The ABS contains sophisticated onboard diagnostics that, when accessed with a Tech 1® or equivalent "Scan'' tool, are designed to identify the source of any system fault as specifically as possible, including whether or not the fault is intermittent. There are over 50 diagnostic fault codes to assist the service technician with diagnosis. The last diagnostic fault code to occur is specifically identified, and specific ABS data is stored at the time of this fault, also, the first five codes set. Additionally, using the scan tool, each input and output can be monitored, thus enabling fault confirmation and repair verification. Manual control of components and automated functional tests are also available when using a scan tool.
Diagnostic Test Drive
When servicing vehicles with the vehicle stability enhancement system (VSES), test drives will be necessary to allow all tests to be run, and all system functions to be enabled and exercised. A test drive may also be required to duplicate specific DTCs covered in this section. The diagnostic system check (including test drive) should be run when vehicle repairs are complete in order to verify the repair. The diagnostic test drive will vary for ABS, TCS, or VSES concerns.
The following is a suggested VSES diagnostic test drive.
- Turn OFF the ignition.
- Reconnect any previously disconnected components.
- Start the engine.
- Install a scan tool.
- With the scan tool, observe the VSES Enabled parameter in the VSES data list. The scan tool will indicate Yes when the steering wheel is centered. Drive the vehicle in a straight line for 45 seconds at 24 km/h (15 mph) while monitoring the scan tool.
- Remove the scan tool.
Drive the vehicle under the following conditions for at least 10 minutes.
- Highway driving:
- Rough roads:
- Perform turning maneuvers (curved freeway entrance ramp, parking lot maneuvers) and high steering angles and yaw rates at low speeds.
- Verify the concern and duplicate driving conditions.
- With the engine still running, use the scan tool in order to check for DTCs. If any DTCs are set repair the appropriate condition as necessary.
Enhanced Diagnostic Information, found in the CODE HISTORY function of the scan tool is designed to provide the service technician with specific fault occurrence information. For each of the first five (5) and the very last diagnostic fault codes stored, data is stored to identify the specific fault code number, the number of failure occurrences, and the number of drive cycles since the failure first and last occurred (a drive cycle occurs when the ignition is turned ON and the vehicle is driven faster than 10 mph). However, if a fault is present, the drive cycle counter will increment by turning the ignition ON and LOCK . These first five (5) diagnostic fault codes are also stored in the order of occurrence. The order in which the first 5 faults occurred can be useful in determining if a previous fault is linked to the most recent faults, such as an intermittent wheel speed sensor which later becomes completely open.
During difficult diagnosis situations, this information can be used to identify fault occurrence trends. Does the fault occur more frequently now than it did during the last time when it only failed 1 out of 35 drive cycles- Did the fault only occur once over a large number of drive cycles, indicating unusual condition present when the fault occurred- Does the fault occur infrequently over a large number of drive cycles, indicating special diagnosis techniques may be required to identify the source of the fault-
If a fault occurred 1 out of 20 drive cycles, the fault is intermittent and has not reoccurred for 19 drive cycles. This fault may be difficult or impossible to duplicate and may have been caused by a severe vehicle impact (large pot hole, speed bump at high speed, etc.) that momentarily opened an electrical connector or caused unusual vehicle suspension movement. Problem resolution is unlikely, and the problem may never reoccur. If the fault occurred 3 out of 15 drive cycles, the odds of finding the cause are still not good, but you know how often it occurs and you can determine whether or not the fault is becoming more frequent based on an additional or past occurrences if the source of the problem can not or could not be found. If the fault occurred 10 out of 20 drive cycles, the odds of finding the cause are much better, as the fault may be easily reproduced.
By using the additional fault data, you can also determine if a failure is randomly intermittent or if it has not reoccurred for long periods of time due to weather changes or a prior repair. Say a diagnostic fault code occurred 10 of 20 drive cycles but has not reoccurred for 10 drive cycles. This means the failure occurred 10 of 10 drive cycles but has not reoccurred since. A significant environmental change or a repair may have occurred 10 drive cycles ago. A repair may not be necessary if a recent repair can be confirmed. If no repair was made, the service can focus on diagnosis techniques used to locate difficult to recreate problems.