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ABS Diagnostic Technology

Bidirectional communication allows you to check the operation of various components.

Back in the early days of anti-lock brakes (ABS), self-diagnostic capability was pretty limited. The early Bosch 2 ABS systems on various import applications provided no fault codes, and self-diagnostics was limited to illuminating the ABS lamp if the module detected a fault in one of its circuits or itself.

To make matters worse, the only test equipment available for these early Bosch systems was a dedicated (and expensive) OEM tester made by Bosch for new car dealers. Each tester was designed for a specific make of vehicle equipped with Bosch 2 ABS (BMW, Mercedes, Porsche, Corvette), so there was no "universal" ABS tester for the aftermarket. Fortunately, the tester wasn't absolutely necessary to troubleshoot problems on these early Bosch 2 ABS systems because most of the circuit tests done by the tester could also be done with an ordinary volt/ohm meter and breakout box. You also needed a service manual, wiring diagrams and sometimes lots of patience (especially if the problem was intermittent). The only test the tester could do that couldn't be done manually was to cycle the ABS solenoids.

The early Bosch ABS tester was not a scan tool because it did not communicate with the ABS module. Rather, it performed a series of step-by-step circuit tests to help a technician isolate any faults. The tests included the ABS relay, voltage supply, valve relay, module diode, wheel speed sensor resistance and voltage output, ABS solenoid resistance and solenoid operation.

As more new ABS systems were introduced, manual flash codes and stored fault codes that could be accessed with a scan tool were added to make diagnosis easier. The Bosch 2E, 2S and 2U ABS systems as well as the Bosch 3 integral ABS system provided both manual and scan tool access to diagnostic codes.

On a typical General Motors application with a Bosch 2U ABS system (1990-'94 full-size rear-wheel drive passenger car), the flash codes can be accessed manually by grounding terminal "H" on the ALDL connector, then counting the flashes of the ABS lamp to determine the code. On these systems, flash codes repeat three times. Codes will stay in memory 50 ignition cycles. But to clear any codes, a scan tool is necessary.

The rear-wheel Kelsey-Hayes RWAL system on older GM, Ford and Dodge trucks (1987 and up) only has the capacity to store one code at a time, but the code can be accessed manually or with a scan tool. The manual procedure on the GM trucks is the same as that for a passenger car, but on Ford and Dodge trucks a pigtail connector has to be grounded to get the codes to flash.

The Kelsey-Hayes 4WAL ABS system (introduced in 1990) and EBC systems (introduced in 1993) have the ability to store multiple codes, as does the ITT Teves Mark 2 ABS system (1985-92 model years). And all except the Kelsey-Hayes EBC systems allow either manual or scan tool access.

Other newer ABS systems provide no manual access to stored codes, which meant a technician had to use a scan tool to read the fault codes stored in the ABS module's memory. These systems included all the Bendix systems (Bendix 6, 9, 10, ABX-4 and Mecatronic found primarily on Chrysler cars and Jeeps), as well as the integral Delco Powermaster III system, and nonintegral Delco ABS VI, ITT Teves Mark 4, and Bosch 5 and 5.3 systems. On some of the GM applications with Bosch ABS/ASR such as 1993 and up Cadillacs and 1992-'94 Corvettes, codes can be read through the electronic speedometer or driver information center by pushing climate control or the trip reset, trip/ODO, and ENG/MET buttons in a specified sequence (the procedure is too long to describe here so refer to a shop manual for the exact procedure).


Although a scan tool is all that's needed to read and clear fault codes on most ABS systems, fault codes alone don't fix the problem. A code only tells you the system has self-diagnosed a problem in a particular circuit or component. It does not tell you what to replace. To fix the problem you have to refer to the step-by-step diagnostic checks in a service manual and isolate the fault by a process of elimination. Jump to conclusions and you may guess wrong.

What's more, intermittent problems may not set a fault code. So if there's no code, don't assume everything is fine. You have to check the operation of the system as well as individual components to figure out what's wrong.

