2001 Cavalier Lacks Spark, PulsePosted 8/11/2005
By Kevin Caple
I'll probably never see one like this again. The car was a 2001 Cavalier that had quit running and was towed in. It had no spark or injector pulse.
The customer had burned out the starter trying to get it started. The tech had the starter rebuilt and reinstalled. He had run through the no-start flow charts, had tried a crank sensor, an ignition module with coils and a different powertrain control module (PCM). Luckily, the technician had a co-worker with the same car and that allowed him the luxury of swapping parts on the problem vehicle with known good parts.
The tech had already ohm-tested the wires from the crank sensor to the ignition module, the module to the PCM, and from the PCM back to the ignition module. He verified the powers and grounds were good to the PCM and ignition module, and that the crank sensor was producing adequate alternating current (AC) voltage.
On this car, the crank sensor sends an analog AC voltage signal to the ignition module with evenly spaced pulses except for one closely spaced signal that indicates where the firing sequence begins. The module converts the AC signal to a digital square-wave signal and sends it to the PCM, and the PCM uses it to signal when to fire the injectors. The PCM sends a digital square wave back to the module on two different wires telling the module which coils to fire and when.
With no spark or pulse, the tech and I focused on three main components: the crank sensor, the ignition module and the PCM. The connecting wires were double-checked. This was not making any sense. We went back to square one and retested all four plug wires for any spark on any wire. Oddly enough, the tech got one spark each time he let off the key.
With the starter engaged, we were greatly stressing the powers and grounds. With that thought in mind, I wondered if we could be losing ground or dropping power when cranking. If the tech let off the starter, was there enough rpm to still make one spark intermittently? With the help of an assistant, he retested all the powers and grounds while the starter was engaged. The powers and grounds tested fine. He even ran redundant grounds and the condition still existed!
The tech did not have a lab scope to look at the crank signal. A scope would have been ideal to see if we were missing the sync pulse on the crank sensor. But as the saying goes, "You fix cars with the tools you have, not with the tools you want." When the tech tested the crank sensor disconnected from the module (no load), the wires tested OK. Although the sensor worked in the co-worker's car, what if the air gap to the crank was just a little different or maybe there was a reluctor issue?
The tech backprobed his AC voltmeter across the two crank sensor wires, and then cranked it. For the first time (over the phone) I could hear the engine turn over. It turned over slower than I would have expected, and the starter sounded labored. I had the tech hook up the battery charger but it made little difference. I asked him to pull the plugs to see if the remanufactured starter would spin the engine freely; it did and now there was spark while cranking. Even cranking slow, there was ample AC voltage from the crank sensor. So I asked him to make a shield for the crank sensor harness from a piece of tin foil, and ground it with a jumper wire to keep electromagnetic interference (EMI) from influencing the crank signal to the module. After he shielded the harness, there was good spark with the plugs installed - no start, but good spark. Further investigation found the timing chain had jumped, which caused the engine to quit and caused the hard crank condition. This in turn caused excessive current draw by the starter, causing EMI interference on the crank sensor and resulting in a no-spark condition.
As technicians, knowing how things are supposed to work lets us perform logical tests in a logical order. We know these tests will be conclusive even when they are used to determine what's not wrong with a particular system.
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