Trailer Diagnostics: Warning Lights

Trailer Diagnostics: Warning Lights

CVW speaks to Drakefield’s Barry Craven about a common trailer diagnostics issue involving warning lights.

Nearly every ABS job I go to, I hear the same thing: ‘I don’t know what the warning light should do.’ Now let’s make this simple; there are only three main light sequences that you really need to be concerned with and they are:

  • ON/OFF at standstill.
  • ON until the vehicle is driven above 7kph then OFF.
  • ON/OFF/ON until the vehicle is driven above 7kph then OFF

With the invention of programmable warning lights in EBS systems, it has become a bit harder to know what yours should be doing, but with a couple of simple tests you can find out.

If you have an ABS socket on both your truck and trailer, start by just connecting this and turn on the ignition. The trailer warning light in the cab should come on. If the light goes ON/OFF then that is the sequence, if the light goes ON/OFF/ON then that is the sequence and if the light just goes ON then drive the truck and trailer above 7kph, if it goes OFF then that is your sequence.

NOTE: If, however, the light stays ON after a test drive then there is a fault in the system and you will have to fix it before you can go any further.

Trailer light

Most new trucks now have an LCD or electronic dash board and it can prove a little tricky to establish which light is actually for the trailer. With each manufacturer having its own type of light, it can be quite confusing, so what I do is use the ISOCHECK lead. This lead is designed to not only test the power from your truck, but it can also be used to show you the warning light function of your trailer.

First, plug the ISOCHECK into the truck’s ABS socket, then turn on the ignition, which will then test all of the power from the truck. If it is all OK then the ISOCHECK will light up GREEN and if there are any faults in the socket the ISOCHECK will light up RED.

Once you have good power from the truck, turn the lead around and connect the ISOCHECK to the trailer’s ABS socket. The warning light in the cab should then come on. At the same time, the ISOCHECK will turn RED to let you know the light is on. If the warning light then goes out, the ISOCHECK will turn back to GREEN. This not only proves you have good power from the truck, it also proves the trailers ABS is working.

The next step is to test drive the trailer, and if the warning light comes on while moving, the ISOCHECK will turn RED. Stop the trailer and remove the ISOCHECK from the ABS socket on the trailer. If the ISOCHECK stays RED the fault is on the truck, if it goes back to GREEN the fault is on the trailer. It’s as simple as that. Now your drivers can tell you where the fault is.

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