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ABS Troubleshooting for Trucks, Trailers, and Buses


STEP 3: Blink code identification and system response

  1. If you already know how to read ABS Blink Codes from the ECU in question, please go straight to STEP 4: Blink code list
  2. If you are unsure how to read ABS Blink Codes from the ECU, start by familiarising yourself with diagram below, and then, where appropriate, follow the step by step procedure in the table
  3. Please ensure all necessary Health and Safety precautions are implemented before commencing any repair work

Haldex ABS Modular ECU



System Response

Notes / Comments



The warning lamp(s) function depends on which power supply is used:

RED warning lamp located on the driver's console of the towing vehicle is operated from the ISO 7638 (or ISO 3731 - 24S) power cable only when the ABS is powered by the ignition switch.

GREEN warning lamp mounted on the headboard of the trailer and is operated from the ISO 1185 (24N) connector when the ABS is powered only from the stop lamp power supply i.e. when the brake pedal is depressed.


On power up of the system, the warning lamp(s) must illuminate in the following sequence in order to show a fault-free system:

ON for 2.5 seconds = Bulb OK and system selfchecking.

OFF for 1 second = System self-checked and preparing to check sensors.

ON until moving = System waiting for vehicle to move above 10 km/h in order to check that the sensors are working.

OFF = Once the vehicle is moving above 10 km/h and the lamp(s) goes (go) out, the electronic system is fully checked.

Note: If a dedicated power source is available to the ABS from the ISO 7638 connector, or to the ISO 3731 (24S), then system integrity will be indicated by the RED cab mounted warning lamp which becomes the primary ABS status indicator, the GREEN trailer lamp becomes the secondary indicator and duplicates the RED cab mounted lamp function when the stop lamp circuit is operated.






During the self-check procedure, the system operates the modulator/s solenoid once. On MODULAR 1 this operation will be audible as one exhaust of air from the modulator (RED channel).

On MODULAR 2 one exhaust of air from each modulator in sequence will be audible (BLUE channel then YELLOW channel).

Once these two checks are made with correct results, no further checks are required.




If the results from STEPS 1 and 2 are not satisfactory, the DDU or INFO Centre should be used to establish the diagnosis.

This information is described visually in the diagram above. (MODULAR 1 system shown only)






The Modular ABS series incorporates a modified lamp flash of three flashes. The lamp will flash when the system is powered up when at rest. This lamp sequence relates to :-

1. Service due - indicates that trailer or system service distance is due. After servicing the trailer or system the next service distance must be reset by using the Info Centre or EOLT program.

2. NON ABS fault - A fault with a device connected to the ABS but not directly effecting the ABS, for example : Reset-to-Ride valve (COLAS).

3. Odometer scaling factor units mismatch - Where the odometer units (miles or km) do not match between service interval and current recording.

4. Code 37 - A diagnostic code sent to the ABS ECU to activate the multiple light sequence.



System Diagnostics

Diagnostic Display Unit (DDU)


An important feature of the MODULAR system is that it provides external diagnostics. The system displays a range of codes, which allow rapid diagnosis of any problem should one occur.

The diagnostic codes can be read from a portable Diagnostic Display Unit (DDU) or a fixed Diagnostic Unit (INFO CENTRE).


The DDU can be connected remotely via the loom assembly diagnostic connector socket mounted in an appropriate position on the trailer chassis as shown in the diagram above.

The DDU gives two types of display:

-Horizontal bars indicate the position of effective speed sensors when the wheels are rotated.

-A two digit display can be interpreted from the codes shown on in the Fault Code List


The INFO CENTRE is a vehicle mounted display permanently connected to the trailer ABS ECU. Typical locations of the diagnostic connector are shown in the diagram above.

The INFO CENTRE provides a multi-digit display of:

-Odometer indicates total, trip, service interval kilometres or miles defined by the ECU and Tyre scale setting.

-Fault codes which can be interpreted by referring to the Fault Code List.

-ABS ECU Information - Serial number, Product code and Configuration code.

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