System Testing

This system contains a sophisticated on-board diagnostics that can by accessed with a bidirectional ``SCAN'' tool. To properly diagnose the ABS system a GM TECH 1 bidirectional scan tool or suitable equivalent is necessary. The scan tool can be used to identify system faults as specifically as possible including whether or not the fault is intermittent. The scan tool can also be used to monitor than input and output signals of the ECU, manually control system components and perform automated functional tests. Using the scan tool, allows for accurate fault confirmation and repair verification.

When performing diagnostic procedures on these systems refer to SCHEMATIC DIAGRAMS for ABS system wiring and ECU connector terminal identification.
In order to properly diagnose and repair this system the following steps should be followed in order. Failure to follow the procedure in order could result in loss of diagnostic data.

Fig. 6 TECH 1 Scan Tool Operation (Part 1 Of 3):

Fig. 6 TECH 1 Scan Tool Operation (Part 2 Of 3):

Fig. 6 TECH 1 Scan Tool Operation (Part 3 Of 3):

1. Connect GM TECH 1 or suitable equivalent to ALDL connector, then read all current and history diagnostic codes. Refer to Fig. 6 for scan tool operation. Be sure to note which codes are current code failures. Do not clear codes unless directed to do so.
2. Read ``CODE HISTORY'' data, note diagnostic codes stored and their frequency of failure. Ensure to note the last failure code entered and the conditions present when the failure occurred. This last failure code should be the starting point for diagnosis and repair.
3. Before beginning service on this system, perform the following preliminary inspection:
a. Check master cylinder for proper brake fluid level.
b. Inspect hydraulic modulator for any leaks or wiring damage.
c. Inspect brakes at all four wheels to ensure that no drag exists and that brakes are operating normally.
d. Check wheel bearings for any excessive wear or damage.
e. Inspect wheel speed sensors for correct air gap, solid sensor attachment, damaged toothed ring or damaged wiring. Refer to COMPONENT REPLACEMENT AND REPAIR/WHEEL SENSOR.
f. Check CV joint for proper operation and alignment.
4. When plugged into the ALDL connector the TECH 1 scan tool can display ABS data, display and clear ABS trouble codes, control ABS components, perform diagnostic testing for intermittent conditions. The TECH 1 uses five different test modes to aid in diagnosis. Use the following TECH 1 diagnostic modes when servicing this system:
a. MODE F0: DATA LIST: In this mode the SCAN tool continuously monitors wheel speed data, brake switch status and other inputs and outputs.
b. MODE F1: CODE HISTORY: In this mode, fault code history data is displayed. This includes how many ignition cycles since the fault code occurred and other ABS information.
c. MODE F2: TROUBLE CODES: In this mode both current and history codes can be displayed or cleared.
d. MODE F3: ABS SNAPSHOT: In this mode, SCAN tool captures ABS data before and after a fault occurrence or a forced manual trigger.
e. MODE F4: ABS TESTS: In this mode, SCAN tool performs hydraulic modulator functional tests to assist in problem isolation during troubleshooting.
5. Refer to DIAGNOSTIC CHARTS for diagnostic circuit check, warning lamp diagnosis and ABS system code diagnosis and repair.
6. If no codes are present, or mechanical component failure codes are present, perform automated modulator test using the SCAN tool to isolate the cause of the problem. If the failure is intermittent and not reproduceable, test drive vehicle while using the automatic snapshot feature of the SCAN tool. Perform normal acceleration, stopping and turning maneuvers. If this does not reproduce the fault, perform an ABS stop from approximately 30-50 mph while triggering an ABS code. If failure still will not reproduce, use enhanced diagnostic information found in CODE HISTORY to determine whether or not this failure should be further diagnosed.
7. Once all failures have been corrected, clear ABS codes.