School Bus J1939 Electrical Fault

We happened to have an electrical fault in one of our International buses. The dash registered a large yellow triangle which is supposed to get the driver’s attention and also a message on the dash panel which said “electrical fault”. If you get lucky like we did it was something to do with the ABS system because the warning lamp for that system was lit up as well. Here is a technical over view of J1939

Getting a reading with the Meritor Wabco software was not successful so obviously there was a communication problem. When ever that occurs we automatically check the wiring resistance at the J1939 twisted pair of wires that are in the diagnostic link harness. They are a green and yellow set of wires that are twisted together and run the whole length of the chassis harness connecting to each electronic control module on the bus.

Those modules are the engine, transmission, antilock brake system, body control modules and also the Instrument control panel. The inputs and outputs are transmitted at lightning speed through out the entire network. For instance when the driver wants to turn on the headlights he clicks on the headlamp switch. That signal goes to the body control module in this case an International bus which makes this command happen through out the network.

If there is a resistance in the J1939 wiring all hell breaks loose. The inputs and outputs don’t jive with the modules and strange things start to happen. Testing these two wires with an ohm meter is the easiest step to take since it can be done at the 9 pin diagnostic link under the dash. The resistance has to be 60 ohms if not there is a wiring problem or a bad terminating resistor.

The terminating resistors (one at each end of the J1939 harness) keeps the circuit intact. They should read 120 ohms when removed from the harness for testing. One resistor could also be missing since there is always one hanging around somewhere in the frame usually close to the module furthest back from the engine. In our case we tested the J1939 wires and got a high resistance of 118 ohms and eventually found a corroded connector going to the rear terminating resistor which caused the excessive resistance.

The terminating resistor was wrapped around the main wiring harness and taped up close to the anti lock brake module which was furthest to the back. It’s a bit of a hunt down for bad connections, wiring and possiblly a bad terminating resistor but with the wrong resistance in the J1939 system you’re going to have trouble in the way of a communication break down between modules.