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Dave's RV-10 Build Log
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Date:  7-17-2019
Number of Hours:  7.25
Manual Reference:  
Brief Description:  CAN Bus Trouble Shooting & Repair

Removed all of the LRU connectors with CAN Bus connections, then removed the terminators at both ends of the CAN Bus. Checked continuity between CAN Hi and Can Lo, with was open circuit so no wires were crossed at a node. Checked continuity of CAN Hi between bus ends and found a good circuit (near zero ohms). Checked CAN Lo and found the same thing. Checked CAN Hi for continuity to ground and found that it was grounded, so that is the problem. Checked CAN Lo to ground which was open circuit so good.

To find the location of the CAN Hi short in a logical manner, I cut out the CAN node in the middle of the bus at the MFD connector which is easy to reach. Then checked for the short in CAN Hi from each end. The short was still present at GSU 25, but absent at the GMU 11 end, so the short is between the MFD and GSU.

Cut out the node in the middle between GSU 25 and the MFD, at PFD-1 since that is easy to reach. Again checked for the short to ground in CAN Hi, and found it was on the GSU side of the bus, cleared from the MFD side. So the short is in either GSU 25, GMA 245, or GAD 29.

Started by removing the back shell on the GAD 29 connector, since it is the easiest of the 3 to access. Checked for the CAN Hi ground and it had disappeared. Pressed on the CAN node in the connector and the ground re-appeared. So the GAD 29 node has the fault. Cut out this node, and checked the CAN Hi at the GSU and it was no longer shorted to ground, confirming this was the bad node.

Re-constructed new nodes to replace the 3 nodes I cut out, checking for grounds and continuity as each node was replaced. Re-assembled all of the connectors, and re-installed the connectors to all the LRUs, with PFD-1 installed but MFD and PFD-2 still disconnected and out of the plane.

Powered up the system, and all the LRUs were now communicating over the CAN Bus with zero errors. Problem Solved!

Looked closely at the defective node cut out, and found a small cut in the CAN Hi insulation only visible using a magnifying glass.
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