Inspection Procedure 15

INSPECTION PROCEDURE 15: Vehicle Shifts Differently with A/C Engaged

Part 1 Of 2:

Part 2 Of 2:

CIRCUIT OPERATION
^ The understanding is that the A/C compressor is operating when the A/C switch is ON and the A/C-ECU (terminal 4) is applying power (voltage) on PCM (terminal 83).
^ When the A/C compressor is engaged, the PCM increases line pressure and briefly delays shift points to compensate for the additional engine load.

COMMENT
The cause is probably a faulty A/C-ECU circuit or a defective PCM.

TROUBLESHOOTING HINTS (The most likely causes for this case:)
^ Malfunction of the dual pressure switch
^ Damaged harness or connector
^ Malfunction of A/C system
^ Malfunction of the A/C-ECU
^ Malfunction of the PCM

DIAGNOSIS

Required Special Tool:
^ MB991958: Scan Tool (MUT-III Sub Assembly)
^ MB991824: V.C.I.
^ MB991827: MUT-III USB Cable
^ MB991911: MUT-III Main Harness B

STEP 1. Using scan tool MB991958, check data list item 65: Dual Pressure Switch.





CAUTION: To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.

1. Connect scan tool MB991958 to the data link connector.
2. Start the engine and run at idle.
3. Set scan tool MB991958 to data reading mode.
^ Item 65: Dual Pressure Switch.
^ When the A/C is in operation, scan tool MB991958 display should be "ON."
^ When the A/C is not in operation, scan tool MB991958 display should be "OFF."
4. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the switch operating properly?
YES: It can be assumed that this malfunction is intermittent.
NO: Go to Step 2.

STEP 2. Check the dual pressure switch.
Q: Does the dual pressure switch pass the checks?
YES: Go to Step 3.
NO: Replace the dual pressure switch.

STEP 3. Check dual pressure switch connector A-25 for loose, corroded or damaged terminals, or terminals pushed back in the connector.





Q: Are the connector and terminals in good condition?
YES: Go to Step 4.
NO: Repair or replace the damaged components.

STEP 4. Measure the power supply voltage at dual pressure switch connector A-25.





1. Disconnect connector A-25 and measure at the harness side.
2. Start the engine and run at idle.
3. Operate the A/C.





4. Measure the voltage between terminal 1 and ground.
^ Voltage should measure battery positive voltage.
5. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the measured voltage battery positive voltage?
YES: Go to Step 11.
NO: Go to Step 5.

STEP 5. Measure the power supply voltage at A/C-ECU connector C-16 by backprobing.





1. Do not disconnect connector C-16.
2. Start the engine and run at idle.
3. Operate the A/C.





4. Measure the voltage between terminal 4 and ground by backprobing.
^ Voltage should measure battery positive voltage.
5. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the measured voltage battery positive voltage?
YES: Go to Step 9.
NO: Go to Step 6.

STEP 6. Check A/C-ECU connector C-16 and intermediate connector C-111 for loose, corroded or damaged terminals, or terminals pushed back in the connector.








Q: Are the connectors and terminals in good condition?
YES: Go to Step 7.
NO: Repair or replace the damaged components.

STEP 7. Check the harness for short circuit to ground between dual pressure switch connector A-25 terminal 1 and A/C-ECU connector C-16 terminal 4.








Q: Is the harness wire in good condition?
YES: Go to Step 8.
NO: Repair or replace the harness wire.

STEP 8. Using scan tool MB991958, check data list item 65: Dual Pressure Switch.





CAUTION: To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.

1. Connect scan tool MB991958 to the data link connector.
2. Start the engine and run at idle.
3. Set scan tool MB991958 to data reading mode.
^ Item 65: Dual Pressure Switch.
^ When the A/C is in operation, scan tool MB991958 display should be "ON."
^ When the A/C is not in operation, scan tool MB991958 display should be "OFF."
4. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the switch operating properly?
YES: It can be assumed that this malfunction is intermittent.
NO: Check the air conditioning system.

STEP 9. Check A/C-ECU connector C-16 and intermediate connector C-111 for loose, corroded or damaged terminals, or terminals pushed back in the connector.








Q: Are the connectors and terminals in good condition?
YES: Go to Step 10.
NO: Repair or replace the damaged components.

STEP 10. Check the harness for open circuit between dual pressure switch connector A-25 terminal 1 and A/C-ECU connector C-16 terminal 4.








Q: Is the harness wire in good condition?
YES: Go to Step 14.
NO: Repair or replace the harness wire.

STEP 11. Measure the switch output voltage at PCM connector C-116 by backprobing.





1. Do not disconnect connector C-116.
2. Start the engine and run at idle.
3. Operate the A/C.





4. Measure the voltage between terminal 83 and ground by backprobing.
^ When the A/C is in operation, the voltage should measure battery positive voltage.
5. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the measured voltage battery positive voltage?
YES: Go to Step 15.
NO: Go to Step 12.

STEP 12. Check intermediate connectors C-111, C-122 and PCM connector C-116 for loose, corroded or damaged terminals, or terminals pushed back in the connector.











Q: Are the connectors and terminals in good condition?
YES: Go to Step 13.
NO: Repair or replace the damaged components.

STEP 13. Check harness for open circuit or short circuit to ground between dual pressure switch connector A-25 terminal 2 and PCM connector C-116 terminal 83.








Q: Is the harness wire in good condition?
YES: Go to Step 14.
NO: Repair or replace the harness wire.

STEP 14. Using scan tool MB991958, check data list item 65: Dual Pressure Switch.





CAUTION: To prevent damage to scan tool MB991958, always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991958.

1. Connect scan tool MB991958 to the data link connector.
2. Start the engine and run at idle.
3. Set scan tool MB991502 or MB991958 to data reading mode.
^ Item 65: Dual Pressure Switch.
^ When the A/C is in operation, the scan tool MB991958 display should be "ON."
^ When the A/C is not in operation, the scan tool MB991958 display should be "OFF."
4. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the switch operating properly?
YES: It can be assumed that this malfunction is intermittent.
NO: Replace the PCM.

STEP 15. Check PCM connector C-116 for loose, corroded or damaged terminals, or terminals pushed back in the connector.





Q: Are the connector and terminals in good condition?
YES: Go to Step 14.
NO: Repair or replace the damaged components.