1991 to 2002 General Motors 3500HD trucks are not merely a heavy duty version of the the 3500 trucks, as in later years. The 1500, 25000, and 3500 trucks carry a chassis designation of GMT-400. The 3500HD chassis and many other components are entirely different, and had their own chassis designation of GMT-455. These trucks were designed to fill a market void for GM in the class 4 truck category.
Offered in both GMC and Chevrolet versions (only name/badging differences), the design goal was to create a class 4 truck in the 15,000 lb GVWR range, have it look as much like the regular pickup truck line as possible, and use as many things from the GM parts bin as possible to keep cost down.
This unique chassis carried the GMT-455 designation, since it was vastly different than the GMT-400 chassis. It was to have larger brakes, heavier duty axles, a heavier frame, and an industry standard straight, flat-top, 34" wide rear chassis for easy body upfitting. It was to have an easier load and entry height than other large trucks, and basically be more marketable to those seeking a big truck with small truck looks and efficiency.
There are obvious visual differences between the GMT-400 and GMT-455 lines, as seen in the picture below. The larger wheels in the 3500HD have 5 holes instead of 4. The bumper sticks out further forward, and has a spacer between bumper and grille. The entire cab sits higher. The fenders have flares above the front tires. The lower door nameplates have "HD" after the 3500 designation. 1995 and later 3500HD trucks have the larger door handles. The GMT455 chassis also has larger 19.5" wheels versus the 16" of the standard 3500 to package the larger brakes.
up until the 1996 model year, only regular cab options were offered from the factory. 1996 and later years had the option of a crew cab. No production extended cab models were ever offered.
There has been much debate whether the crew cab option was provided directly off the standard factory line, or produced as a standard cab and then sent to another plant for conversion into the crew cab. If this latter option was the case, the cab would have been chopped behind the rear doors and the back part added on, with the frame also being cut and extended. If the former option, then a complete crew cab from the medium duty Kodiak/Topkick line would have been substituted, as it also appears to have been an option in that line as well.
Documentation of 3500HD crew cab trucks seems to indicate they have the original two door VIN, which would indicate they were manufactured as regular cabs and then modified. Using this method, GM would have cut the regular cab 3500HD, and extended it, preserving the VIN plate on the original sheet metal and sidestepping some VIN tampering laws. If the trucks were manufactured as original crew cabs, unmodified, why would they have kept the two door VIN?
Factory documentation specifies that these trucks used the same cab as the Kodiak/Topkick medium duty trucks. Experience indicates that 3500HD and 1500/2500/3500 cab body mount parts are the same.
With the changeover to heavy duty truck door handles in 1994, the medium duty crew cab rear doors appear to have used the same handles as the front, but without the lock cylinder hardware attached inside the door. We have not yet seen a dedicated handle for the rear doors without the lock cylinder hole.
We know for certain that no special crew cab frames were made. All were cut and spliced longer under the cab. 3500HD crew cabs have the wrong wheelbase specified versus what is measured, meaning they came off the factory line originally at one length and were modified at some point.
But were the cab units themselves cut and spliced? Here is what we know and what we have heard:
-We know the same stampings behind the rear door were not used between regular and crew cabs, due to the lack of the 'character line' even with the rear door window on teh crew cab. This is clearly a different sheet metal stamping. This lack is visible on all crew cabs whether GMT-455 or GMT-400 versions.
-We know the rear floor section of the regular cab was not reused for the crew cab 3500HD, or indeed any crew cab. The cab rear floors are different.
-We know that the same cab rear panels were used on both regular cabs for the light duty and medium duty trucks. This is verifiable through photographs.
-We know for certain the same doors were used interchangeably, as well as glass, mirrors, and as many common parts as possible.
-We know upfitters were making aftermarket crew cab conversions for the medium duty Topkick/Kodiak line prior to 1996, when the 3500hd was offered with a factory crew cab.
-Centurion did some regular to crew cab conversions on GMT-400 1500 series trucks by splicing cabs together to preserve the original firewall/VIN plate.
-Rumors say stock light duty crew cab carpet kits do not fit a 3500HD without modification.
We are currently doing continuing research on this topic. We don't know for certain at this time what method was used to create the factory 3500hd crew cabs. If you have any information or documentation on this topic just let us know here.
-Combination welded and riveted frame, extensive weld fixturing and robotic welding.
-135.5", 159.5", and 183.5" wheelbases, all with 34" frame rail centers from cab backward.
