Dave MacEslin

I continue to trouble shot and learn about the many sensors built into a car to meet today's emmison standards. My 99 JGC keeps stalling and has a code for crankshaft position sensor that I had a garage replace (I don't know what kind they used) and I still have my problem. So I am looking at CPS. I have the AUTOTAP diagnostic tool which gives me lots of great information. The tool shows both fuel trims maxed out with only 11 inches of mercury so it looks like I have a vacuum leak but I can not understand why this would pop a code for CKPS. The tool tells me the CPS and CKPS edge counts. First what is an edge counter? The CPS edge count builds up to around 90 and then drops to almost 0 instantly in a repeatable pattern. The CKPS fluctuates slightly but remains pretty constant. Why would they be reporting different types of data. How can I use this data to help me diagnose my issue. I will check for vacuum leaks this weekend

Dave

dave1123

The cam sensor determines fuel injector timing and is adjustable by the PCM within a certain range. If it's not within that range, the PCM has to guess and varies back and forth from rich to lean, hoping to find the right setting. Both the crank sensor and cam sensor count index notches or windows in their respective systems to figure out what the engine is doing. There are windows in the flexplate that disrupts the magnetic field for the crank sensor and notches in the cam sensor rotor to signal which cylinder is supposed to fire and when. You'll have to see the FSM for the setting procedures for the cam sensor, but the data that will come up is a + or - value. Turn the sensor housing until it reads 0. If the crank sensor isn't a Mopar, the magnetic field created may not be strong enough for it the send the proper signal.

97grand4.0

X2 on what Dave said. I will add the "edge count" term, I believe refers to the actual edge of the waveform as viewed on a scope, as in rising and falling edges, or leading and lagging edges, of the square waves generated by the cam and crank sensors.
You have a count rising and falling to 0 which suggests to me that the two are not in sync with each other, meaning one or the other is not happening when it should.
From my experience with both sensors, one, neither one is very reliable at setting the code and two; the code itself can refer to either sensor.

Note that the Grand cherokee crank sensors and cam sensors are different from the Cherokee sensors. If you had the problem and they replaced the crank sensor, and the problem continues, then I would look to the cam sensor. And I would put in a junkyard one first and maybe finally before laying out $100 somewhere. You are fortunate it threw that code at all. This definitely means one or the other sensor, though. As Dave said, Mopar cps and ckps sensors use much stronger magnets and generate a better signal. There are videos online demonstrating this. I have had the cam sensors from AZ, new distributors, etc, fail in no time. However I think that just made it not start, not stall out like you're experiencing. Check voltages to the sensors while cranking...maybe pcm

dave1123

I've never seen a failing crank sensor throw a code except after it totally failed. However cam sensors do all the time. I like the idea of trying a junkyard cam sensor to see if it makes a difference. It's just 2 screws to change one. Don't pull the drive or you'll be retiming it.

97grand4.0

Yeah the cam sensors from aftermarket aren't worth the paper they're printed on, even worse than the crank sensors IMO.
I actually have a crank sensor from autozone that was installed before I learned of the magnetism, and knock on wood it's still fine in the xj.

dave1123

Well the cam sensor in his 99 is quite different from the one in your 97, but still, you are right.