Multiplex Wiring Troubleshooting on a School Bus

Mechanics Hub - July 30, 2014

Whenever we had wiring problems it was a hunt down from the beginning but once you find good information showing you how to carry out a step by step process it's much easier to be successful to find the fault. We were working on a Thomas C2 conventional school bus at the time and what happened was a break down in communication between the ABS Module and the rest of the system.

The communication network is called the J1939 which is 2 wires that transmit inputs, outputs and signals between all modules that control ABS brakes, transmission, lights, heater motors and switches. The J1939 is lightning fast and since it runs through the “backbone” as the they call it.... any kind of interruption will cause a failure for certain circuits to work properly.

There is a way to check the circuits and it's easier than you think. With our ABS module not communicating the first thing to check is the resistance of the J1939 wiring. They are a pair of wires yellow and green that are both battery and ground that run parallel to each module in the bus electrical system. When you disconnect the main battery and check any J1939 connector you should get 60 ohms.

If you do not get 60 ohms there is a problem with either the wiring or one of the terminating resistors at both ends of the J1939 harness. They are there to minimize reflection of data.... if one fails the resistance will read 120 ohms. The reason 60 ohms is the desired reading is because both resistors are connected in parallel which signifies they are both healthy.

So what we did is check the resistance at the diagnostic link (where you hook up the laptop under the dash) at pin C and D. We got 116 ohms so there was a problem in the multiplex wiring. Every connector going to each module read the same result so the first thing to do was to check the 2 terminating resistors. The proper reading of each resistor by itself is supposed to be 120 ohms until they are plugged into the system.

We finally hunted down along the frame and found one end of the J1939 resistor connector. There was one problem resistor to be found. It must have fallen out of the plug somewhere along the way which caused the ABS light on the dash to stay on which of course disabled the entire ABS brake system. The D.O.T boys don't like that scenario so it had to be repaired immediately. All that had to be done was to insert a new terminating resistor and right away the multiplex system was back to normal at 60 ohms resistance.