Just FYI, I stole the following from a very reputable Cummins source and this was specifically for a XJ build, but....
"For what it's worth, the way we wired our black TJ was to keep the stock PCM and use the Cummins tachometer signal wire connected to the jeep's cam position signal. This is the signal that excited the PCM to think there is a running 4.0L in it. From there, the factory dash is reading engine speed from Crank Position Sensor on bellhousing and driving the dash tach with that.
If you're not running the 4L flywheel, then you may need to make a tone ring to mimic the tooth count and put it on the front of the Cummins crank and mount the 4L crank position sender to that for your dash tach. I'm guessing you could also splice the Cummins tach signal to both give input into the cam sensor and crank sensor but from what I'm reading you may need an interface for as the 4L crank may be looking for 3 pulses per rev whereas the 2.5L jeep engine looks for 4... Not sure if the 4Cyl PCM is plug and play with your Jeep's harness...
From our manual:
"If the user chooses to integrate the factory tachometer, the R2.8 CM2220 R101B wiring harness includes a tachometer output signal wire. The signal wire (white wire, labeled Tach, pin 9) exists in one of two states, source or sink. The ECM toggles the state rapidly to produce a square wave with a variable frequency and a fixed nominal duty cycle of 50 percent. Four pulses are produced for each crankshaft revolution, for an output frequency (Hz), per the following formula: • Engine rpm x 4 pulses per revolution x 1 minute per 60 seconds
There is a minimum allowable frequency for the square-wave, which corresponds to a lower threshold of the engine speed which can be reported to the tachometer. The ECM provides a single pin output interface for the tachometer output with the following requirements in the table below. Item Requirement Operational States Two: SOURCE and SINK Modes One: pulse width modulation (PWM) Minimum Source Voltage (@Maximum Source Current) 6.0 volts
Item Requirement Maximum Source Current 5 mA Minimum Load Resistance 1100 ohms Maximum Sink Voltage (@Maximum Sink Current) 0.5 volts Maximum Sink Current 5Ma Frequency Range 2-8500Hz" "