The A/C cut-out relay is a normally closed relay. The ECM opens the relay by providing a ground on circuit No. 449. This occurs during wide open throttle, an overheat condition, and momentarily during engine start-up on a warm engine.

NOTE: Test numbers refer to test numbers on diagnostic chart.

The A/C system must be fully charged so that the pressure cycling switch contacts are closed.
If ECM monitors Wide Open Throttle (WOT) simulated by fully depressing the TPS plunger, the A/C cut-out relay Quad Driver in the ECM is turned on, energizing the relay. This will cause current interruption to A/C clutch. Anytime TPS is fully depressed, long term ECM memory must be cleared to allow resetting of the TPS auto ranging feature.
Grounding the ALDL "test" terminal should turn on the A/C cut-out relay and Quad Driver in the ECM, allowing current to flow through the test light.
Test determines if relay is at fault or ECM circuit operation is incorrect.
If the test light is on, the circuit is either grounded or ECM Quad Driver is on. The ECM will have to be replaced if the test light goes off when ECM connector is disconnected because the Quad Driver in the ECM is faulty.

Fig 1: Flow Chart & Schematic C10C, A/C Cut-Out Relay

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.

Safety Warning

Chart C10C - A/C Cut-Out Relay

Use the Manual With the Right Hardware

When to See a Mechanic

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