24x crank sensor

bought the 97 lumina used and it had a few problems but I managed to straighten out all but 1.The computer puts the code for the 24x crank sensor all the time.I had it on the scanner and the rpm for the 24x is allways out of wack(sometimes reading 40,000 rpms).I have changed the sensor and the harmonic balancer with no luck.I did the wire test to check the PCM with the scanner and everytime I put 12v on the signal wire the rpm would jump.I jumped all the crank sensor wires to the PCM with jumpers and still same problem.Last thing I tried was a used PCM from another 97 lumina and the MIL was off for about 1/2 hour then came back on(havent had a chance to get it back on the scanner yet)but now after about 2 mins of run time the battery light will come on in the dash although the car is charging fine,old pcm it dosent come on but other pcm makes the car run alot better.I had stalling with original pcm but the replacement the car dosent stall.Anybody with some new suggestion to try would be greatly appreciated.

Are you sure your charging system is working correctly? I think I've heard before that if a voltage regualtor or an alternator was going whack, that it would cause all the sensors not to get enough voltage to operate correctly, and send false readings. Correct me on this Jeff. Just a thought.

40,000/24x = 1670 rpm?

PCM grounds check OK?

Is the signal wire supposed to carry 12v?

Did you try swapping chips in the PCMs and see what happens?

charging is ok I have checked this its above 13.5v.RPM rang should be around 700-800 same as the idle and the only removable chip in the pcm is the knock module and I did change it.Grounds I didnt check but the ground would have to be in the wiring harness(no ground strap or wire to the actual case and its mounted in plastic),I will check grounds next.I had to install the original PCM again because the other didnt run good at highway speeds.So now the battery light is ok again but the stalling is back.Funny thing is the stalling is really bad in reverse and rarely happens in drive and it didnt happen with the replacement PCM.Guess Im looking at getting a new PCM.

Ok. Seen grounds do some really bizarre things. Ran across a couple that wouldn't run right unless the scanner was hooked up, then everything showed fine, remove it, and they'd stall. Turned out the PCM was pulling a ground through the loop created by the scanner being plugged in.

Hate electronics. Actually not, always a challenge. My motto is verify circuit integrity before assuming anything, and avoid all the tail-chasing.

well I found the trouble It was the ground connections under the rad support.Seems they used a crimp connector to connect several grounds together and they were corroded.Had to replace some of the wire but now the MIL is out and the car runs smooth.