Component Diagnostic Charts/Tests

CIRCUIT DESCRIPTION
ECM control of the A/C clutch improves idle quality and performance by:
- Delaying clutch apply until the idle air rate is increased.
- Releasing clutch when idle speed is too low.
- Releasing clutch at wide open throttle.
- Smooths cycling of the compressor by providing additional fuel at the instant clutch is applied.

Turning on air conditioning supplies CKT 59 battery voltage to the clutch control relay and terminal "BC9" of the ECM connector. After a time delay of about 1/2 second the ECM will ground terminal "GF1" of the ECM connector, CKT 459, and close the control relay. A/C compressor clutch will engage.

TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Checks for low refrigerant as cause for no A/C.
2. This and following tests check for faulty A/C control relay.
3. Checks for faulty cycling switch.
- Solenoids and relays are turned "ON" or "OFF" by the ECM, using internal electronic switches called "drivers." Each driver is part of a group of four, called "Quad-Drivers." Failure of one driver can damage any other driver in the set.

Solenoid and relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM 'driver." Using an ohmmeter, check the coil resistance of the A/C relay before replacing the ECM.

DIAGNOSTIC AIDS
Before replacing ECM, use ohmmeter and check resistance of each ECM controlled relay or solenoid coil. Refer to ECM QDR Check, CHART C-1A in "Electronic Control Module (ECM) and Sensors." See ECM wiring diagram for coil terminal identification for solenoid(s) and relay(s) to be checked. Chart C-1A Park/Neutral Switch Diagnosis
Replace any relay or solenoid that measures less than 20 ohms.