This technical case study was submitted by Brad Halse from B&L Mechanical Services. Thanks Brad!
"This week I had a 2011 Toyota Coaster bus in from another shop that had the ' check engine' & 'ABS' lights on. They also couldn't communicate with their scan tool.
With the G-scan connected, the ABS system had about eight codes present. After replacing a faulty LHR wheel speed sensor & repairing a poorly contacting harness connector near the ABS module (found by load-testing power & ground sides) the system was back up and running.
Now to the ECU communications, my first test is always to check resistance across CAN high & low and should show 60 ohms.
This test revealed 120 ohms indicating one of the terminating resistors was open circuit.
With a diagram from the Toyota Australia Portal I found one BUS was connecting the ECU and Turbo Control Actuator module.
When I tried to remove the plug from the TCA (Turbo Control Actuator) the wiring simply pulled out from the back of the plug, bingo!! Turns out the turbo had been removed at some stage to repair an exhaust leak.
To reconnect the wiring to the plug, they 'soldered' the wires to the crimps (and did a very poor job of it). With the wires removed from the TCA (Turbo Control Actuator), I cycled the ignition & now had communications with the ECU.
With a new plug from Toyota & four replacement female terminals from an old Toyota harness, it now had full communication, no codes & was now showing 60 ohms across the CAN BUS."
Brad also kindly forwarded a before and after trace of the CAN circuits from his picoscope as shown below.
CAN HI & LO (Fault Present)
CAN HI & LO (After Repair)
Mechanic Top Tip
It's important to remember that the terminating resistors in a CAN network can reside within a control unit or in the harness!
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