J1939 Termination Resistance Drift: The 4.7 Ohm Hot vs Cold Difference That Kills Communication
A 2021 Kenworth T680 rolled into a Midwest fleet shop with a CAN bus fault that had already burned five days of diagnostic time. The truck had a PACCAR MX-13, 187,000 miles on the odometer, and a cab-side termination assembly that still looked factory-fresh. The shop had swapped the ECU, the instrument cluster, and the […]
Why a 50 MHz Oscilloscope Misses CAN Bus Glitches (and What a 100 MHz Scope Reveals)
In March, a North American commercial vehicle ECU supplier sent us a 42-second capture. The bus ran 500 kbit/s J1939, twelve nodes, 8.2 meters of twisted pair, with 120 Ω termination at both ends. By the time we saw it, the customer had already replaced two transceivers and rewritten the driver. The 50 MHz oscilloscope […]
Single-Ended vs Differential ADC in Automotive ECUs: Why Your MAP Sensor Reading Is Lying to You
I remember the exact bench setup. It was a humid Tuesday in July, and we had a customer’s ECU on the harness simulator. The firmware team was chasing a ghost—a manifold absolute pressure reading that wandered by twelve kilopascals at idle. Twelve kPa. On a naturally aspirated engine, that’s the difference between a smooth idle and a stumble that […]
Pressure Washing Your Harvester? Why IP67 Connectors Are the Minimum Standard for Forestry CAN Networks
I have watched a harvester operator finish a shift, open the side door, and point a hot-water pressure washer straight at the valve block. He was not doing anything careless by forestry standards. He was cleaning the machine. The CAN bus did not go down immediately. It went down the next morning, then came back after the sun warmed the connector […]
J1939 Termination Resistor Drift in Extreme Temperatures: Measurement Protocol for Forestry Applications
A few years ago, I spent a week at a logging operation in northern British Columbia. The mornings were around -38°C, and every day the same harvester would throw CAN bus errors until the cab warmed up. After three days of swapping ECUs, checking wiring harnesses, and reloading software, we finally measured the terminating resistor with the connector cold-soaked. […]
Forestry CAN Bus Diagnostics: A Field Engineer’s Step-by-Step Guide to SPN 639 FMI 9 Troubleshooting
It was 14°F and the snow was coming down in thick, wet clumps somewhere in a spruce block north of Thunder Bay. I was standing on the track of a 2018 Ponsse Ergo harvester that had decided to park itself mid-cycle. The head wouldn’t feed. The dash showed one active fault: SPN 639 FMI 9. The operator, a […]