Antilock Brake System (ABS) Indicator Illuminates When Power Takeoff (PTO) is Engaged (Replace Electronic Brake Control Module EBCM))

This bulletin is being revised to include additional information. Please discard Corporate Bulletin Number  02-05-27-001 (Section 05 - Brakes).

Some customers or dealers may comment that the ABS indicator illuminates when PTO is engaged. The vehicles affected by this condition mainly operate as stationary support units with the engine at fast idle and the PTO engaged.

Effective with the initial release of the Bendix® EC-17 ABS ECU, a diagnostic feature was incorporated referred to as an "All-Sensors Backed-Out" condition. This condition occurs when the ECU detects an indication of road speed from another vehicle system (an engine or transmission, for example) while receiving no wheel speed input from any of the installed ABS wheel speed sensors. This causes the ECU to log sensor faults on individual wheels (known as "Sensor Start" faults). The fact that the ECU has logged "Sensor Start" faults on all wheels at the same time, based on a road speed broadcast from another vehicle electronic module, causes the ECU to make a diagnostic judgement that all ABS wheel speed sensors are backed-out (from the exciter ring).

The ECU receives this road speed information via one of the serial communications links (J1587 or J1939). The ECU only conducts this test once per power cycle. Once the ECU detects valid wheel speed during a power cycle, it will no longer test for this condition for the remainder of the power cycle.

The "All-Sensors Backed Out" diagnostic has been implemented to detect conditions where excessive air gap exists on all wheel speed sensors (for example, a mechanic performs a brake job and fails to push the sensors in). This condition would lead to illumination of the ABS indicator lamp, indicating loss of ABS functionality.

When a vehicle is placed in the PTO mode, the engine will drive an auxiliary device (such as a pump) rather than the drive wheels. If the vehicle is wired in such a manner that the engine or transmission incorrectly supplies an indication of road speed to the ABS ECU (when in fact the vehicle is stationary), the ABS ECU will diagnose an "All-Sensors Backed Out" condition. This situation is caused by the placement of the vehicle road speed sensor (engine or transmission) in a way that it will generate an indication of road speed when the vehicle is stationary in the PTO mode. No vehicle system should broadcast an indication of road speed when the vehicle is stationary.

When the ECU enters the PTO mode, the ABS ECU sets the "All-Sensors Backed-Out" condition, illuminating the ABS indicator lamp. Although certain earlier versions of ABS ECU's would latch on the ABS indicator lamp in this instance (after three occurrences), requiring the ECU to be reset, the current release of the EC-30 ECU will automatically clear the sensor faults and turn off the indicator lamp when the vehicle is returned to normal operation (taken out of the PTO mode) and driven (causing the ECU to detect valid wheel speeds). These sensor faults are never latched (require a reset).

Four new Bendix® EC-30 EBCM part numbers have been released to correct this condition. The new modules will not prevent the illumination of the ABS indicator lamp when the vehicle is placed into the PTO mode, but the indicator lamp will turn off upon return to normal, non-PTO operation. If diagnosis leads to the EBCM as the cause, replace the EBCM on the affected vehicle. Refer to SI Document ID #725298 for EBCM replacement procedures. Refer to the Parts Information table below for vehicle and parts usage.

Effective March 1, 2001, all towing vehicles must control an in-cab trailer ABS warning lamp. Trailers built after this date will transmit the status of the trailer ABS unit over the power line carrier to the towing vehicle or tractor. Special wiring and a connector are provided with the applicable module part number for this purpose.

The trailer ABS ECU broadcasts the PLC signal. The Bendix® EC-30 EBCM supports PLC communications in accordance with SAE 2497.

*This column represents the number of teeth on the wheel speed sensor tone ring on the front axle.

Parts are currently available from GMSPO.

For vehicles repaired under warranty, use: