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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: Fault 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 Modal 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 each modulator solenoid separately once. This operation will be audible as an exhaust of air from the valve. If more than one modulator is fitted, the exhaust noise from each valve will be separated by appproximately 1 second.

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. (MODAL 4S/3M configuration shown).


System Diagnostics

Diagnostic Display Unit (DDU)



The ECU has microcomputer controlled fault detection circuits which log faults to a stored memory that may be accessed via an onboard fault display (the TODD) or with the use of a hand held diagnostic display unit (the DDU) see above diagram.

The electrical connections to the ECU include power supplies, sensors, modulators (valve(s)) and the diagnostic output. These connections are made using female terminals located in "plug on" connector housings which are coded with a coloured identity band to match the corresponding colour coding of the ECU terminal groups.


For further information refer to the Configurations and Wiring Diagrams for the Haldex ABS Modal system.

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.

Haldex ABS Modal System Configurations


S - Refers to a sensor.
M - Refers to a modulator

S1A/S1B, S2A/S2B and S3A/S3B identify the left and right hand of sensor pairs and the order in which they are installed on the trailer.

Consider sitting on axle 1 with axle 2 behind you, then sensor group "A" will always be to your left and sensor group "B" will always be to your right. You will note that for this to be true you will be facing forwards for full trailers and backwards for semi or centre axle trailers.

The configuration reference indicates component quantities in the system. Hence a 6S/3M system will have 6 Sensors and 3 Modulators. The sensor identity is relevant to the position in which its' output is displayed on the TODD or DDU, when checking sensor function. The sensor identity varies between different configurations dependant on the number of axles and sensor pairs fitted.

NOTE: These identities should not be confused with axle numbers, which differ between full trailers and semi or centre axle trailers.

ECU Refers to the electronic control unit of the MODAL ABS system.

Axle numbering:

Semi-Trailer and centre axle trailer axles are numbered from the rear axle forwards hence axle 1 is the rear axle.

Full trailers (Drawbars) have their axles numbered from the front of the trailer rearwards so that the front axle is number 1. This numbering system caters for self steering axles which will always be number 1. If axle 1 is a self steer axle it must always be controlled by the RED modulator channel operating on a “select low” principal, i.e. the ABS channel controlling that axle reacts to the first locking wheel and controls both wheels of the axle to that level of tyre to road grip.

It is not possible to provide individual wheel control on self steer axles as split friction conditions would result in vehicle instability due to the differing tyre to road grip across the axle, which would cause it to self steer.



Each modulator channel is colour coded as follows.


The RED channel sensing is always from sensors identified as S1A and S1B which will also be colour coded RED at the ECU connection. This channel provides sense low operation as independant wheel control ABS cannot be applied to self steer axles. ECU connector colour code is red.

The BLUE channel sensing is always from sensor(s) identified as S2A or S2A and S3A together: ECU connector colour code is blue.

The YELLOW channel sensing is always from sensor(s) identified as S2B or S2B and S3B together: ECU connector colour code is yellow.

MODAL ABS is capable of controlling 1, 2 or 3 channels of braking effort sensed by 2, 4 or 6 sensors and may therefore be installed in a range of different configurations as 2S1M, 2S2M, 4S2M, 4S3M or 6S3M depending on the type of trailer and the degree of ABS control specified by the operator or trailer manufacturer. The number of control channels is determined by the number of modulator(s) electrically connected to the ECU. The table below shows how to determine the number of controlled channels for your installation.


Modulator cable . . . . . . . . . . . . . . .Number
of ABS connected to ECU: . . . . . . . .control
Red (RD) only . . . . . . . . . . . . . . . . 1
Yellow (YE) & Blue (BU) only . . . . . .. 2
Red (RD) Yellow (YE) Blue (BU) . . . ._3

The number of wheel speed sensors connected to the ECU may be 2, 4 or 6 but the combination of modulator solenoid cables and wheel speed sensor cables will always be as shown in the table below.


Modulator cable . . . . . . . . . . . . . . .Sensors
is connected to: . . . . . . . . . . . . . . .connected
Red (RD) . . . . . . . . . . . . . . . . . .S1A and S1B
Blue (BU) . . . . .. . . .. . . S2A and possibly S3A
Yellow (YE) . . . . . . . . . . S2B and possibly S3B



Haldex ABS Modal System Configuration Codes


NOTE:- Where sensed axles are also lift axles, the sensors will be disabled when the axle is lifted, these are indicated in parentheses in the above diagram.

In this event the configuration and wiring diagrams should be checked against your installation to ensure that the actual pipework and wiring conforms to the allowable configuration details.


When the ECU is initially powered up, a system configuration code is briefly displayed on the TODD or DDU, which should be confirmed as correct for the particular installation, by referring to the configuration codes listed as C0 to C9 in the diagram above.

If a vehicle has its modulator piped incorrectly, solenoid wiring interchanged between solenoids, or sensor wiring interchanged between sensors then it is possible that a correct configuration code may be displayed but that poor ABS performance is evident. This will occur due to incorrect cross connection of sensors and modulators at the ECU.

When the ECU receives a locking wheel signal it attempts control of that wheel, but if sensors or modulators are incorrectly connected either electrically or in the case of the modulator pneumatically, the wrong wheel will be controlled, resulting in poor ABS and brake performance. In such circumstances the MODAL system may generate a fault code indicating slow wheel recovery group i.e. A fault code number such as, 41, 42, or 43. (see Fault Code List)


In the event of needing to identify the configuration, match your trailer details to one shown in Fig 8.4. i.e. Type of trailer, No.of axles, No.of modulators fitted,and No.of sensed wheels. (see also Figs 8.1 and 8.2)

Having determined this information, Figs 8.5 to 8.10 can be used to determine the configuration layout applicable to your trailer.

Having determined your trailer layout, refer to the corresponding section diagram from Figs 8.11 to 8.15 which show the correct checkout procedure in the order:

1. Lamp sequence.
2. Modulator blowdown check
3. Sensor output indication on TODD or DDU

when a wheel or wheels are rotated.

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