Point by point resistance and voltage checks can help you isolate problems with or without codes, but such tests can be very time-consuming and may not always reveal a problem. Dynamic tests are also impossible with this approach because you have no means of exercising or testing the operation of the ABS solenoids and valves. Such components may show the correct voltage and resistance readings, but may not be functioning correctly.

Many of the newer ABS systems also provide more sophisticated diagnostics including the ability to store snapshot data associated with a fault code, code history and the ability to take two-way inputs from a scan tool to exercise various components in the ABS system such as the solenoid valves, relays, pump motor, ABS warning lamp circuit, etc. To diagnose these systems, you need a more sophisticated type of scan tool or a dedicated ABS tester that allows bidirectional (two-way) communication between the ABS module and scan tool. This allows you to exercise the various solenoids (a procedure which is often necessary to bleed the brakes on these vehicles), and to check the operation of the various components.

Here are three such dedicated ABS testers you should know about:


One of the newest testers is OTC's ABS Reader. This hand-held tester includes "GM-approved" software for full bi-directional control on 1991-'94 Delco VI and 1988-'91 Powermaster III ABS systems. The tool can also be used to troubleshoot Bosch, ITT Teves, Kelsey-Hayes and Mazda MECS ABS systems.

The ABS Reader has an easy-to-read four line, 19-characters per line LCD display, with nine input buttons. The display shows diagnostic codes with descriptions along with live data stream information on ABS sensors and switches, plus record and playback of data stream. It also allows bi-directional inputs for the Delco ABS systems which provides the following:

  • Powermaster III; manual control, hydraulic control, Powermaster bleed, pump run tests (run cycle time, total time, leak down, cycles without brake), system identification, data stream and fault codes;
  • Delco VI; manual control, hydraulic control, electromagnetic brake (EMB), motor test, gear tension relief, relay test, voltage load test, lamp test, system identification, motor rehome, data stream and fault codes;
  • Mazda MECS; wiggle test plus fault codes;
  • Bosch and Teves; data stream and fault codes;
  • Kelsey-Hayes; fault codes only;

On the Delco VI system, the manual control test checks each of the three motors to determine if they are operating properly. The EMB test and motor test can help you find problems with the motor pack. The gear tension relief test can find a defective hydraulic modulator. The relay test checks to see that the relay is supplying power to the ABS system. The motor rehome test opens up the ABS circuits so the modulator can be bled. The hydraulic control test lets you check the operation of all three ABS channels (left front, right front and rear) to detect problems. The voltage load test checks system voltage to verify that it is within specifications. The lamp test checks the lamp circuit and driver module. The system identification test provides information on the ABS control module, eliminating the need to check part numbers visually.

The OTC ABS Reader retails for $449 (tool and manual only P/N #3758), or $599 with adapter cables and carrying case (P/N #3757). For further information, contact your OTC distributor or call 1-800-533-6127.

Information on the ABS Reader was provided by OTC.


The ABS-TECH by Edge Diagnostic Systems is a dedicated computer-based ABS analyzer for diagnosing all current import and domestic ABS systems. The ABS-TECH diagnostic software module is actually an "add-on" enhancement for the SIMU-TECH computerized engine control system diagnostic pod that works in conjunction with a personal computer. The ABS-TECH system has been on the market for about five years, and is currently used by some OEMs for ABS diagnosis as well as Midas and Goodyear.

One of the many features offered by this easy-to-use menu-driven system is the ability to perform an automated full system sweep of ABS sensor, actuator and signal circuits in about 60 seconds. The test results are then displayed with "pass" or "fail" ratings for each component based on vehicle specifications stored in the ABS-TECH data base. This feature allows a technician to pinpoint electrical problems quickly, saving valuable diagnostic time.

Another unique feature of the ABS-TECH software is that it can do a fully automated dynamic ABS test, eliminating the need to test drive the vehicle. It does this by inputting simulated vehicle and wheel speed sensor signals into the ABS module, which allows the technician to see how the system responds during a simulated ABS stop.

The system can also cycle and test all of the ABS solenoids for checking individual ABS hydraulic circuits.

