91 YJ engine swap 2.5 to 4.0-PCM question

[Mudtug]

Hi All,
I have a 91 YJ that currently has a 4” lift, 33” tires. It came with a 2,5L,Ax-15 and NP231 running 4:11 gears in the Dana 30 front and 35 rear.

i want to swap the engine and tranny out with a 4L i6 and AX15 (reusing the Np231). The engine I have is out of a 97 TJ. I understand the engine mounts will change. Like my exhaust will change as well obviously.i have the powertrain wiring harness for the 97 (cam pos sensor, crank position sensor, speedo, etc.).

The question is; will the PCM /ECM from the 97 TJ work with the other two body harnesses? I know the TJ uses a digital 2 wire CCD to run all the gauges on the dashboard, pretty sure the 91 does not use that same technology. OR do I need a PCM from a YJ series jeep?

the YJ is 30 years old, so no emissions inspections are required, thankfully.
Would appreciate some input from the mechanical gurus on this forum.

thank you,
Mike

 

[machoheadgames]

No, 97 PCM will not work, at least not without a decent amount of wiring work but I believe you'd lose the tach regardless.

The 91 was an odd year in that it went fuel injection but kept the older style gauges that 87-90 had. 92 released a new set of all electronic gauges. The YJ uses SCI, not CCD, but I believe the only gauge it affects is the tach. Not sure if the 91 tach was updated to be run off the PCM or not. The 87-90 tachs were driven by the ignition coil if I'm remembering right.

91 Speedo is driven directly a speedo cable, the speedo cable attaches to a hybrid speed sensor at the transfer case which has two wires coming off it, those two work to provide a speed reading to the computer which helps with setting the idle for given conditions like coming to a stop, etc.

92-95 speedos are driven by a splice directly off of the speed sensor wire that feeds the PCM speed input.

To get it to work on the 97 PCM, you would need to rework the TJ engine harness to connect to the YJ body harness. The TJ body harness would not be fun to make work on the YJ body because of different routing, different locations for things, etc.

The only way I know how to make a YJ go OBDII and retain its tach gauge, is to get wiring and PCM out of a 96 XJ which was OBDII but still SCI. Then it would run the tach, and of course the other gauges all still work because they were already working independent of SCI anyways. You could also go aftermarket gauges, but that's a whole 'nother thing I'm not going to get into. But the tach is the big one in question, nothing else relies on PCM.

Best way to do this would be stay with OBDI, get the 4.0 PCM and wiring harness, move the motor mounts, and just plug in the new harness. Of course all the other stuff like radiator hoses, fan, shroud, etc are all a given no matter what.