Timing Reference?

[scottyb]

Where does the computer get it's timing reference?

My assumption is that it gets it from the crank sensor. I figure since the motor will run after unplugging the cam sensor, then it only uses the cam sensor to determine #1 TDC on startup. Cam and crank need to be in sync for correct firing sequence, but crank is used one the sequence is established.

OR, does it adjust the crank refererence based on the cam sensor at startup? In other words, if you remove and re-install the cam sensor with a 3 degree change, does this change carry over to the timing?

 

[TurboDave]

Your first assumption is pretty much correct. Timing reference is taken from the crank sensor. The cam sensor and crank sensor work in unison during start up to determine correct injector sequencing.

So altering the cam sensor wont have any affect on ignition timing.

 

[ijames]

The ecm uses the cam sensor for injector timing and continuously resyncs the crank sensor with the cam sensor for ignition timing, so if the engine is running and you unplug the cam sensor the ecm will set a code and drop into batch injection mode and keep running. However, if the engine is running and you turn the cam sensor too far the ecm will resync to an incorrect cylinder and the timing will jump 120 degrees and the engine will miss/pop/die/whatever (not just theory; I've seen it with a timing light).

 

[scottyb]

I took the cam sensor out the other day to assess the excessive noise. Before I removed it, I marked it and noted it's position. After I was done, I just popped it back in and eye-balled the mark without going thru the set procedure. If I get my mark off by a bit, does it change the timing? Does it just need to be "in the ballpark" to sync to the crank, and the crank sensor/chip controls the timing regardless if the cam sensor is changed by a few derees?

It runs fine. I plan to order a new cam sensor because of the noise.

 

[ijames]

If the engine cranks and goes through the rpm range without a hiccup then it's close enough for ignition purposes - you are firing the right cylinder at the right time controlled by the crank sensor and aren't close enough to the window edge to ever slip into the next window. You might notice a slight change in idle quality, gas mileage, and emissions if the cam sensor is off because of the injector timing, but then again some people have driven for years with the sensor 180 degrees out and never noticed anything (this works because of the waste spark ignition system).