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Inspection Procedure 27: A/C Switch System <Manual A/C>
 
 
 
 
CIRCUIT OPERATION
  • Battery voltage is applied to the A/C switch connector terminal from the ECM connector terminal AC/S.
 
 
 
COMMENT
  • When the air conditioning switch is turned on, the ECM will receive the A/C ON signal from the switch. The ECM uses this signal to control the A/C compressor relay.
 
 
 
TROUBLESHOOTING HINTS (The most likely causes for this case:)
  • Failed A/C switch of heater control unit
  • Open/short circuit or harness damage in A/C switch circuit or loose connector contact
  • Failed ECM
 
 
 
 
DIAGNOSIS
Required Special Tools:
  • MB992744: Vehicle communication interface-Lite (V.C.I.-Lite)
  • MB992745: V.C.I.-Lite main harness A
  • MB992747: V.C.I.-Lite USB cable short
  • MB992748: V.C.I.-Lite USB cable long
  • MB991958: Scan Tool (M.U.T.-III Sub Assembly)
    • MB991824: Vehicles Communication Interface (V.C.I.)
    • MB991827: M.U.T.-III USB Cable
    • MB991910: M.U.T.-III Main Harness A (Vehicles with CAN communication system)
STEP 1. Using scan tool (M.U.T.-III), check data list.
cautionTo prevent damage to scan tool (M.U.T.-III), always turn the ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool (M.U.T.-III).
(1) Connect scan tool (M.U.T.-III). Refer to HOW TO CONNECT THE SCAN TOOL (M.U.T.-III) .
(2) Turn the ignition switch to the "ON" position.
(3) Check the following item in the data list. Refer to Data List Reference Table .
  1. Item 76: A/C switch
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is the A/C switch condition normal?
It can be assumed that this malfunction is intermittent. Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions .
Go to Step 2.
 
STEP 2. Check the A/C switch of the heater control unit.
Check the A/C switch (Refer to GROUP 55A - Heater Control Unit - Air Conditioning Switch Continuity Check ).
Is the A/C switch condition normal?
Go to Step 3.
Replace the heater control unit.
 
STEP 3. Measure the voltage at heater control unit connector.
(1) Disconnect the connector and measure at the harness side.
(2) Turn the ignition switch to the "ON" position.
(3) Measure the voltage between terminal AC/S line and ground.
  • Voltage should be battery positive voltage.
(4) Turn the ignition switch to the "LOCK" (OFF) position.
Is battery positive voltage (approximately 12 volts) present?
Go to Step 5.
Go to Step 4.
 
STEP 4. Check of open circuit, and short to ground in AC/S line between heater control unit connector and ECM connector.
Is the harness wire in good condition?
Replace the ECM.
Repair the connector(s) or wiring harness.
 
STEP 5. Check of open circuit, short circuit, and damage in ground line between heater control unit connector and body ground.
Is the harness wire in good condition?
Go to Step 6.
Repair the connector(s) or wiring harness.
 
STEP 6. Check of damage in AC/S line between heater control unit connector and ECM connector.
Is the harness wire in good condition?
Go to Step 7.
Repair the connector(s) or wiring harness.
 
STEP 7. Using scan tool (M.U.T.-III), check data list.
(1) Turn the ignition switch to the "ON" position.
(2) Check the following item in the data list. Refer to Data List Reference Table .
  1. Item 76: A/C switch
(3) Turn the ignition switch to the "LOCK" (OFF) position.
Is the A/C switch condition normal?
It can be assumed that this malfunction is intermittent. Refer to GROUP 00, How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions .
Replace the ECM.