Introduced in 1992, HUMMERs are powered by a diesel, gas, or turbo diesel engine. All use either a 3 or 4 sp automatic transmission. They have a full-time 4wd system with torque split evenly front to rear. The transfer case is lockable and has a low range. The differentials are Torsen torque sensing differentials (not a limited slip unit). The halfshafts connect the diffs to the geared hubs. The geared hubs allow the halfshafts to enter above the wheel centerline and provide a gear reduction at the wheel. Wheels are either a single piece or two piece steel wheel, shod with 37x12.5/16.5 Goodyear MT or GS-A tires.
Engine
HUMMERs can be powered by one of 4 engines. The original engine is a Detroit Diesel (GM) 6.2L V8 diesel. Most military HMMWVs use this engine. 150 hp, 250 lbs-ft. It is also used in the 92-93 HUMMERs.
In 94, AM General switched to the Detroit Diesel (GM) 6.5L V8 diesel. 170 hp, 290 lbs-ft. It is available in the 94-97 HUMMERs (except for 96-7 California HUMMERs).
Starting in 95, a 5.7L gas GM V8 TBI was offered as an optional engine. 190 hp, 300 lbs-ft. It is a 4 bolt main, marine motor. It was discontinued in late 96 because of availability.
In 96, the Detroit Diesel (GM) 6.5L V8 turbo diesel was introduced into the HUMMER line. 190 hp, 385 lbs-ft.
In 97, the Detroit Diesel (GM) 6.5L V8 turbo diesel has increased power to 195 hp and 430 lbs-ft of torque.
The engine is cooled by a large 4 core radiator, canted behind the hood top grill (before the air lift hook). The oil, transmission, and power steering coolers are also there. In 97 the power steering cooler is placed vertically behind the fan shroud.
Transmission
HUMMERs using the 6.2L engine (92-93) are mated to a GM TH400 ("3L80") 3sp automatic transmission, the heavy duty version of the ubiquitous TH350. It has 3 forward gears and 1 reverse. It can be locked in 1st, 1-2, or 1-2-3 (D). The ratios are 2.48, 1.48, and 1.00. Reverse is 2.08.
From 94 on, all HUMMERs use the GM 4L80E 4sp automatic. This is the same as the HD automatic transmission in GM trucks. It can be locked into 1st, 1-2, 1-2-3 (D), or 1-2-3-4 (OD). The ratios are the same as the TH400 except for a 0.75 overdrive.
Max input torque is 440 lbs-ft. The torque converter is locking with a ratio of 2.2:1.
Transmission fluid goes through a cooler integrated with the engine oil cooler. From the cooler, the fluid goes though an intercooler inside the transfer case, and then back to the transmission.
Transfer case
All HUMMERs use a New Process Gear (NPG) 242 2sp transfer case. It is a full time transfer case incorporating an open differential between the front and rear driveshafts (DS). It has 4 modes, High (H), High Locked (HL), Low (locked) (L), and Neutral (N). The ratios are 1:1 in high, 2.72:1 low.
The input shaft and rear output shaft are co-linear. The front is a left handed output, chain driven.
In H, the normal operating position, torque is evenly split front and rear via the transfer case's differential. This center differential is open and allows the front and rear driveshafts to turn at different rates to allow turning on high traction surfaces. In HL, the differential is locked (manual locker), forcing the front are rear driveshafts to turn at the same rate. This is equivalent to a part-time 4wd system. The L position forces the input shaft to turn a planetary gear assembly at the front of the case. The ring gear is machined into the front case. Low adds a gear reduction of 2.72:1. The differential is always locked in low. The N position disconnects the input from the output shafts totally. The differential is open in this position.
A sensor at the rear of the transfer case (TC) is used to determine speed. Another sensor determines if the TC diff is locked or unlocked, illuminating a status light.
The TC is cooled via transmission fluid flowing through an intercooler inside the TC. The transmission and transfer case fluids do not mix.
The engine, transmission, and transfer case are rigidly connected together. The assembly is canted slightly to the right.
Differentials
The differential case is an AMC (Dana) 20, hypoid (top entry), same as in some older Jeeps (AFAIR). The differential has a ratio of 2.73:1. HUMMERs use Zexel-Gleason Torsen torque sensing/biasing differentials. These are not a limited slip design or locking design. The bias ratio is 3.8:1 AFAIK. Since the suspension is independent, the differential case is connected to the frame.
Brakes
HUMMERs use 4 wheel disc brakes, inboard mounted, with the rotors sandwiched between the differential output flange and halfshaft flange. The rotors are 10.5 inches. Parking brake actuates rear calipers. The power steering pump and fluid provide boost to the dual reservoir master cylinder.
