ABS diagnostic capabilities vary from system to system and vehicle to vehicle. Therefore, service personnel usually need more than one type of diagnostic equipment for ABS testing, sources said. Most service-age vehicles have diagnostic connectors of some kind, but all connectors don't provide a data stream. Data stream is a sequence of electrical pulses flowing from the diagnostic connector into an interfaced scan tool. Grossly simplified, these pulses describe the information sensors are sending into the computer and the commands the computer sends out to its actuators.
In an ABS system, the wheel-speed sensors are the most important sensor input to the ABS computer.
Meanwhile, commands to the electrical-hydraulic (EH) actuator are the most important ABS computer outputs. The EH actuator prevents wheel lockup during panic stops by modulating brake application pressure.
When neither a data stream nor a diagnostic connector are available, technicians can monitor ABS system activity by back probing the appropriate terminal of the ABS computer harness connector. Or they can connect a breakout box between the computer and the vehicle's wiring harness.
The breakout box, which has a panel of numbered electrical terminals on it, eliminates the tedium of identifying the correct computer wire and connecting a voltmeter or oscilloscope to it.
Instead, the technician uses a reference guide to find the numbered breakout box terminal representing the circuit he needs to test. Then he connects a test lead to that terminal and proceeds with the diagnosis.
Jeff Masterman, a technical trainer for Standard Motor Products and its EIS Brake Parts division, cautioned tire dealers and technicians that all scan tools are not created equal. Functionality varies so much from one brand to another that some scan tools cannot read any or all of an ABS data stream-when that data stream's provided. Plus, some scan tools issue ABS test commands others do not.
For example, the Delco VI integral ABS system appears on many of General Motors Corp.'s smaller vehicles. When a technician wants to bleed the rear brakes on a Delco VI-equipped car, he must first reposition some EH actuator pistons in their upward position. If he plugs a Tech 1 (a General Motors-specific scan tool endorsed by the automaker) into the car's diagnostic connector, he can do this with the
touch of a button.
But without a Tech 1, he has to drive the car around the block in order to reposition the EH actuator pistons, Mr. Masterman explained.
On many ABS systems of the mid- to late-1980s, technicians can access ABS codes by connecting the appropriate test terminals with a jumper wire or special jumper ``key.'' Always reference the proper service manual beforehand because many systems of this era don't store fault codes after you shut off the ignition switch! When in doubt, always retrieve the trouble codes before shutting off the vehicle, he warned.
Later, more-refined ABS systems allow you to recall trouble codes on command with a scan tool. However, the only way to access trouble codes on some Jeep ABS systems is with a Chrysler-specific scan tool called the DRB-II, he said.
System-specific breakout boxes are offered for some ABS setups, but these may not be the best value, Mr. Masterman said. In some cases, techs can update existing breakout boxes to test ABS systems.
For instance, many techs already own breakout boxes that interface with Ford Motor Co. engine control computers because Ford is the most common service-age vehicle lacking a data stream. The breakout box maker may offer a test cable that connects its product between the ABS computer and the vehicle's wiring harness.
Mr. Masterman said another cost-effective option comes from Thexton Manufacturing Co. of Minneapolis. Thexton offers two different conversion connectors that adapt common Ford breakout boxes to popular Bosch and Teves ABS computers.
According to a Thexton designer, Thexton ABS adapters No. 132 and 134 fit most of the Bosch and Teves ABS computer applications technicians encounter.