Inspection Procedure 27

A/C Load Signal Circuit:

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.