The inboard location provides some protection from contamination. The bottoms of the rotors are about 17 inches above the ground.
The 12,100 GVWR 97 models have 12" rotors instead.
Geared Hubs
Halfshafts (HS) transmit the acceleration/braking torque to the hubs. The HS are a 2 constant velocity (CV) joint design. The two rears are symmetrical while the left front is short and the right front is long.
The geared hubs are the housing for two straight cut spur gears. The HS enter about 4 inches above the centerline of the wheel spindle and turn the upper spur gear. The upper gear turns the lower gear which is connected to the spindle. The gears provide a reduction of 1.92:1.
The gear reduction has several significant benefits. Since the final drive reduction is done at the hubs, the differentials can be smaller and use a higher gear ratio (numerically low), reducing stress. Furthermore, the HS see only about half of the torque, allowing smaller, lighter parts. Another benefit is that brake torque gets multiplied also, increasing braking effectiveness.
Geared hubs allow the HS to be above the wheel centerline, allowing the differentials to be placed higher up without a severe HS angle. The HS are also more protected along with the brakes. A side effect is that there is nothing inline with the spindle and allows the Central Tire Inflation System (CTIS).
Wheels
Up until 96, there were only two wheel types, a single piece and a two piece steel wheel. The single piece was just like a traditional 16.5 inch rim. The two piece wheel is a split rim design. The split rim allows the insertion of the runflat assembly or a rubber spacer to provide bead locking. Under high side loads, a bead can slip away from the edge of the rim, off of the bead seat area, and allowing all the air to escape. The beadlock looks like a stiff rubber band running between the wheel lips. The beadlock cannot be stretched over an assembled or one piece rim.
In early 96, a new single piece steel wheel was introduced. It is called the 1 piece, Heavy Duty because it has a slightly higher load capacity than the original 1 or 2 piece wheels. However, all the rims have sufficient capacity for full GVW operation. The HD 1 pc. replaces both the single and two piece wheels. They can be equipped with a beadlock for the inner bead only or with a beadlock/runflat device.
Runflat System
The HUMMER runflat system consists of rubber spacer assemblies placed inside each wheel. The system provides the ability to drive on one or more flat tires for up to 30 miles at 20 mph. However, the tire may be ruined beyond repair. The system is derived from the military runflat system.
There have been three versions of the runflat assembly. It is easier to describe the second version (94-95) first. It consists of a base that looks like the beadlock spacer, a large flat rubber band. Offset slightly from the center is a 5" high ridge about 2" wide at the top. In cross section, the assembly looks like an upside-down "T". This assembly fits inside the tire. When a tire goes flat, the tread rests on the runflat spacer instead of the rim. The tire height is the same as if you had about 4 psi in the tires.
The original runflat assembly (92-93) was a two piece affair. There was an "L" shaped half and small bead lock shaped spacer. The spacer butted up against the "L" part to make the runflat "T" shape. Both the original and second design required two piece wheels. The assembly was first compressed and fit inside of the tire. The inner tire bead was then placed on the rim half. The other half was then bolted on, squeezing the tire beads and runflat assembly between the rim halves.
In 96, the latest runflat design can be fit onto a new single piece wheel. The second generation runflat is cut in two to give "C" shaped parts. These are then placed inside a half seated tire and bolted together. The other tire bead is then forced over. It provides bead retension for the inner bead only.
Tires
Up to 94, all HUMMERs came with Goodyear Wrangler MT tires 37x12.5/16.5 (37 inches tall, 12.5 inches wide, 16.5 inch rim). These are an off-road tire with deep lugs suitable for mud, snow, etc. In 95, AMG began offering the Goodyear Wrangler GS-A tires with the same dimensions as an option on diesel HUMMERs and standard on the gas versions. The GS-A perform better on the road (and sand) and are quieter.
Fording
The geared hubs, winch, differentials, transmission, and transfer case are all sealed. They are equipped with vent lines that go to the air intake system (at hood level). Diesels also have a sealed starter. They are able to operate in 30 inches of water, mud, whatever, continuously. There is no possibility of contamination. Gas HUMMERs do not have a sealed starter and are limited to 24 inches.
Military HMMWVs have a snorkel kit which extends the exhaust and air intake to roof level. They can operate in 60 inches of water. Civilian HUMMERs cannot use the snorkel kit. The dash, fuses, alternator, power steering cap, transmission controller, radio, and engine controller (TD and gas) are not waterproof.
Suspension
HUMMERs have 4 wheel independent suspension with upper and lower A arms, coil springs over hydraulic shocks. The suspension members are symmetrical front and rear, e.g. the lower right rear is the same as the lower left front.
