Powertrain Controls - ECM/PCM

A/C Compressor Clutch Circuit Diagram:

CIRCUIT DESCRIPTION
The A/C pressure sensor provides a voltage signal that indicates the A/C high side pressure to the PCM. The PCM applies a 5 volt reference voltage to the senor. The sensor divides the current from the reference signal between ground and a signal wire back to the PCM, varying the voltage signal to the PCM. The signal voltage varies directly with the high side pressure from about 0.1 volt at 0 psi, to about 4.6 volts at 450 psi.

PURPOSE AND OPERATION
The A/C pressure sensor responds to changes in the A/C system high side pressure. This signal indicates the approximate load on the engine from the A/C compressor, and is compared to signals from other inputs to determine IAC valve pintle position for correct idle speed control. This signal is also used to control A/C clutch and engine cooling fan operation. When the A/C is operating (A/C request), the PCM responds to the pressure sensor signal as follows:

1. Signal voltage very low (0.1 volt or less for more than 5 seconds) with A/C turned "ON".
a. A/C compressor clutch disengages.
b. Code 66 sets.
2. Signal voltage in the middle range.
a. Cooling fan turns "ON".
b. PCM increases IAC valve counts to compensate for additional engine load.
3. Signal voltage high (4.6 volts or more for more than 5 seconds).
a. A/C compressor clutch disengages
b. Code 66 sets.

TEST DESCRIPTION

Code 66 Diagnostic Chart:

Numbers below refer to circled numbers on diagnostic chart.

1. This step checks that the signal from the pressure sensor is reaching the PCM, and that signal is within the normal range for the test conditions. Normal voltage under these test conditions is 0.1 - 1.8 volts.
2. Checks to see if high voltage signal is result of faulty sensor, a short to voltage in signal circuit (circuit 732), or a faulty PCM.
3. Checks wiring, connections, reference voltage, and PCM. Bridging reference voltage circuit to signal circuit should cause signal voltage to measure same a reference voltage (5 volts.)
4. Checks reference voltage from PCM. Determines if low voltage reading in previous step is due to open or short in reference circuit 474.

DIAGNOSTIC AIDS
Code 66 sets when the signal voltage is measured outside the normal range for the sensor and is not due to a refrigerant system problem. If problem is intermittent, check for opens or shorts in wiring harness, or poor connections. If OK, replace pressure sensor. If code 66 recurs, replace PCM.