Trucks, Jeeps

1997 and later Jeep Wrangler Powertrain

AMC 4-liter I-6 engine

The 4.0-liter OHV SMPI I-6 engine was extensively modified in 1996 to make it smoother and quieter than in 1995. It also has more torque at low and mid-range speeds. The broadened torque curve enhances standing start and low-speed performance without penalty to fuel economy. The result was 222 lb-ft of torque at 2800 rpm and 181 hp at 4,600 rpm.

Highway fuel economy increased by three miles per gallon with manual transmission and one mile per gallon with automatic due to lower wind and rolling resistance. City fuel economy is also two mph better with manual transmission compared to 1995.

Structural Components

Through FEA (Finite Element Analysis), the lower portions of the cylinder block have been strengthened and stiffened to reduce noise due to structural vibration. A main bearing brace provides major stiffening of the crankshaft support structure, also to reduce noise. The brace is a one-piece stamped steel "ladder" bolted to the bottoms of the main bearing caps.

An isolated cylinder head cover system blocks the transmission of valve train noise. There is no metal-to-metal contact between the cover and the head. The new cylinder head cover is stamped steel. A silicone bead gasket with a steel backbone seals the head-to-cover joint, reducing the potential for leaks. The cylinder head sealing surface is also redesigned to work with the new sealing system. Rubber grommets with compression limiters isolate the fasteners from the cover to block the passage of noise while providing ample clamping force for excellent sealing.

Dynamic Components

A new camshaft profile and new valve springs provide a substantial increase in low-speed torque and quieter valve train operation. Valve train noise is reduced by "softening" the opening and closing ramps on the cam to reduce valve closing velocity. The new cam profile also provides a smoother idle.

New lightweight cast aluminum pistons significantly reduce engine noise and vibration. They have contour-machined skirts that adapt to all operating temperatures, run quietly, maintain low oil consumption, and minimize the possibility of scuffing. New piston rings accompany the new pistons.

Engine Adaptation

A molded, dual-durometer duct delivers air from the air cleaner to the throttle body. The duct has a rigid center and soft accordion-pleated cuffs. Flexibility of the cuffs provides easy installation over the throttle body inlet and the air cleaner outlet and accommodates relative motion between these parts. The cuffs are clamped to the throttle body and the air cleaner to provide a tight seal that contributes to low induction noise and keeps dust from bypassing the air cleaner.

A new direct-mount power steering pump contributes to improved accessory belt life by reducing the belt span and providing more accurate alignment of the pump pulley. The pump mounts on a machined boss cast onto the intake manifold.

AMC 2.5 liter four-cylinder engine 

The 2.5-liter OHV SMPI I-4 engine was modified in 1996 to make it smoother and quieter than in 1995. It also delivers more torque at low and mid-range speeds. The broadened torque curve enhances standing start and low speed performance without penalty to fuel economy.

The result was 140 lb-ft @ 3500 rpm and 120 hp @ 5400 rpm. Highway fuel economy improved by one mile per gallon with both manual and automatic transmissions due to lo wer wind and rolling resistance.

Dynamic Components

A new camshaft profile and new valve springs, the same as those used on the 4.0-liter engine, provide a substantial increase in low-speed torque and quieter valve train operation. Valve train noise is reduced by "softening" the opening and closing ramps on the cam to reduce valve closing velocity. The new cam profile also provides a smoother idle.

New lightweight cast aluminum pistons significantly reduce engine noise and vibration. They have contour-machined skirts that adapt to all operating temperatures, run quietly, maintain low oil consumption, and minimize the possibility of scuffing. New piston rings accompany the new pistons.

Engine Adaptation

A new direct-mount power steering pump contributes to improved accessory belt life by reducing the belt span and providing more accurate alignment of the pump pulley. The pump mounts on a machined surface cast onto the intake manifold.

 The engine air intake system uses a "quarter-wave" tuning chamber in the intake duct and a Helmholtz resonator mounted atop the throttle body to reduce induction noise and provide a pleasant sound. The quarter wave tuning chamber dampens unpleasant sounds that have frequencies that are a fractional multiple of the basic induction frequency. The tuning chamber is a closed-end tube paralleling the intake duct that terminates in an elbow leading to a port in the side of the duct. The length, diameter and port location determine its tuning effect. The duct and tuning chamber are molded in one piece. Flexible, accordion-pleated cuffs integrated with the rigid duct and tuning chamber unit clamp over nipples on the body-mounted air cleaner housing and the engine-mounted resonator. The cuffs provide a tight seal that contributes to low induction noise and keeps dust from bypassing the air cleaner. The accordion pleats accommodate relative motion between engine and air cleaner.

