Diagnosis & Testing: Notes

NOTE: Tech 1 scan tester is recommended when testing and diagnosing 4WAL system, but is not necessary.

The Brake Pressure Modulator Valve (BPMV) contains a self-diagnostic capability to detect system failures. When a fault code is set, the BPMV may disable 4WAL system and illuminate ANTILOCK light for duration of ignition cycle. Fault codes stored by BPMV can be displayed using on-board diagnostics or Tech 1 with appropriate cartridge.

NOTE: All 2WD Astro and Safari vehicles use 3 wheel speed sensors. All-Wheel Drive (AWD) Astro and Safari vehicles use 4 wheel speed sensors.

Before diagnosing 4WAL system, perform a comprehensive visual inspection of system by checking wiring harness connectors for looseness and harness routing (pay particular attention to wheel speed sensor wiring harness routing), applicable fuses in fuse block and ground connections. Ensure brake fluid level in master cylinder reservoir is full.

Start 4WAL system diagnosis with appropriate FUNCTIONAL TEST chart under TROUBLE SHOOTING CHARTS. See Figure or Figure . If failures are found during FUNCTIONAL TEST, you will be directed to enter diagnostics to retrieve codes or perform symptom diagnosis. FUNCTIONAL TEST may indicate 4WAL system is functioning properly.