One of the tricks to swapping in a 4.0L is figuring out how to give the computer a vehicle speed signal (VSS), especially if the recipient is equipped with a mechanical speedometer and you want both to work. The solution is to nab one of these gizmos at your local wrecking yard, which retains the mechanical drive for the speedo but also has the necessary sensor for the computer built-in. Renix-injected Cherokees and YJs allegedly have them, but we didn’t have any luck finding the sensor on either one. We nabbed this one, plus a spare from an early ’90s Dakota, at the local Pick-a-Part. You’ll need to splice in the sensor’s connector (make sure you grab it), but the color codes from the 4.0L harness to the connector will likely match. We also needed a 7/8-inch to 3/4-inch adapter to hook up the CJ speedometer cable, which we sourced from a speedometer specialty shop online for under $15.