With Scan Tool

The Electronic Control Unit (ECU) performs self-diagnostics and can detect and often isolate system failures. When a malfunction is detected, the ECU sets a Diagnostic Trouble Code (DTC) that represents the malfunction, turns on the BRAKE lamp (in most instances), and will disable Rear Wheel Anti Lock (RWAL) functions as long as the BRAKE lamp remains on.

The ECU performs an automatic self-test when the ignition switch is turned to the "ON" position. The ECU cycles each valve to check component operation. If it detects an error, it sets a Diagnostic Trouble Code (DTC) as described above.

TECH I DIAGNOSTICS

CAUTION: When using the Tech 1 for RWAL system testing, make sure you are using the RWAL cartridge that includes the model year you are working on. Refer to the RWAL Cartridge Operator's Manual for additional information about the cartridge.

The Tech 1 has 4 test modes for diagnosis of the RWAL systems. These test modes are:

MODE F1: FUNCTION TEST
This test mode is used to perform a functional test of the system. The Tech 1 will simulate an acceleration and deceleration signal that causes the ECU to cycle the RWAL system. If the ECU stores a DTC during the test, the Tech 1 will display it at the end of the test. If a DTC is set before you begin the test, the Tech 1 will display the DTC and you will not be able to continue until the DTC is cleared.

MODE F2: TROUBLE CODE
This mode allows you to access DTCs and offers some diagnostic information on the DTCs.

MODE F3: MISCELLANEOUS TEST
This mode allows you to do specialized testing of the Digital Ratio Adapter Controller (DRAC) and Stop lamp Switch (Brake Switch).

MODE F4: VSS MONITOR
This mode allows you to monitor the vehicle speed signal as the ECU sees it. This can help you determine if a malfunction relates to the speed signal.
- Select F4: VSS Monitor from main menu.
- Speed will be displayed MPH and KPH.

F1: BRAKE SWITCH TEST
This test allows you to determine if there is a malfunction in the stop lamp (brake) switch circuit. On 4WD vehicles, this mode will also allow you to check the front axle and transfer case switched circuits.
- Select F3: Misc. Tests from main menu.
- Select F1: Brake Switch Test.
- Complete the test by following the direction of each screen.