Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSMITSUBISHI2005LANCER L4-2.4L SOHCREPAIR AND DIAGNOSISPOWERTRAIN MANAGEMENTCOMPUTERS AND CONTROL SYSTEMSTESTING AND INSPECTIONSYMPTOM RELATED DIAGNOSTIC PROCEDURESINSPECTION PROCEDURE 27
2005 Mitsubishi Lancer L4-2.4L SOHC
Inspection Procedure 27
2005 Mitsubishi Lancer L4-2.4L SOHCSECTION Inspection Procedure 27
Improper Idle Speed When the A/C is Operating (A/C load signal)
CIRCUIT OPERATION
- The ECM or PCM increases the engine idle speed by driving the throttle actuator control motor when the automatic compressor-ECU sends a "A/C on" signal to the ECM or PCM .
- The A/C-ECU detects how the air conditioning is applying load to the engine and converts the information to a voltage signal (High voltage = low load, Low voltage = high load). This voltage signal is called "A/C load signal". The ECM or PCM receives this A/C load signal from the A/C-ECU through terminal No.78 and determines the idle-up speed according to the high or low air conditioning load.
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
- Malfunction of the A/C control system.
- Open or shorted circuit, or improper connector contact.
- ECM failed.
- PCM failed.
DIAGNOSIS
STEP 1. Check connector C-137 at ECM or C-138 at PCM for damage.
Q: Is the connector in good condition?
YES: Go to Step 2.
NO: Repair or replace it. Refer to Harness Connector Inspection. Then confirm that the malfunction symptom is eliminated.
STEP 2. Measure the output voltage at ECM connector C-137 or PCM connector C-138 by backprobing.
1. Do not disconnect the connector C-137 or C-138 and measure at the harness side.
2. Start the engine and run at idle.
3. Turn the A/C switch "ON".
4. Measure the voltage between terminal No.78 and ground.
- If atmospheric air temperature is 15°C (59°F) or less, the voltage should be 1 volt or less.
- If atmospheric air temperature is 18°C (65.4°F) more,the voltage should be battery positive voltage.
5. Turn the A/C switch "OFF".
6. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the measured voltage within the specified range?
YES: Replace the ECM or PCM. Then confirm that the malfunction symptom is eliminated.
NO: Refer to Heating and air conditioning - Introduction To Heater, Air Conditioning And Ventilation Diagnosis.
A/C Load Signal Circuit:
CIRCUIT OPERATION
- The ECM
- The A/C-ECU detects how the air conditioning is applying load to the engine and converts the information to a voltage signal (High voltage = low load, Low voltage = high load). This voltage signal is called "A/C load signal". The ECM
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)
- Malfunction of the A/C control system.
- Open or shorted circuit, or improper connector contact.
- ECM failed.
- PCM failed.
DIAGNOSIS
STEP 1. Check connector C-137 at ECM
Q: Is the connector in good condition?
YES: Go to Step 2.
NO: Repair or replace it. Refer to Harness Connector Inspection. Then confirm that the malfunction symptom is eliminated.
STEP 2. Measure the output voltage at ECM connector C-137
1. Do not disconnect the connector C-137
2. Start the engine and run at idle.
3. Turn the A/C switch "ON".
4. Measure the voltage between terminal No.78 and ground.
- If atmospheric air temperature is 15°C (59°F) or less, the voltage should be 1 volt or less.
- If atmospheric air temperature is 18°C (65.4°F) more,the voltage should be battery positive voltage.
5. Turn the A/C switch "OFF".
6. Turn the ignition switch to the "LOCK" (OFF) position.
Q: Is the measured voltage within the specified range?
YES: Replace the ECM or PCM. Then confirm that the malfunction symptom is eliminated.
NO: Refer to Heating and air conditioning - Introduction To Heater, Air Conditioning And Ventilation Diagnosis.
RENDER: 1.0x
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.