Diagnosing Trailer Brake Issues

Diagnosing Trailer Brake Issues

Eclipse Automotive Technology is a supplier of multi-brand diagnostics in the coach and bus industry. Here, the company demonstrates how its Test Pad Extreme tablet, together with Jaltest software, can diagnose a complex trailer brake fault.

The increase in embedded technology in commercial vehicles is well known, and has provided real benefits to vehicle operators over recent years. This same technology is increasingly used in trailers. This has led to trailers becoming more complex and requiring more advanced diagnostics to identify and correct faults. Here, Dave Rees, Technical Director of Eclipse, explains how the Eclipse Jaltest equipment is used to quickly identify defects on trailer systems.


A trailer has been brought into the workshop with braking problems that appear to be a result of imbalanced and locking up brakes. On inspection, there are no signs of any visible damage or mechanical failures that would cause this problem. A manual test is also carried out on the brake chambers to ensure that when the brakes are applied, the required pressure is being delivered.

Jaltest diagnostics

Jaltest is connected to the trailer to read the ECU memory and identify the cause of the pressure application fault. An initial diagnosis is carried out to check whether any current or historic faults are logged that may guide the engineer to the right area. A current fault is logged in the ECU’s memory that relates to a pressure sensor plausibility error. Using the Jaltest component information and freeze-frame data, it is possible to see when the error last occurred, instilling the technician with confidence that this is the error to be investigated.

Fig 1. Trailer pressure in an unloaded state with the footbrake applied.

Measurements, parameters and system checks

It is likely that the pressure sensor is the cause of the fault. However, due to the fact that it is an internal component and a replacement valve assembly would be very expensive, it is important to confirm this hypothesis. Using Jaltest to run a series of tests and live data readings, the pressures can be viewed to ensure that the right figures are being produced.

Fig.1 is an example of the trailer pressure in an unloaded state with the footbrake applied. The input pressure is correct, however, modulator 21 is reading 2 bar below the level at which it has been set.

To check the full range of the sensor’s capability, it is possible to perform a number of brake pressure simulation tests and checks. These simulate the minimum to maximum braking forces, as well as controlling the suspension percentage load, ensuring the pressures are adjusted accordingly. On the graph opposite, both pressure readings from output pressure 21 and 22 can be seen. Output 21 stops increasing at 3 bar, whilst output 22 continues to rise with the increase in pressure demand. This confirms, together with the fault code, that the sensor internal to the valve is faulty and the assembly needs to be replaced.

Jaltest Diagnostics can then be used to carry out the replacement procedure because it has the capability of copying the ECU parameters and pasting them onto the new control unit. Even if the new control unit has a non-compatible software variant, it is still possible to program the new ECU by manually inputting the trailer data using the information off the old ECU.

Below is an example data sheet from the Jaltest software, which can be used to program a new control unit:

With the new valve fitted and programmed using Jaltest, the technician can carry out an EOL (End Of Line Test) to verify that all the pressure readings are correct, and that the new ECU passes all the tests. Trailer programming can be a complex and involved procedure, and if the technician is unsure, then the new ECU could be programmed with incorrect data, making the trailer unsafe. The Eclipse Automotive Vehicle Support department understands the problems faced by the technicians carrying out this procedure and can guide them through the process.

Eclipse Automotive Vehicle Support

This service offers customers a telephone support service manned by professionals who have extensive diagnostic experience. They are backed up by databases of historic vehicle faults and manufacturer information that, together with their knowledge, enables them to guide technicians regarding diagnostic procedures. The support staff are able to link into the Eclipse tablet in the workshop or wherever the technician happens to be working on the vehicle.

For more information on Eclipse, click here.


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