Other features you'll find with the ABS-TECH are:

  • A single point connection. The cable connects between the ABS module and wiring harness (which does require different adapters for various applications);
  • A fully self-configuring seven-trace lab scope for viewing live signal output waveforms. Up to 75 different signal inputs can be shown, seven at a time on the screen. There is also a stored library of known "good" waveforms for comparison purposes. The software allows you to record, play back, save and print all signals displayed;
  • Built-in custom meters, including volts, ohms, amps and frequency for pinpoint testing. Data can be displayed various ways; analog (waveform, bar chart, variation chart), digital (frequency, dwell, voltage, duty cycle, pulse width), and switch (on/off);
  • Online help covering recommended ABS brake bleeding procedures, control module locations, signal descriptions and more.

The ABS-TECH is available through Snap-on distributors, and sells for around $2,000 as an add-on (if you already have SIMU-TECH) or from $8,000 to $12,000 for the complete system. Contact your Snap-on distributor or call 1-800-746-8832 for more details.

Also available from Edge Diagnostics is their hand-held Personal Automotive Computer (PAC) diagnostic tool, which combines the functions of a scan tool with a four-channel lab scope. This unit can also be used to read ABS codes and display signal waveforms, but lacks the diagnostic capability of the ABS-TECH system.

Information on the ABS-TECH was provided by Edge Diagnostic Systems.


Snap-on's Fast-Track ABS Troubleshooter (MT25002794) provides access to vehicle scan codes on domestic vehicles. Specific systems covered include GM (Bosch 2U, 2S and 3, ITT Teves Mark 2, Delco Powermaster III, Delco VI, Kelsey-Hayes RWAL and 4WAL), Ford (Teves Mark 2 and 4, Kelsey-Hayes RABS, and Sumitomo), Chrysler (Bendix 4, 6 and 10, Kelsey-Hayes RWAL, Bosch 3), Jeep (Bendix 9). For Asian import applications, there is a separate combination cartridge kit MT25004495. Adapter cables for domestic and import vehicles are available separately (cables can be purchased or leased from Snap-on). Introduced in 1995, the Fast-Track ABS troubleshooter cartridge is an add-on enhancement for Snap-on's MT2500 hand-held Scanner and Domestic Combination Primary cartridge. Depending on the application, the Fast-Track ABS Troubleshooter can display codes, system data including wheel speed sensor readings, lamp and pump motor status, and valve positions, and perform functional tests for activating various ABS valves, relays and motors, and cycle the ABS solenoids when bleeding the brakes. The tester can handle brake bleeding on Delco VI, Bendix 4 and ABX-4 and ITT Teves Mark 4 applications, actuator tests on Chrysler Bendix 4, 6 and 10 and Teves Mark 4 applications, plus many other tests.

Each ABS menu in the Troubleshooter is organized to include:

  • Warning lights and initial checks - often overlooked checks that can lead to misdiagnosed problems, system specific lamp diagnostics and preroad test checks.
  • ABS brake service caution areas - things to avoid or be careful about when servicing the brakes, plus brake specifications.
  • Code tips - extensive, system specific code descriptions with code set conditions and repair tips.
  • Symptom tips - descriptions of and repair tips for frequently occurring problems that may not always set a code;
  • Tests and procedures - detailed subsystem test information such as wheel speed sensor, pump and valve assembly testing, including brake bleeding procedures;
  • Training information - description of how system works.

The Fast-Track ABS Troubleshooter cartridge, which comes with an ABS diagnostic reference manual, and four manuals covering GM, Chrysler, Ford and Jeep, sells for around $1,500 and is available through Snap-on distributors. For more information, call 1-800-424-7226.

Information on the Fast-Track Troubleshooter was provided by Snap-on.

Editor's Note: There are other scan tools that can read and clear ABS fault codes. Scan tools offered by Autodiagnosis, Inc., Edge Diagnostic Systems, Fluke Corporation, OTC Division of SPX Corporation, Thexton Manufacturing and Vetronix Corporation are profiled in the ABS Tester Buyer's Guide which ran in the March 1997 issue of BRAKE & FRONT END. This article focuses on three dedicated ABS testers that allow bidirectional communication between the ABS module and scan tool.

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