An Oldie but a Goodie?

 

Talk about shaking the cobwebs out from deep in your memory, Tyler had a mint-condition, 1987 “C” car with a 3.8 sequential fuel injected engine and C3I (Computer Controlled Coil Ignition) system. Everything about the car was beautiful—including the misfire, occasional stalling and backfire.

“It happened shortly after we rebuilt the engine,” Tyler said. We replaced the injectors, the ECM, the crank sensor once and the cam sensor twice. Oh—and the MAF sensor. That didn’t correct the issues and even the C3I ignition module.”

“Were they all OEM parts?”

“They were known good parts taken off a couple of engines,” Tyler said.

I wrote that down.

“Any suggestions?” He asked.

“Yeah—it probably isn’t an injector, ECM, crank or cam sensor, ignition module or MAF,” I said. “And I doubt if you have a bad plug wire,” I said, noticing a new set on the car. When you think about it, my job is relatively easy. I only get called after a lot of components were replaced and systems have been checked. But sometimes, you just must walk away from a vehicle that has been causing grief to you. It happens to all of us.

They checked the grounds, power feeds, battery, fuses and did a visual inspection. “There just isn’t that much scan tool data to look at for help,” Tyler said. “There are no misfire counters or anything.”

He was right. On-board diagnostics back in 1987 were sparse as were the data lists available. “I used a spark-tester tool, everything fired,” he said. “I also used an injector tester noid light.

“I even checked to make sure that the ECM and C3I module were at the same ground level,” he said.

Over lunch, I remembered my history. The purple/white wire provided a digital input from the ignition control module to the PCM that is used to calculate RPM and crankshaft position. The PCM compares pulses on this circuit to any that are on the Reference Low circuit, ignoring any pulses that appear on both (passive filtering). Since this was a Sequential Fuel Injected engine, once the PCM saw a certain RPM, a single injector would be triggered every 120-degrees of crankshaft, in the firing order.

The ground that Tyler mentioned is a PCM input or more correctly a ground “offset” circuit connected between the PCM and the ignition module. This is a low reference circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the ignition control module. Although this circuit is electrically connected to the PCM, it is not connected to ground at the PCM. The PCM compares voltage pulses appearing on the IGN reference input to those on this reference low circuit, ignoring pulses that appear on both, which is likely due to EMI (aka “noise”) thus allowing this interference to be filtered out and ignored.

The tan/black wire was the bypass line. Above a certain RPM, the PCM applies the 5V bypass to connect the Electronic Spark Timing circuit to the coil primary control field-effect transistors.

Once 5V has been picked up by the IGN control module it will “switch” to EST or Ignition Control (IC) mode and connect this signal to the primary FET circuits controlling each waste-spark coil pack.

When I got back to his place I decided to use my oscilloscope connected to the brown/white wire at the cam sensor which goes into a buffer amplifier in the ignition module. The signal had a fair share of noise on it. I observed that the signal was going low between the normal pulses and it was assumed to be an extra low pulse as interpreted by the C3I module. The module ended up re-sequencing its coil firing order around the false pulse, resulting in the coil now firing 120-degrees BTDC. My inspection camera revealed that the insulation was shaved off from the cam sensor signal wire under the power steering pump. I pointed it out to Tyler.

“Wow—was there ever a time when you got stumped?”

“You know, Tyler, the only thing more dangerous than ignorance is arrogance. I’ve made a ton of mistakes and I’ve learned from each and every one.

This was a perfect example of following a diagnostic process along with an understanding of the fundamental circuits and what they do.

Had Tyler’s shop performed a thorough visual inspection, there is no doubt in my mind that he would have found it.

“I’m thinking about teaching part-time,” Tyler said, “Any tips on how to teach young adults?”

Don’t teach them,” I said, “Instead, why don’t you provide the right conditions in which they can learn?”

Tyler shrugged his shoulders. “But I don’t have a teaching degree,” He said.

“You’ve got experience, right, Tyler? Well, actual experience creates knowledge that is both respected and valued by others. The experiences you have in life provide the best lessons for others. The experience that you have is rich in knowledge and that is the most compelling and authentic way to make a difference in the lives of your students. And that advice is priceless.”

“I wish your bill was charge-less!”