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ABS Troubleshooting for Trucks, Trailers, and Buses


How does the ABS lead work?

Yet again this week I have done several jobs where I've been asked the same question over and over again, namely, "How does the ABS lead work?"

Everyone seems to have their own opinion as to how it works, ranging from, "If it's not plugged in, your trailer brakes won't work", to, "It's only there to provide back up power anyway". Both incorrect.

The truth is the ABS lead, or ISO 7638 to give it it's proper title, is essential to making your trailer ABS function correctly. If you have the connections on your truck and trailer, they must be connected, and must be working correctly. You should also be looking at the ABS light in the cab, and not the one on the trailer headboard.

Remember your pin

So, how does the ABS lead actually work? Below is a diagram showing the pin numbering system used on the ISO 7638, and below it a table describing the functionality of each pin.


Description / Function


In most trucks is permanent live


Is switched ignition live


Is common return to battery negative


Is common return to battery negative


The most important - is the warning light control wire

How pin 5 works

It is often thought that pin 5 is controlled by the truck's ABS system, or, even by the truck itself, both of these are incorrect.

How pin 5 actually works is quite simple: Power from the battery via the ignition switch goes to a fuse, then to the warning light in the cab, and then on to pin 5. When the ABS lead is connected to the trailer, the trailer ABS system reads 24 volts coming down pin 5 and then lowers the voltage inside the trailer ecu to below 5 volts (or to ground in some cases), which then brings the ABS light on in the cab. When the vehicle is driven away, and the trailer ABS system is happy with the wheel speed signals, the ECU then raises the voltage back up to 24 volts and the warning light in the cab goes out. If a fault appears when the vehicle is driven, the ECU lowers the voltage again bringing the light back on.

Check the power

An easy way to check the power from the iso socket is to use a multimeter and test between:

Pins: 1 and 4
Pins: 2 and 3
Pins: 4 and 5

You should have three lives and two earths. Alternatively, you could use an ISO tester such as the Drakefield DRA07T to do all these tests for you. In which case, you merely have to plug the DRA07T into the truck, and read the LED display on the side of the unit.

Most new trucks don't actually have a warning light as such, instead the dash is an LCD display which shows a picture of a trailer, or trailer warning, but pin 5 still works in the same way. pin 5 is always controlled by the trailer ECU.

Pins 6 and 7

The next things to mention are pins 6 and 7 in an EBS lead. These are the "CAN" connections, "CAN" stands for Control Area Network, which in simple terms is a connection between two or more ECUs allowing them to talk to each other.

When you have an EBS truck and an EBS trailer, the braking is done through the CAN lines. When you press the brake, the truck sends a CAN signal to the trailer ECU to tell the brakes to apply, if the CAN signal fails, the brakes will still work using the air pressure signal through the service line.

Some people have asked me, "If you have an ABS truck and EBS trailer, do the trailer brakes still work properly?" The answer is yes; all it means is the truck is unable to send the CAN signal to the trailer, the brakes, however, will work as normal.

Finally, the last thing to mention is that the ABS light should be out.

What industry people are saying...


“I recommend the SensorMaster for time saving and accurate diagnostic readings. This saves both time and money to my company and my customers.”

— J. Lane, Lane Trailer Services


“Overall, we have found the SensorMaster to be very useful, especially in a time saving capacity.”

— CVW Review, May 2007


“For ease of use, speed, and accuracy of results, you'll find the SensorMaster hard to beat as an ABS diagnostic tool”

— P. Edwards, Swedish Truck . . . . Parts


“A worthwhile addition to any commercial vehicle workshop, especially for the accurate diagnosis of sensor related problems.”

— M. Toghill, Knorr-Bremse


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