24X/1X Signals to the PCM

[DavidBraley]

I wasn't sure where on the forum to post this question, so hopefully this place is OK. I have a potentially really dumb question, so please be kind...

I'm trying to understand the reason why the Gen 3 Ls 24X crankshaft, 1X camshaft signals to the PCM are separate from each other. The camshaft is tied to the crankshaft through the timing chain. I also know the engine is what is called an interference valve-train fit (meaning that if the timing chain breaks, the motor is in big trouble fast with pistons hitting open valves). Because of this, the PCM has no way of protecting the engine from a camshaft that suddenly stops.

I can completely understand why the crankshaft position sensor and camshaft position sensor would be need to be separate in an engine with variable valve event timing.

I read somewhere that the reason was because the PCM would then be able to detect misfires. This made no sense to me. Is there that much slop in the timing chain to cause the cam to bounce back and forth between a couple of degrees relative to the crankshaft during a misfire that the PCM detects it? If so, I'm impressed with the GM engineers...

[Gampy]

Stroke detection ... I'm sure there are more/other reasons.

[delcowizzid]

It's so it knows where number 1 cylinder is in the firing order on the power stroke so it can inject at the right time and fire the coil on the correct cylinder. Without it you would only be able to run waste spark and batch fire fueling not full sequential

[DavidBraley]

It's so it knows where number 1 cylinder is in the firing order on the power stroke so it can inject at the right time and fire the coil on the correct cylinder. Without it you would only be able to run waste spark and batch fire fueling not full sequential

Now that makes perfect sense to me after looking at the timing diagram I recently found below. For the first 360 degrees of crankshaft rotation, the camshaft sensor sends a high signal to the PCM. The second 360 degrees of crankshaft rotation the camshaft sensor transitions to a low signal sent to the PCM. On that falling edge of the camshaft sensor from high to low, the PCM knows the #1 cylinder is at top-dead-center on the compression stroke and is ready to be fired. Except, the PCM actually fires #1 just before the high to low transition from the camshaft sensor, but I'm guessing that's all calculated internally as ignition advance.

I can see now why these two signals most definitely need to be separate.

Thank you!!!

CKP_CMP_waveform_relationship_001.jpg (84.61 KiB) Viewed 2245 times