Chrysler Repair: Diagnostic trouble code P 1398, crankshaft bearings, crank sensor

Question
Hi Roland,
My 96 Grand Cherokee got rejected because of check engine light and code P1398. Did soe research, means "crank position sensor" probably defective. The car runs perfect and gets good MPG, so it's probably BS. Do you know anything about this?

Answer
Hi Brian,
The code does not mean specifically that the sensor is defective. It is that the system in the pcm that checks for misfires is not able to take into account machining differences between the cylinders(which causes variations in the rpm [that it calculates]so that it can't truly measure misfires by variations in rpm,which is what it needs to do to detect and to reduce emissions).
The possible causes are:
position sensor improperly installed (you will want to get a new spacer paper for the tip of the sensor, then reinstall it with the tip placed against the flex plate... the paper self destructs but establishes the interface spacing that is necessary for proper operation. Maybe a dealer will have such a spacer),

variation between the sensor target and the flexplate that it looks at (due to damage to slots in plate or foreign material),

severly worn crankshaft bearings,

wiring connector to sensor,

low compression in one or more cylinders,

faulty sensor,

faulty pcm.
So you might want to run thru those items in order to see if you can find something before replacing the sensor. If none can be found, then replacing the crank sensor may be the thing to try. You can erase the old code by disconnecting the battery for a few minutes. Do that after you try a correction so as to be sure that you aren't looking at the old code.
Roland