 The resonator is a molded plastic housing the volume of which is tuned to damp out air intake pulsations. The top surface of the resonator is ribbed for stiffness.

Fuel injection 

The engine is equipped with a returnless fuel injection system to help meet evaporative emission requirements. This requires new fuel rails, fuel lines and vapor lines. Higher system operating pressure than in 1995 -- 49 psi (338 kPa) -- assures a constant supply of fuel at the injectors.

A new MAP (manifold absolute pressure) sensor, which is used as part of the engine control system with both engines, mounts directly to the throttle body to simplify assembly and improve control system response compared to the remote-mounted sensor used previously.

air cleaner

The air cleaner housing sits atop the right front wheelhouse. The inlet snorkel for the air cleaner curves upward, its radiused opening just below the inner surface of the hood. This location minimizes the possibility of water ingestion into the engine during water fording. The air cleaner has a large, rectangular, pleated paper air filter.

five-speed manual transmission

The 1997 Wrangler retains the same five-speed manual transmission used in the prior model without change. Unique ratios are provided to complement the performance characteristics of each engine.

three-speed automatic transmission

Hydraulically operated three-speed automatic transmissions with electronically operated torque converter clutches are available with both engines. A new torque converter that provides more torque multiplication at low speed for better launch feel and low-speed acceleration is coupled to the 4.0-liter engine. A stall torque increase of 14% is accompanied by a substantial reduction in slippage, which helps improve fuel economy and reduces engine "flare" at launch. Redesign of both stator and impeller blades provides the combination of beneficial changes.

transfer case

The robust Command-Trac® transfer case, which provides part-time four-wheel drive with two speed ranges, remains functionally unchanged from 1995. In 4WD low range, the transfer case multiplies transmission torque output by 2.72 times. This gives the four-cylinder Wrangler enough torque to start from a standstill on a 30° slope and to climb an 18° slope (32% grade) easily in third gear. Sealing integrity of the transfer cases is enhanced by eliminating the extension housing for the rear yoke. A lip seal at the rear of the main housing now experiences only rotary motion of the output shaft. Plunging motion of the yoke relative to the output shaft due to suspension action occurs inside a convoluted rubber boot that rotates with the drive shaft.

axles

A new, more robust rear axle -- the Dana 44 -- will be available in mid-year on Sport and Sahara vehicles. It will be standard at high altitude and optional nationwide. It includes a 3.55 gear ratio and Trac-Loc® differential. This axle has larger ring and pinion gears and larger axle shafts than the standard axle. It also includes unit bearing construction that provides a high lateral load capacity.

 A new front axle differential carrier places the drive pinion below the axle center line to avoid drive shaft interference with the floor pan and to reduce universal joint angles for smoother, quieter operation. The pinion is also shorter to further reduce joint angles. Seamless axle tubes provide increased strength compared to the welded construction used in 1995. Axle ratio availability is the same as in 1995.

trailer towing

Trailer towing capacity remains at the Class I level -- 2000 pounds (900 kg) -- with a 300 pound 136 kg) tongue weight limit with the 4.0-liter engine and with the 2.5-liter engine with manual transmission as in 1995. With the 2.5-liter engine and automatic transmission, the limit is 1000 pounds (450 kg).

Engine computer (PCM)

Wrangler shares a new PCM (powertrain control module) with other Jeep and Dodge Truck vehicles. Called JTEC (Jeep-Truck Engine Controller), it provides both engine and automatic transmission control functions. It also provides the sensing and functional analysis capabilities required by the OBD II diagnostic system. JTEC includes new fuel injection, ignition timing and idle speed control software that result in better driveability and reduced emissions.

New larger Group 34 batteries offer up to 20% higher cold cranking capability than in 1995. Two ratings are available: 500 CCA (cold cranking amperes) and 600 CCA. Both batteries have removable vent caps because the heat and rough terrain driving these vehicles experience makes replenishing the water supply a necessity. The batteries are protected from excess underhood heat by an simple and effective insulator consisting of 3 layers of black plastic "bubble wrap". The wrapper is heat staked on the edges and in the corners to slip neatly over the battery as a unit.

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