Alien Makes and Models

“This one is serious, RJ,” the owner of a 2014 Kenworth said. “The EGR was non-functional, no DEF injection, reduced engine torque along with the MIL and STOP ENGINE light. Where do you start?”

The truck was a 2014 Kenworth, W900 Cummins ISX 15 CM 2350 truck. This one had a fault code 175 SPN 3464 FMI 3. The code indicated that the signal coming out of the throttle actuator is high. Considering that the throttle actuator was replaced three times, I strongly suspected it wasn’t the fault. I checked the terminals at the actuator and the ECM with my 40x magnifier, 25mm.—great for spotting terminal fretting.

 

We started by verifying the concern, followed by a thorough visual inspection. Research revealed no bulletins, although the Kenworth had a work history. The ECM was replaced and programmed. The throttle actuator was replaced three times, and a terminating resistor was replaced. I decided to decode the CAN bus packets, although I strongly suspected where the problem was—at the terminator that was replaced. I wanted to see how the data packets looked.

“What are we looking at? T.D. asked.”

“It is a Stuff Error, T.D. There is an issue with the terminating resistor that they replaced.”

This bit stuffing scheme is used to guarantee enough edges in the bit stream to maintain synchronization within a frame. If a listening device detects 6 bits of the same value, then it must have been the case that synchronization was lost and what was received was NOT what was sent! This is precisely what can happen with improper termination. Some bits in the frame are transmitted and received correctly, but the entire frame (and the integrity of all the bits in that frame) is not maintained and received at the listening CAN interface. 

Whoever worked on it before disconnected a 120Ω resistor, removed the other one and added two 120Ω resistors in series at one end. I didn’t know why. We repaired that issue.

Someone replaced the throttle actuator three times for a reason. My oscilloscope revealed AC noise to the neighborhood of 700 mV and that is a huge neighborhood.  The ECM was sending a regulated voltage signal out—dirty with AC noise of about 650 mV AC P-P. It was time to check the power feeds going into the ECM. Bingo!  The ECM power feed had about 500 mV AC ripple (the battery acts as a dampener, smoothing out AC current so a bad battery can cause strange symptoms in a vehicle). I had that condition on an Impala once. I checked the alternator with my scope: 1.2 volts AC!

Finally, the ECM ground had excessive voltage drop. After all the repairs, the Kenworth ran great. T.D. was happy to get it back on the road.

“Sorry you didn’t see any green aliens when you were here,” T.D. laughed.

“Did you ever see a UFO, T.D.?”

“No, but others around here have. There has been reports of saucers, cigar-shaped UFO, triangle-shaped, diamond-shaped and round ones,” He said. “We have a fair share of them.”

“You know what has always bothered me about the different shapes? I’ve always wondered if aliens have a choice of buying different models,” I said.

“What do you mean, RJ?”

“Well, we can buy an SUV, or a compact, or a sports car—and we can buy a Ford, Chrysler or GM; or imports. Judging by all the different UFOs, I just assumed that the aliens have choices of what to fly to our planet, T.D.”

He just shook his head.