GVWR is 10,300 lbs, a Class 3 truck. Curb weight varies from 5,850 for a gas 2 door to 6,550 lbs for a diesel wagon. Payload capacities vary from 4,450 lbs to 3,750 lbs. All have a maximum GCWR of 14,800 lbs, which I believe is the limit for Class 3. Hence, towing varies from 8,950 lbs to 8,250. Curb weight does not include any options. Rocker panel protection adds about 400 lbs alone.
97 introduced two optional GVWR: 10,300 and 12,100. The payload for a 2 door increases to 5,550 lbs.
As a Class 3 truck, HUMMERs are exempt from many DOT safety regulations such as passive restraints and third brake lights. They have to have padded dashes, collapsible steering columns, steel roofs and doors, and lights. The lights on the cab and tail are required by law. All the funny marker lights are also required by law.
Overall ground clearance is 16 inches. The lower control arms slope down slightly from about 12 inches inside the wheels. The bottom of the control arm is about 10 inches above the ground. The arms are box steel, with full TIG welds - extremely durable.
Central Tire Inflation System
CTIS is an option on HUMMERs. The system consists of an underhood air compressor, deflator, selector valve, pressure gauge, front and rear air lines, and wheel connects.
A single air line runs from the compressor to the deflate valve to the input port on the selector. The selector is a 3 way, 3 port valve. Each wheel has a connector which adds or subtracts each wheel from the system. They are controlled from outside the vehicle. Normally they are just left connected. The two fronts wheels are connected together via an air line to one port of the selector valve. Rears are the same connecting to the 3rd port on the valve. Note that left to right pressures are equalized on each axle.
From within the cab, the selector valve can be adjusted to connect the input line to the front air system, rear air system, both, or neither. When set to the both position, front and rear pressures equalize. A gauge with two needles shows the front and rear air pressures.
To inflate, the selector is adjusted and the compressor is turned on. When the desired pressure is reached, the compressor is manually turned off. Deflate is exactly the same except the deflate valve is opened. When the desired pressure is reached, the switch is released. The system can be operated at any speed. In 94+ models, there is a connector at the compressor to inflate other tires, accessories, etc.
In 96, the selector valve was changed from a manual valve to an electronic system. Instead of a rotary switch, there is a rocker switch with 3 positions. Up is front, center is both, and down is rear. There is no off position.
Normal pressure is 26 psi front, 28 rear. Pressure can be increased for load up to 45 psi. Normal offroad pressure is around 20 psi. Sand, mud, deep snow, can be tackled at around 12 psi. Pressure can be lowered to 10 psi for a huge footprint temporarily. An alarm sounds if pressure falls below 8 psi.
Winch
The winch is a Warn Mil-12000. It has a 12,000 lbs, single-line, first wrap rating. It differs from the standard Warn M12000 winch. The industrial motor and control pack are sealed and connect into the HUMMER vent system. The winch can operate under 6 ft of water. It is equipped with Warn's ECL (Electronic Current Limiter) circuitry and Thermal Cutoff systems for protection. It comes with 75 ft of 7/16 EIPS wire rope (M12000 comes with 100ft of 3/8).
The winch shares the same basic parts as the military winches. However, the motor operates at 12V not 24V. The military units are 6,000 or 9,000 lbs. and use 3/8 EIPS cable (15,000 lbs rating).
Rocker Panel/Driveline Protection
Rocker panel protection consists of two shaped plates that fit under the vehicle between the frame rails and the doors, protecting the floor panel from damage. Up until 96, they were available in steel or aluminum. In 96+, they are all aluminum (steel too heavy). The aluminum is about 3/8 inch thick and is extremely tough. They are replaceable.
Rocker panel protection is actually a derivative of the military mine protection kit. The military kit consists of the steel panels to provide occupant protection. Somebody thought it was useful for the civilian versions and it became an option in 93 (?).
Driveline protection provides three tubular steel cages, covering the bottom of the front diff, rear diff, and engine pan through transmission. It also adds a skid plate on the fuel tank, front engine (or winch), and transfer case. The cages are about 15 inches off the ground.
Body
All HUMMERs share the same frame, suspension, and powertrain as all others in the same vintage. Engines and transmissions have changed over the years. Most HUMMERs also share the same body.
The HUMMER hood is hinged at the front, swinging up and away from the windshield. It is a composite consisting of fiberglass and plastic.
The body tub, rear fenders, tailgate, and bed are 6061T6 aluminum (aircraft quality, heat treated, tempered). The aluminum panels are bonded together and then riveted (2800 rivets total). The doors, windshield frame, and roof (if equipped) are steel.