Abs & ebs problems

this is this months article about iso7638 sockets ( ABS sockets)

ISO 7638

Yet again this week I have done several jobs where ive been asked the same question over and over again, how does the ABS lead work?
Everyone seems to have there own ideas how it works ranging from if it’s not plugged in your trailer brakes wont work to it’s only a back up power anyway!! Both wrong!

The truth is the ABS lead or iso 7638 as it is actually called is essential to make your trailer ABS work correctly,
and if you have the connections on your truck and trailer they must be connected and must be working correctly,
this also means the ABS light you should be looking at is in the cab and not the one on the trailer headboard.

This is where the problems start, how does the ABS lead actually work, well this is how:
Pin 1 in most trucks is permanent live
Pin 2 is switched ignition live
Pin 3 is common return to battery negative
Pin 4 is common return to battery negative

And most importantly and the most misunderstood is
Pin 5 which is the warning light control wire,
Some people think pin 5 is controlled by the trucks ABS system or even by the truck itself, both of these are wrong, how it actually works is quite simple, a basic way of explaining it is this:

Power from the battery via the ignition switch goes to a fuse then to the warning light in the cab then to pin 5, when you connect your ABS lead to the trailer the trailer ABS system sees 24 volts coming down pin 5 and then lowers that voltage inside the trailer ecu to below 5 volts which then brings the ABS light on in the cab, when you drive 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, then if a fault appears when you are driving the ecu lowers the voltage again bringing the light back on,(this is only a basic description of the warning light circuit).

An easy way to check the power from the iso socket is to Get a multimeter and test between:
Pin 1 & 4
Pin 2 & 3
Pin 4 & 5
You should have 3 lives and two earths.
Or you could use an iso tester (see pic) which will do all these tests for you, just plug the iso tester into the truck and look at the LED display on the side.

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 the same way,
It’s always controlled by the trailer ecu.

The next thing to mention is pin 6 and 7 in an EBS lead, these are the “CAN” connections “CAN” means control area network which in simple terms is a connection between two or more ecu’s allowing them to talk to each other,
when you have a EBS truck and a EBS trailer the braking is done through the “CAN” lines, basically 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 via the air pressure signal through the service line.
Some people have said to me that If you have a ABS truck and EBS trailer then the trailer brakes wont work properly, this is not the case, all it means is the truck cant send the “CAN” signal to the trailer, the brakes work as normal.

And the last thing to remember is when you’re driving the ABS light should be out.