NOTE: To identify wire colors and modules on serial data links, see appropriate wiring diagram in DATA LINK CONNECTORS article in WIRING DIAGRAMS.

Perform Automatic Level Control diagnostic system check. See AUTOMATIC LEVEL CONTROL (ALC) DIAGNOSTIC SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM. After performing diagnostic system check, go to next step.
Turn ignition off. Disconnect ALC sensor connector. See Fig 1 . Measure resistance of Brown/White wire between ALC sensor connector terminal "B" and DLC terminal No. 8. See Figure . If resistance is near 0.5 ohms, go to next step. If resistance is not near 0.5 ohms, go to step 4 .

Fig 1: Locating Automatic Level Control (ALC) Sensor
Courtesy of GENERAL MOTORS CORP.
Inspect for poor connections at ALC sensor connector. If fault was located and repaired, go to step 5 . If fault was not located and repaired, go to step 6 .
Locate and repair open in Brown/White wire between ALC sensor and DLC. After repair, go to next step.
Reconnect all disconnected connectors. Install scan tool to DLC. Turn ignition on, with engine off. Attempt to establish communication with ALC sensor. If scan tool communicates with ALC sensor, system is okay. Test is complete.
Inspect ALC fuse (20-amp) in fuse block for an open. Test battery positive voltage circuit (Red wire) at ALC sensor harness connector terminal "A". Check ground circuit (Black wire) at ALC sensor harness connector terminal "E" for an open. If fault was located and repaired, go to step 5 . If fault was not located and repaired, go to next step.
Test serial data circuit (Brown/White wire) between DLC terminal No. 8 and ALC sensor harness connector terminal "B" for a short to ground. See Figure . If fault was located and repaired, go to next step. If fault was not located and repaired, go to step 9 .
Repair short to ground in serial data circuit (Brown/White wire) between DLC terminal No. 8 and ALC sensor harness connector terminal "B". After locating and repairing fault, go to step 5 .
Test serial data circuit (Brown/White wire) between DLC terminal No. 8 and ALC sensor harness connector terminal "B" for a short to voltage. If fault was located and repaired, go to next step. If fault was not located and repaired, go to step 12 .
Repair short to voltage in serial data circuit (Brown/White wire) between DLC terminal No. 8 and ALC sensor harness connector terminal "B". After locating and repairing fault, go to next step.
Replace ALC sensor. See appropriate ELECTRONIC LEVEL CONTROL article in SUSPENSION. After replacement, go to step 5 .
Reconnect ALC sensor and any other disconnected components or electrical harness connectors. Turn ignition off. Install scan tool to DLC. Turn ignition on. Attempt to establish communication with ALC sensor. If communication with ALC sensor is established, go to next step. If communication with ALC sensor is not established, go to step 11 .
Communication error recorded is probably an intermittent condition. Fully test all connections and harnesses for poor connections and intermittent conditions. For more information, go to PRELIMINARY INSPECTION under TROUBLE SHOOTING. After fault is located and repaired, go to step 5 .

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

Scan Tool Does Not Communicate With ALC Sensor

Use the Manual With the Right Hardware

When to See a Mechanic

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