why a cam sensor on 2.5 DOHC or any other

[jim milewski]

is it to check for jumped belt/tooth on valve timing?

[johnceggleston]

is it to check for jumped belt/tooth on valve timing?

 

there are lobes on the back side of the driver side cam sprocket. the cam sensor ''senses'' them as they rotate past. the ecu uses the signal from the cam sensor to trigger the fuel injection.

 

the same is true for the crank sensor and crank sprocket. but they trigger the spark plugs to fire.

 

if the cam or the crank jumps a tooth or two, the fuel and spark do not occur at the correct ''time'', either too early or too late. same for the valves since they operate with the cam sprockets.

Edited September 30, 2011 by johnceggleston

[idosubaru]

ecu uses the signal from the cam sensor to trigger the fuel injection......crank sensor and crank sprocket. but they trigger the spark plugs to fire.

 

i've wondered this before too - why two sensors? the cam and crank are fixed in relation to each other via a fixed timing belt so why not use one sensor? i can sort of fabricate an explanation in my mind....

[Nug]

They have two for a very fundamental reason. The crank sensor tells the ecu when to fire the plugs, what the rpm is, etc. But the engine needs two rotations to complete a single power stroke. For example, if the #1 piston is compressing its fuel air mix, the plug should fire when the piston reaches the end of its stroke. But it's confused, so instead, the plug is told to fire at the top of the exhaust stroke.

 

It needs a method of deciphering which stroke is correct. The cams turn at half speed, so if the ecu knows to look for a cam pulse at a certain time in relation to a crank pulse, it knows where it's at.

 

Wow, that was a convoluted, terrible answer. Sorry.

[Nug]

Once the engine is started, the cam sensor can be unplugged and it will continue to run. The ecu needs to compare the time between the two signals before it can initiate fuel injection and ignition.

 

Pretty sure.

[johnceggleston]

Wow, that was a convoluted, terrible answer. Sorry.

 

but your point is correct. the crank rotates twice for every one rotation of the cam. so if the fuel injection ''lobes'' were on the crank, the ecu wouldn't know which rotation is was actually on.

 

and if the spark ''lobes'' were on the cam ..... well maybe that would work. unless it was just too complicated . there probably needs to have some distance in between the lobes.

 

and having 2 sensors on one sprocket just seems silly.

Edited October 1, 2011 by johnceggleston

[Fairtax4me]

They have two for a very fundamental reason. The crank sensor tells the ecu when to fire the plugs, what the rpm is, etc. But the engine needs two rotations to complete a single power stroke. For example, if the #1 piston is compressing its fuel air mix, the plug should fire when the piston reaches the end of its stroke. But it's confused, so instead, the plug is told to fire at the top of the exhaust stroke.

 

It needs a method of deciphering which stroke is correct. The cams turn at half speed, so if the ecu knows to look for a cam pulse at a certain time in relation to a crank pulse, it knows where it's at.

 

Wow, that was a convoluted, terrible answer. Sorry.

 

What about the "Wasted Spark" cars that have the one coil pack in the middle? They fire both plugs at the same time. One is at the top of the compression stroke while the other is at the bottom of the exhaust stroke.

[idosubaru]

right on, i see it now with the two crank rotations per cam rotation. thanks.

[jim milewski]

those were good explanations, not convuluded at all, but i think the wasted spark holds true as there are two leads per coil. Fuel injection on my sons IROC is pretty unique, two pulses every 720 degrees, one on the back of the valve, the other when it's open

[kanurys]

Also, since each of 2 coils runs 2 cylinders, one spark per coil is wasted each crank rotation. Because of the 2 coil system it can fire every crank rotation and be triggered off of the crank. These engines might need 4 coils or another method to trigger the spark from the cam sensor. (multiply by 2?)

[987687]

Also, since each of 2 coils runs 2 cylinders, one spark per coil is wasted each crank rotation. Because of the 2 coil system it can fire every crank rotation and be triggered off of the crank. These engines might need 4 coils or another method to trigger the spark from the cam sensor. (multiply by 2?)

 

Well, the turbo engines actually do have a coil for each plug.

 

But in the case of wasted spark system, yes, it can fire every rotation. But that's not the issue. In terms of that, you don't need a cam sensor. BUT, as others have stated, you still need the cam sensor for fuel injection.

Since there is one injector per cylinder, unlike spark, the computer needs to know if TDC means intake or exhaust stroke.

 

And this brings me back to the turbo motors with coil on plug. These technically shouldn't need a crank sensor, right? Since the fuel and ignition need to be exact, and nothing is wasted, it should all be able to be sensed off the cam. For RPM, just multiply that signal by two.

[kanurys]

Thanks for clarifying. That's kind of what I was trying to get at in a more convoluted way of speaking. I didn't know that COP system was on the turbos, though.

[1-3-2-4]

Once the engine is started, the cam sensor can be unplugged and it will continue to run. The ecu needs to compare the time between the two signals before it can initiate fuel injection and ignition.

 

Pretty sure.

 

On what car is this? As soon as I pull my cam sensor plug the car dies.