When servicing the ABS-VI, the following steps should be followed in order. Failure to follow these steps may result in the loss of important diagnostic data and may lead to difficult and time consuming diagnosis procedures.
- Connect a bi-directional scan tool, as instructed by the tool manufacturer, then read all current and historical diagnostic codes. Be certain to note which codes are current diagnostic code failures. DO NOT CLEAR CODES unless directed to do so.
- Using a bi-directional scan tool, read the CODE HISTORY data. Note the diagnostic fault codes stored and their frequency of failure. Specifically note the last failure that occurred and the conditions present when this failure occurred. This last failure should be the starting point for diagnosis and repair.
Perform a vehicle preliminary diagnosis inspection. This should include:
- Inspection of the compact master cylinder for proper brake fluid level.
- Inspection of the ABS hydraulic modulator for any leaks or wiring damage.
- Inspection of brake components at all four (4) wheels. Verify no drag exists. Also verify proper brake apply operation.
- Inspection for worn or damaged wheel bearings that allow a wheel to wobble.
- Inspection of the wheel speed sensors (where possible) and their wiring. Verify correct air gap range, solid sensor attachment, undamaged sensor toothed ring, and undamaged wiring, especially at vehicle attachment points. The sensors for the rear wheels are built into the rear wheel hub/bearing assemblies and cannot be inspected or serviced.
- Verify proper outer CV-joint alignment and operation.
- Verify tires meet legal tread depth requirements.
- If no codes are present, or mechanical component failure codes are present, perform the automated modulator test using the Tech 1®, T-100® or equivalent scan tool, to isolate the cause of the problem. If the failure is intermittent and not reproducible, test drive the vehicle while using the automatic snapshot feature of the bi-directional scan tool.
Perform normal acceleration, stopping, and turning maneuvers. If this does not reproduce the failure, perform an ABS stop, on a low coefficient surface such as gravel, from approximately 30-50 mph (48-80 km/h) while triggering any ABS/ETC code. If the failure is still not reproducible, use the enhanced diagnostic information found in CODE HISTORY to determine whether or not this failure should be further diagnosed.
Once all system failures have been corrected, clear the ABS codes. The Tech 1® and T-100®, when plugged into the Data Link Connector (DLC), becomes part of the vehicle's electronic system. The Tech 1® and T-100® scan tools can also perform the following functions on components linked by the Serial Data Link (SDL):
Display ABS data
Each test mode has specific diagnostic capabilities which depend upon various keystrokes. In general, five (5) keys control sequencing: YES, NO, EXIT, UP arrow and DOWN arrow. The FO through F9 keys select operating modes, perform functions within an operating mode, or enter trouble code or model year designations.
In general, the Tech 1® has five (5) test modes for diagnosing the anti-lock brake system. The five (5) test modes are as follows:
MODE FO: DATA LIST -In this test mode, the Tech 1® continuously monitors wheel speed data, brake switch status and other inputs and outputs.
MODE F1: CODE HISTORY -In this mode, fault code history data is displayed. This data includes how many ignition cycles since the fault code occurred, along with other ABS information. The first five (5) and last fault codes set are included in the ABS history data.
MODE F2: TROUBLE CODES -In this test mode, trouble codes stored by the EBCM, both current ignition cycle and history, may be displayed or cleared.
MODE F3: ABS SNAPSHOT -In this test mode, the Tech 1® captures ABS data before and after a fault occurrence or a forced manual trigger.
MODE F4: ABS TESTS -In this test mode, the Tech 1® performs hydraulic modulator functional tests to assist in problem isolation during troubleshooting. Included here is manual control of the motors which is used prior to bleeding the brake system.
Press F7 to covert from English to Metric.
As with most electronic systems, intermittent failures may be difficult to accurately diagnose. The following is a method to try to isolate an intermittent failure especially wheel speed circuitry failures.
If an ABS fault occurs, the ABS warning light indicator will be on during the ignition cycle in which the fault was detected. If it is an intermittent problem which seems to have corrected itself (ABS warning light off), a history trouble code will be stored. Also stored will be the history data of the code at the time the fault occurred. The Tech 1® or equivalent scan tool must be used to read ABS history data.
INTERMITTENTS AND POOR CONNECTIONS
Most intermittents are caused by faulty electrical connections or wiring, although occasionally a sticking relay or solenoid can be a problem. Some items to check are:
- Poor mating of connector halves, or terminals not fully seated in the connector body (backed out).
- Dirt or corrosion on the terminals. The terminals must be clean and free of any foreign material which could impede proper terminal contact.
- Damaged connector body, exposing the terminals to moisture and dirt, as well as not maintaining proper terminal orientation with the component or mating connector.
- Improperly formed or damaged terminals. All connector terminals in problem circuits should be checked carefully to ensure good contact tension. Use a corresponding mating terminal to check for proper tension. Refer to "Checking Terminal Contact" later in this information for the specific procedure.
- GM's J 35616-A Connector Test Adapter Kit must be used whenever a diagnostic procedure requests checking or probing a terminal. Using the adapter will ensure that no damage to the terminal will occur, as well as giving an idea of whether contact tension is sufficient. If contact tension seems incorrect, refer to "Checking Terminal Contact" later in this information for specifics.
- Poor terminal-to-wire connection. Checking this requires removing the terminal from the connector body. Some conditions which fall under this description are poor crimps, poor solder joints, crimping over wire insulation rather than the wire itself, corrosion in the wire-to-terminal contact area, etc.
- Wire insulation which is rubbed through, causing an intermittent short as the bare area touches other wiring or parts of the vehicle.
- Wiring broken inside the insulation. This condition could cause a continuity check to show a good circuit, but if only 1 or 2 strands of a multi-strand type wire are intact, resistance could be far too high.
When diagnosing an electrical system that uses Metri-Pack 150/280/480/630 series terminals (refer to Terminal Repair Kit J 38125-A for terminal identification), it is important to check terminal contact between a connector and component, or between inline connectors, before replacing a suspect component.
Mating terminals must be inspected to ensure good terminal contact. A poor connection between the male and female terminal at a connector may be the result of contamination or deformation.
Contamination is caused by the connector halves being improperly connected, a missing or damaged connector seal, or damage to the connector itself, exposing the terminals to moisture and dirt. Contamination, usually in underhood or underbody connectors, leads to terminal corrosion, causing an open circuit or an intermittently open circuit.
Deformation is caused by probing the mating side of a connector terminal without the proper adapter, improperly joining the connector halves or repeatedly separating and joining the connector halves. Deformation, usually to the female terminal contact tang, can result in poor terminal contact causing an open or intermittently open circuit.
Follow the procedure below to check terminal contact.
- Separate the connector halves. Refer to Terminal Repair Kit J 38125-A, if available.
- Inspect the connector halves for contamination. Contamination will result in a white or green buildup within the connector body or between terminals, causing high terminal resistance, intermittent contact or an open circuit. An underhood or underbody connector that shows signs of contamination should be replaced in its entirety: terminals, seals, and connector body.
- Using an equivalent male terminal from the Terminal Repair Kit J 38125-A, check the retention force of the female terminal in question by inserting and removing the male terminal to the female terminal in the connector body. Good terminal contact will require a certain amount of force to separate the terminals.
- Using an equivalent female terminal from the Terminal Repair Kit J 38125-A, compare the retention force of this terminal to the female terminal in question by joining and separating the male terminal to the female terminal in question. If the retention force is significantly different between the two female terminals, replace the female terminal in question, using a terminal from Terminal Repair Kit J 38125-A.