Electronic Control
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Engine Computer (ECU)

The '87-'90 4.0L engines were run by Renix (Renault-Bendix) engine controllers, the '91-'95 engines were run by Single Board Engine Controllers (SBEC), and the '96 and later engines were run by Jeep Truck Engine Controllers (JTEC). The JTEC is the only controller that can be reprogrammed. The others require you to physically change a chip which involves unsoldering and resoldering.

JET performance manufacture a Power Control Module for '91-'95 OBD I Jeeps and for '96-'06 OBD II Jeeps that piggybacks between your stock ECU and wiring harness. It intercepts the stock codes and replaces them with performance versions. Stage 1 is for stock engines, Stage 2 is for engines with a low restriction exhaust and 180 degree thermostat. They claim a 10hp/18lbft gain at the rear wheels on a Jeep Wrangler 4.0L automatic. In reality performance gains do not live up to the hype and there have been mixed reviews about this product.

Hypertech have the Max Energy Power Programmer for the 1996-04 Jeep Wrangler, Cherokee, Grand Cherokee 4.0L. It carries part no. 52500 and is available at Summit Racing. They claim a 12hp/13lbft gain (premium fuel) from this product but in addition, it has an adjustable rev limiter, adjustable top speed limiter, and reads diagnostic trouble codes. It also stores the OEM programming and will restore it if you wish to use the Programmer on another vehicle.

The Unichip is an in-line, state-of-the-art computer upgrade that tunes stock and custom engine components to perform to their potential, safely and efficiently. The plug and play unit '05-'06 TJ is added on to your existing PCM, requires no wiring, and can be preprogrammed for your custom needs. This unit will correct pinging on MPI applications as well as increase performance and fuel mileage. It also comes with an optional software and cable kit that allows new programs to be emailed and installed by the user using a laptop computer, eliminating costly shipping charges.

Chip manufacturers make wild claims of big horsepower/torque gains by installing their performance chips on normally-aspirated EFI engines. This simply doesn't happen in the real world. At best, small HP/TQ gains are seen on the dyno. Some chips even cause a HP/TQ loss! The main benefits of performance chips seem to be elimination of the top speed governor, raised rev-limiter, and harder gear shifts in automatic transmissions.

Leigh Performance MAP Adjuster

This device adjusts the injector pulse width by modifying the output voltage from the manifold absolute pressure (MAP) sensor to the ECU. Increasing the pulse width adds more fuel and eliminates the surging and bucking of lean-running modified EFI Jeep 4.0 engines. Matches fuel curve to airflow changes such as high-flow TB, ported head, header, longer duration camshaft. Easy Installation; Includes Module and connectors with pre-wired splicing setup. If you're confident in your own electronic skills, you can copy Dino's homebrew MAP adjuster instead.

Adjustable Crank Position Sensor

You can make your stock CPS adjustable by slotting the bolt holes. This allows more ignition advance for increased horsepower and is particularly useful at high elevations where the engine is less sensitive to detonation due to the thinner air. Since naturally-aspirated engines produce approximately 3% less HP for every 1000ft elevation, the small gain in performance produced by this device is welcome.

Relocate MAT sensor

Relocating the Manifold Air Temperature (MAT) sensor to the cold air intake allows it to pick up ambient air temperature instead of intake manifold air temperature, which as much as 60*F higher. Lower temp. signals are sent to the ECU which then enriches the fuel/air mixture at WOT (wide open throttle) and slightly advances the ignition timing. The result is increased horsepower/torque if previous airflow-enhancing mods (intake, exhaust) have caused the engine to run lean. The benefit is greater in hot weather conditions.

Coolant Temperature Sensor (CTS)

The CTS is a device that monitors the coolant temperature. On the '91 and later Jeep 4.0L engine, it is mounted in the thermostat housing (see photo below). As the coolant temperature increases, the resistance of the sensor decreases, providing a varying output voltage to the ECU. The ECU uses information from the CTS to adjust air/fuel mixture, ignition timing, and to switch on the electric cooling fans.
The electric fans are programmed to switch on when the coolant temperature reaches ~105*C (~220*F) and off again when it falls back to ~95*C (~200*F). The ECU can be tricked into switching the fans on at ~100*C (~210*F) and off at ~85*C (~185*F) by splicing in a 4.0kohm resistor across the two wires going to the CTS. Click on the photo below for further details.

APEXi Super AFC NEO Fuel Computers 401-A018

Stroker engines or engines that have had a supercharger/turbocharger kit bolted on cannot rely on the stock engine computer alone to provide optimum air/fuel ratios under light load/full load conditions across the rpm range. One device that can deliver the goods is the A'PEXi Super AFC NEO Fuel Computer. The universal unit is suitable for the Jeep 4.0/stroker engine and carries part no. 401-A018. Aaron Clements (aka MrShoeboy) installed an earlier version of this unit on his 4.6L stroker-powered  '91 XJ with excellent results. Here's his write-up:

Apexi SAFC 2 Write Up

Split Second FTC1 Fuel/Timing Calibrator

The FTC1-201 is a plug-and-play Fuel/Timing Calibrator for 4.0L Jeep applications.   It controls an additional injector in order to provide fuel in boost.  Both the additional injector pulse width and timing retard are laptop programmable.  The unit comes pre-programmed with a base map for a typical supercharger application.   It also has an enrichment function that adds fuel in boost at part-throttle.  The FTC1-201 is compatible with the 3-plug, JTEC PCM modules found on 1997 through 2004 model years. The R4 software is included with this unit.