A/c-Heater System Diagnosis - Automatic Chevrolet Corvette C4

CODE 01 & 02 AIR MIX DOOR ACTUATOR CIRCUIT FAULT

Feedback Potentiometer (POT) sensor measures position of air mix door. A/C-heater programmer uses this information to drive air mix valve motor. Low feedback voltage represents hot air mix door position and high feedback voltage represents cold air mix door position. If Codes 01 and 02 are set, A/C-heater programmer will enter a default value of 128 counts (mid position), so driver may still control full hot and full cold positions.

Note. Test numbers refer to test numbers on diagnostic charts.

1. This test eliminates possibility of false code being set. With ignition switch in OFF position, remove courtesy fuse, radio fuse or negative battery cable and install after 1 minute. This will clear A/C-heater programmer memory.
2. This test will determine if the A/C heater programmer is commanding the air mix door to travel its full range. If parameter 34 is not within specified range, full hot and full cold, replace A/C heater programmer.
3. This test visually confirms that the air mix door is actually moving.
4. This test is to determine if the air mix door motor potentiometer is within the specified range for full hot and full cold door position. If air mix door motor is not within specified range, step 6) should be performed before replacing air mix door motor.
5. This test will determine if the air mix motor is receiving operating voltage from the programmer. If approximately 8 volts are present at C295, air mix door motor is suspect.
6. This test will determine if air mix door is misaligned causing air mix door motor potentiometer to be outside specified range.
7. This test will determine if there is output voltage at A/C-heater programmer to activate air mix door. If voltage is present at A/C-heater programmer and not at C295, an open exists in the circuit. If voltage is not present at A/C-heater programmer, replace programmer.

Diagnostic Aids

If Code 01 or 02 are intermittent, check connectors and wiring harness for possible open or short. If air mix door has more than 5 mm of free play, replace heater case assembly.

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CODE 03 & 04 OUTSIDE AIR TEMP SENSOR CIRCUIT FAULT

Outside air temperature sensor is a thermistor that controls signal voltage to A/C-heater programmer. A/C-heater programmer supplies and monitors voltage (about 5 volts reference) on circuit No. 735 to sensor. When outside (ambient) air temperature is cold, sensor resistance is high and A/C-heater programmer will see a high signal voltage. If outside (ambient) air temperature is hot, sensor resistance is low and A/C-heater programmer will see a low voltage.

Note. Test numbers refer to test numbers on diagnostic charts.

1. This test will determine if outside air temperature sensor is shorted or open internally. If temperature-to-resistance relationship is not within specified range, replace sensor.
2. This test will determine if there is an open or short in air temperature circuit. If no open or short is present and temperature-to-resistance relationships are within specification, replace A/C-heater programmer.

If resistance readings are within range at the sensor but out of range when checked at the A/C-heater programmer, check all connections for water intrusion or corrosion. If no problem is found after performing steps 1) and 2), check for intermittent open or short while wiggling harness and monitoring resistance at A/C heater programmer harness terminals D16 and C16.

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CODE 05 & 06 IN-CAR TEMP SENSOR CIRCUIT FAULT

In-car temperature sensor is a thermistor to control signal voltage to heater and A/C-heater programmer. A/C-heater programmer supplies voltage (about 5 volts/reference) to sensor circuit No. 734. When inside temperature is cold, sensor (thermistor) resistance is high. Therefore, the A/C-heater programmer will see a high signal voltage. If inside air temperature is high, sensor (thermistor) resistance is low. Therefore, heater and A/C-heater programmer will see a low voltage.

Note. Test numbers refer to test numbers on diagnostic charts.

1. This test will determine if sensor is open or shorted internally. If temperature-to-resistance relationship is not within specified range, replace sensor. 2)This test will determine if there is an open or short in the sensor circuit. If no short or open is present and temperature-to-relationship is within the specified range, replace the A/C-heater programmer.

If resistance readings are within range at the sensor but out of range when checked at the A/C-heater programmer, check all wiring harness connectors between the sensor and A/C-heater programmer. If no problem is found after performing steps 1) and 2), check for intermittent open or short while wiggling harness and monitoring resistance at A/C heater programmer harness terminals C16 and D15.

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CODES 07 & 08 SUN LOAD SENSOR OPEN 07/SHORTED 08

Solar sensor is a reverse bias photo diode that measures sun load. In AUTO mode, sun load sensor inputs data to the A/C-heater programmer on sun load conditions.

Note. Test numbers refer to test numbers on diagnostic charts.

1. This test will determine if sensor is providing correct data to the A/C-heater programmer.
2. This test will determine if 5 volt reference is available to the sensor. If 5 volts is not available to sensor, an open or short in circuit No. 590 or the A/C-heater programmer is at fault.
3. This test will determine if the sensor ground circuit is open.
4. This test will determine if a short to 5-volt reference is present in ground circuit, causing an open Code 07 (no current flow).

Bright sunlight will cause low counts and darkness will produce high counts, in parameter 04. If no code is present after performing step 4), an intermittent open or short is present. Enter parameter 00 and wiggle sensor wiring harness connectors to see if code resets.

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CODE 09 LOW FREON DETECTED

In Cycling Clutch Orifice Tube (CCOT) air conditioning system, a cycling low side pressure switch informs A/C-heater programmer via Central Control Module (CCM) of direct operating condition of compressor (cycling). When refrigeration system becomes low on freon, ON/OFF frequency of cycling switch increases. If compressor on-time becomes too short, A/C-heater programmer will trip a flag (flash LED) and shut compressor off until system is recharged/repaired and battery reset.

It is possible for A/C-heater programmer logic to be lost if 15-amp CTSY (courtesy) fuse, 10-amp radio fuse, or negative battery cable are installed with ignition switch in ON position.

Note. Test numbers refer to test numbers on diagnostic charts.

1. This validates need for further system testing by removing negative battery cable and installing after 1 minute to clear a possible false code.
2. Code 09 indicates A/C system is low on refrigerant. Add one pound and leak check system. If no leak is found, proceed to Step 3).
3. This step will charge system as necessary, so clutch cycles can be monitored. Excess cycling is usually due to plugged orifice. If clutch will not engage, connect a jumper wire across pressure cycling switch connector to check for an open. Compressor clutch should engage.

If A/C system pressures are in range after checking A/C system for insufficient cooling, and Code 09 is still present, problem exists in driveability or emissions.

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CODE 10 OR "- - -" UART OR E & C BUS FAILURE

UART is composed of two serial data lines leading from Central Control Module (CCM) to A/C-heater programmer. Data is sent and received by both A/C-heater programmer and CCM. UART information is sent by A/C-heater programmer to A/C-heater control assembly over E and C bus (Entertainment and Comfort Data Line).

Code 10 indicates A/C-heater programmer detects a fault in serial data it is receiving. Check for short or open in UART data lines to CCM.

Three dashes "- - -" displayed by A/C-heater control assembly indicates E and C bus failure has occurred. Check for an open in circuit between A/C-heater programmer and A/C-heater control assembly, or short in data line output from A/C-heater programmer.

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