Hello All! 1997 XJ. I have been struggling with wiring issues. I have purchased a new ECM (forgive me if my jargon is amateur...I am an amateur with a desire to get deeper into trouble! LOL!) The code is PO121. The jeep has gremlins but I have been systematically rebuilding the wiring harness...I could be the problem, I know.... I have been testing wiring back with an Ohms reader and this is my conclusion: MAP sensor connector, when testing the Brown-Yellow back to the ECM main connector (Series A) returns this result. Pin A4 returns good tone but when I check the other pins, just to be thorough, I get a reading of 67.7 on Pin 26. I feel that this is the issue for the rough idle and senseless shifting at freeway speeds due to the crankshaft position sensor. Thoughts...and thanks for your time. See pix.

MAP sensor connector

ECM A series Pin connector

Ohms readings

 

Here are the pinouts for the 2 connectors for additional reference. Note that the sensor return (circuit K167) is a ground common to most of the sensors and passes through a number of connectors and splices so it a good thing you have tone. However, according to the chart and your pic, pin 26 is not used. Pin A27 (circuit K1) goes to the middle pin on the MAP. Pin A17 provides the 5 volt reference to all the sensors that require it. Same thing about multiple connectors and splices.