What specific role does the CMP signal play?
06-08-2013, 08:21 AM
#1
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What specific role does the CMP signal play? Not sure where to put this, so feel free to move it.
I'm putting together my own LT1 24x conversion, using my own crank trigger wheel. The car runs, but I haven't finished the cam sensor reluctor and housing yet, so I have no camshaft position signal, and am obviously using only CKP as of right now.
My question is, what does the PCM use the CMP signal for, exactly? I know it helps with quicker starts, and I've heard that without it the injectors are batched fired and fuel mileage suffers. Also read somewhere that ignition timing isn't as precise. Is this true? Are there any other side affects from running a failed (or in my case, missing) cam sensor?
I don't plan on running it without CMP long term, I'm really more curious as to what exactly the signal is used for.
Thanks for looking!
I'm putting together my own LT1 24x conversion, using my own crank trigger wheel. The car runs, but I haven't finished the cam sensor reluctor and housing yet, so I have no camshaft position signal, and am obviously using only CKP as of right now.
My question is, what does the PCM use the CMP signal for, exactly? I know it helps with quicker starts, and I've heard that without it the injectors are batched fired and fuel mileage suffers. Also read somewhere that ignition timing isn't as precise. Is this true? Are there any other side affects from running a failed (or in my case, missing) cam sensor?
I don't plan on running it without CMP long term, I'm really more curious as to what exactly the signal is used for.
Thanks for looking!
06-24-2013, 06:00 PM
#3
ckp tells position of the crank and counts off. cmp tells it when to start counting.
ex: no1 cyl is on tdc but on what stroke? intake or power? cmp will confirm
which one it is.
ex: no1 cyl is on tdc but on what stroke? intake or power? cmp will confirm
which one it is.
06-25-2013, 03:02 PM
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So then if there's no CMP signal then the pcm must go into some "wasted spark" mode, where it fires the plug just before the intake stroke, and just before the power stroke?
06-30-2013, 11:16 PM
#5
If there is no CMP signal while starting, the LS1 PCM will guess at the correct setting (1 chance in 2, or 50%). If the engine responds (rpm increases), the guess was right and the PCM uses this setting from then on. If the engine does not respond with an increase in RPM, the guess was wrong and the PCM uses the opposite setting. After the engine is running, the CMP is not needed and nothing is affected.
07-08-2013, 08:16 AM
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Thanks for the response, that makes more sense. I finally got hptuners running and changed the IFR so that the car doesn't flood itself, and it runs surprisingly good! I must say it's tempting to run it just the way it is, considering all the work that will go into making a trigger wheel and sensor housing.
