Easy Money Impossible

 

 

"I've checked everything and everything checks out fine," Darrell said about the pickup truck. "It came in for a misfire, according to the customer; but the fact is, the torque converter clutch keeps cycling intermittently."

 

When I get an email, I research technical service bulletins and other documents, and Safercar.gov and for programming updates. I may look at online forums as well. I found nothing on the truck. "The truck has been around to several top shops in the area," Darrell said. "The transmission was replaced three times, the Powertrain Control Module was replaced three times, the coolant sensor was replaced twice, as was the throttle position sensor and mass airflow sensor. And new plug wires were installed."

 

"Were they all OEM new parts?" I asked. "Every one of them, yes."

 

"Good," I said, "At least you've ruled out a lot of possible causes." I wrote it all down in my pad. "You said it's intermittent. Tell me about it," I said, asking all the questions. Darrell said engine temperature didn't matter. Ambient temperature made no difference. Neither did rainy weather. It happened mostly under steady cruise conditions on the highway. "The thermostat was also replaced," Darrell said. I wrote it all down. I did a visual inspection for obvious conditions and wrote my findings down. I checked all fuses and ran a ground credibility check. I wrote all of my findings down. There was an old MAF sensor DTC. A road test did not duplicate the concern. By this time I decided to go to lunch. During lunch, I typically look over all the notes that I took, reviewing what I've learned. I showed Darrell what my visual inspection found.

 

"You know what EMI is, right?" Darrell nodded. "Electromagnetic Interference," he said.

 

"Inductive coupling takes place with magnetic fields. Current flowing in one conductor will create a magnetic field (left hand rule or right hand rule, dependent upon which hand you use to write with) that then induces current flow on a nearby conductor. Then you have conductive coupling.

If something creates a high frequency pulse, such as a loose battery cable, the current pulse caused by the connection will be seen on the wires. Capacitive coupling is another way."

 

"I wish I paid more attention in school," Darrell said. "I remember the instructor said something about it. Please explain."

 

"When a conductor has alternating current flowing within it, a potential difference will exist between it and any nearby conductors. This creates an electric field, or capacitive coupling. The wiring will see this potential difference and current flow will occur."

 

After lunch, we went back to the shop and road tested the truck again. This time, it acted up. While watching the TCC PWM solenoid, we saw that the duty cycle was bouncing all over the place as the TCC cycled on and off. "The PCM is commanding the TCC on and off. It can't be another bad PCM--can it?"

 

Remember what the visual inspection caught? The wiring harness moved too close to the spark plug wiring after the clip that held it in place unlatched. EMI cot into the TCC wiring, confusing the PCM.

 

"I guess they clip those wiring harnesses for a reason," Darrell said.

 

"I guess they do," I replied. "EMI stands for Easy Money Impossible."