is changed. The switch has failed.
Repair: Replace the switch. After replacing the switch, verify that the operation of the new switch is correct before operating the machine.
STOP
Test Step 2. CHECK THE SWITCH INPUT CIRCUIT FOR AN OPEN
The key start switch and the disconnect switch remain in the OFF position.
- At the machine harness connector for the switch, connect one end of a jumper wire to contact 3 (wire 567-WH(White)). Connect the other end of the jumper wire to a good ground contact at the frame.
- Disconnect the machine harness connector J1 from the ECM.
- At the machine harness connector for the ECM, measure the resistance between the input contact of the switch at contact J1-24 (wire 567-WH(White)) and a good ground contact on the frame.
Expected Result:
The resistance measurement is less than 5.0 ohms.
Results:
- OK - The resistance measurement is less than 5.0 ohms. The input circuit of the switch in the machine harness is not open. Proceed to Test Step 3.
- NOT OK - The resistance measurement is greater than 5.0 ohms. There is an open or a bad connection in the input circuit of the switch in the machine harness.
Repair: A resistance measurement that is much greater than 5.0 ohms but less than 5000 ohms could indicate that one of the connection points in the circuit is not open but is not a good connection.
Refer to the complete Electrical System Schematic of the machine. Check all of the connectors that are in the input circuit of the switch. Ensure that all connections are clean, that all connections are secure and that all connections are in good condition. Repair the machine harness or replace the machine harness.
STOP
Test Step 3. CHECK FOR A SHORT TO ANOTHER VOLTAGE SOURCE
The key start switch and the disconnect switch remain in the OFF position. The machine harness connector J1 remains disconnected from the ECM.
- Disconnect the machine harness connector J2 from the ECM.
- At the machine harness ECM connector J1, measure the resistance from input contact of the switch at J1-24 (wire 567-WH(White)) to all of the other J1 and J2 contacts that are used for the ECM.
- While observing the resistance measurement, wiggle the wires and move the machine harness around at the ECM connectors and at the switch.
Expected Result:
Each resistance measurement is greater than 5000 ohms at all times during the manipulation of the machine harness wires and connectors.
Results:
- OK - Each resistance measurement is greater than 5000 ohms. The machine harness is not indicating a short to another voltage source. Proceed to Test Step 4.
- NOT OK - Each resistance measurement is not greater than 5000 ohms. There is a short in the machine harness. The short is between the wire for the input circuit of the switch (wire 567-WH(White)) and the circuit with the low resistance measurement.
Repair: Refer to the complete Electrical System Schematic of the machine. Examine all of the connectors in the defective circuit for points in the machine harness where a short could occur.
For information on testing for short circuits and finding short circuits, refer to Testing and Adjusting, "Wiring Harness (Short Circuit) - Test". Repair the machine harness or replace the machine harness.
STOP
Test Step 4. CHECK THE OPERATION OF THE SWITCH
- Inspect the harness connectors. Clean the contacts of all of the harness connectors.
- Reconnect all harness connectors. Make sure that the connectors are fully seated. Observe that the clips for each connector are completely fastened.
- Turn the disconnect switch and the key start switch to the ON position.
- Check the operation of the switch.
Expected Result:
The switch is operating properly.
Results:
- OK - The switch is operating properly. The operation of the switch is correct at this time.
Repair: The initial problem was probably caused by a poor electrical connection or a short at one of the harness connectors that was disconnected and reconnected. Resume normal machine operation.
STOP
- NOT OK - The circuit for the switch is still not operating correctly. The problem has not been corrected.
Repair: Check the input circuit of the switch again. Use the Caterpillar Electronic Technician (ET) service tool, if available, in order to perform a wiggle test on the machine wiring harness. The wiggle test can detect momentary or intermittent shorts or opens in the machine wiring harness.
If the cause of the problem can not be found, the ECM may have failed. Verify that all possible causes of the problem have been investigated before replacement of the ECM.
Prior to replacing an ECM, always contact your dealership's Technical Communicator for possible consultation with Caterpillar. This consultation may greatly reduce repair time. If the cause of the diagnostic code is not found, replace the ECM. Refer to Testing and Adjusting, "Electronic Control Module (ECM) - Replace".
STOP