Believe it or not, there was a time when the mere idea of fuel injection very nearly caused a riot in Jeep's ranks. Jeep, said the faithful, had always been about rugged simplicity, DIY fixes with hacksaws and duct tape, and electronics that could be replaced with parts from a lawnmower. That all changed going into the 1980s, and it didn't take long for Jeep's mud-encrusted contingent to realize that electronic fuel injection wasn't just tolerable off-road, it was better. Fuel-injected vehicles could climb and descend grades that carbureted ones couldn't, and started every time owners turned the key. Granted, some still bemoaned that electronic systems didn't work well while submerged -- but there's just no making some people happy.
Have the new ECM programmed for your vehicle's identification number and mileage. Many automotive repair shops, automotive parts stores, and dealerships have the specific tools necessary to reprogram the ECM for a fee. Most can do it when you buy the new ECM, so have that information with you.
Open the hood of the 1998 Jeep Grand Cherokee. Loosen the 10-mm nut on the negative battery cable pinch clamp and pry the battery cable clamp free from the battery. Move the battery cable away from the post to avoid contact while working on the vehicle.
Siphon the coolant reservoir bottle empty with the vacuum-operated fluid evacuator. The coolant reservoir bottle attaches to the passenger-side fender near the rear of the engine compartment.
Remove the hose from the bottom of the coolant reservoir bottle. Using a 10 mm socket, remove the bolts attaching the coolant reservoir bottle to the fender. Remove the coolant reservoir bottle.
Disconnect the three square electrical connectors from the ECM. The ECM is located on the firewall, directly behind where the coolant reservoir bottle sat. Pinch the tabs to release and pull directly outwards on the connectors to disengage the harness.
Remove the three 8 mm bolts retaining the ECM to the firewall and remove the ECM.
Slide the replacement ECM into place. Install the three 8 mm bolts and tighten them hand-tight, or to about 5 foot-pounds, with a 1/4-inch ratchet or torque wrench. Plug in the three ECM harness plugs.
Slide the coolant reservoir bottle back into place. Install the 10 mm retaining bolts, and tighten the bolts just snug. Re-attach the coolant reservoir hose. Refill the coolant reservoir with the antifreeze that was siphoned out.
Reinstall the battery cable. Start the engine. Close the hood. Drive the vehicle for several miles to allow the ECM to relearn adaptive engine and transmission control.
