Electronic Heartbeat?

“It’s a 2016 International and it was parked out back for the weekend when a storm blew in,” Ketchum Jessup said as we stood by the truck. “I was havin’ me a cup of coffee, watching the storm, when a bolt of lightning zapped my truck. Struck the mirror on the driver’s side and blew the glass out,” he said, gnawing on a plug of tobacco. “After that, the truck wouldn’t even crank over.”

I looked at the documentation of other repairs. Every module on the J1939 data link had been replaced: the ABS ECU, Auxiliary Gauge Switch Pack ECU, the Body Controller, the ECM, the Electronic Gauge Cluster and the Pyrometer Ammeter Module. On the second visit to the shop, the Body controller, the terminating resistors and the J1708 data link had been replaced.

“It ran for a little while and then the Engine Stop warning turned on,” Ketchum said. “So I had a mobile repair truck come out.”

I saw that the Body Controller was replaced again, Ignition 1 relay and the Auxiliary Power relay. The BC supplies ground to the auxiliary power relay when the theft deterrent is active. Then the key is ON, the auxiliary relay is energized and the relay opens. When the relay opens it allows power to be fed to the Ignition Relay 1, the Crank Inhibit relay and the ECM. If the auxiliary power relay is energized, the vehicle won’t run.

“Now the gauges tick-tock as if they were on a clock or heartbeat or something when the key shuts off. Sometimes it starts, sometimes it doesn’t,” he said.

I checked for recalls, campaigns, safety-related reports and the like, but nothing came up during my research. I checked connections at the battery as a standard procedure. I checked all the fuses.

“Those are all basic checks,” Ketchum said.

“Exactly,” I responded. I used my inspection camera mad carefully checked all ECU connections, including the door pods. I visually inspected all ECU grounds. I was going over all worked performed by the other shops. You have to. The one thing that you fail to check will be the thing that gets you.

When I decided to check the terminating resistors, that’s when my ohmmeter readings were all over the place, as if I had current flow after everything powered down. I disconnected the battery and checked the terminating resistors. Individually, they measured 120Ω, which was good. But when I checked the J1939 backbone, my ohmmeter indicated OL.

 I reconnected the battery and started the truck. It ran fine for a few minutes and then the Engine Stop light appeared. I turned the key off and the gauges would jump periodically. I disconnected the battery and measured the resistance of the J1939 backbone again. Again it returned OL. Was the data bus open?

I manually ranged my digital meter to the 4KΩ scale. I measured the bus again and the resistance slowly climbed until it returned OL on the display. I reversed the meter leads at the diagnostic link connector, and the reading on the ohmmeter red a negative resistance, counting down to 0.00Ω. It reversed polarity and counted up again, until reaching OL. Now I knew what was happening.

My meter’s 9V battery was charging a capacitor somewhere on the circuit. When I reversed the lead connections, the capacitor discharged and then charged up again. Once the voltage reached the same value on each side of the dielectric, the capacitor was fully charged. The meter displayed OL. I decided to unplug each ECU, one at a time. I was also cross-checking each number. The fourth ECU I disconnected caused the odd reading stop.

Modern heavy-duty trucks can be described as rolling computer networks with seemingly endless possibilities for data collection and transmission. Many different devices found on trucks have the ability of collecting and storing this data when an event occurs. These devices are collectively referred to as Heavy Vehicle Event Data Recorders (HVEDRs). In most cases, however, an HVEDR is not a device but a software application added to an existing device that has an original purpose other than collecting and storing incident data.

Data collected and stored by HVEDRs can be useful when analyzing a truck crash. The most useful data comes from trip/event data recorders on engine Electronic Control Modules (ECMs) and wireless fleet management systems (a.k.a. tracking/ communication systems, mobile resource management systems, and telematics systems). These systems can record vehicle speed, brake usage, vehicle-use histograms, vehicle position history, active and historic diagnostic trouble codes, and more. An endless number of devices used on trucks that have a HVEDR function and no two devices record the same data. Therefore, an HVEDR is like a Christmas present where you really don't know what you're going to get until you open it. You may think you know what is in the box and sometimes you are right but often it is a surprise.

This truck went to a bodybuilder at some point. The ECU was added that acted as a dedicated HVEDR. It must have contained a capacitor that charged and discharged. I opened the ECU. I found a choke that was charging a capacitor due to a bad conductive trace so I removed the cap, repaired the trace, installed a new cap, and checked it again. The capacitor was now functioning as designed.

Ketchum held the cap in his hand. That little thing called all this grief?”

“To you and me both,” I said.

I'm pointing to the cap.