-Design specification was low 30.3" frame height for easier loading than heavy trucks.
-Designed for a 9,785 lb. payload including occupants, body, cargo, optional equipment, etc.
-Radiator support and grille riser/filler piece are unique to GMT-455 chassis, to allow the use of a 19" x 32" x 2-1/8" radiator (larger than full-size pickups). This allows the truck to function without an auxilliary cooling fan even under idle-soak conditions with a 37" x 3-1/4" grille opening.
-Notes: after first few years extra support plates were added on frame behind the cab, it was found for uses like rollback bodies the frame would be too stressed with the counterweight on the rear. In 1994 models GM started adding frame re-inforcement plates behind the cabs, this being the weakest point.
-Front suspension used 57" tapered leaf springs at 571 lbs/in., spaced 841 mm apart with a stablizer bar. Also lateral track bar and steering damper.
-35 mm shocks in front (match rear).
-Rear suspension used 56" springs, multi-leaf two stage. First stage 771 lbs/in., second stage 2000 lbs/in.
-No rear stabilizer bar, 35 mm shocks in rear.
-Front to rear ride balance of 1.3:1.
-(Thanks to one of our readers Patrick who helped clarify data on the brakes and some other information!)
-Front disc brakes 1991-1994 (thru VIN# RJ107932): disc brake rotor diameter of 14.25" (361.6mm) (same as P-chassis).
-Front disc brakes 1994-later (after VIN# RJ107932): disc brake rotor diameter of 13.85" (351.8mm).
-Rear disc brakes 1991-1994 (thru VIN# RJ107932): disc brake rotor diameter of 13.75" (349.3mm) (same as P-chassis).
-Rear disc brakes 1994-later (after VIN# RJ107932): disc brake rotor diameter of 13.59" (345.2mm).
-Initial units used a mechanical height-sensing load proportioning valve from axle to frame that allowed more brake fluid to pass to the rear brakes when the chassis was loaded. It is unknown how far into production this system was used, but we know for sure into 1993.
-3.375" brake single-piston diamter on all calipers (same as P-chassis).
-The same calipers using single pistons were used on front/rear, with the exception sides were not interchangable left to right.
-A later 'tow package' option reportedly came with duel piston calipers on the rear. We do not know what year the tow package was available but reports indicate 99-02 had it as an option. Reportedly the dual piston is the same on right and left. Even though they did not come on the front, they do fit if someone wishes to do so.
-Early years had rear-wheel ABS only, later years had four-wheel ABS.
-9"x3" prop-shaft mounted park brake, same as used on medium duty school bus chassis but activation system modified for use of pedal instead of lever. Turnbuckle adjustment system to compensate for wear.
-19.5" x 6" wheels, hub piloted, 10 lugs on 7.25" bolt circle, 5.25" hub bore, stock are Accuride 27775 steel. 10 lugs used on rear, 5 lugs used on front. Wheels used a lug reinforcing plate between wheel and nuts.
-All tire and wheel combinations balanced to same specification as light duty trucks, no more than 6 oz. out of balance prior to final correction of 0 oz. of imbalance.
Janesville, WI plant:
In 1990 the GMT455 assembly line started at the Janesville plant in the tire warehouse. The line started with eighteen employees on chassis and final production. Five field-test vehicles went to customers around March 1990, with limited production beginning in June. Twenty pre-production prototypes were tested to 750,000 miles.
The first regular production truck offically rolled off the line on October 9, 1990, a Tuesday (according to local news reports from Thursday Oct. 11, 1990). Production started at 5 trucks per day on the assembly line, escalating to 22 to 26 trucks per 10-hour shift (only one shift was scheduled initially).
The cab unit was also produced at Janesville, being the same as the medium duty version, and made on same assembly line as medium duty trucks.
Flint, MI plant:
The Flint 3500hd line started in 1997. Only Summit White cabs sold for domestic use were produced here. All other 3500hd units were still produced at the Janesville plant.
-Only available with manual transmission. Since the initial units were not available with manuals (those being not production-ready), no initial units have the 5.7L engine.
-Throttle body injection.
-190hp at 4,000 RPM.
-300 ft-lbs. at 2,400 RPM.
-Notes: this engine option is hard to find. Actual production numbers are unknown at this time, but reportedly very low.
-Throttle body injection.
-230hp at 3,600 RPM.
-385 ft-lbs. at 1,600 RPM.
-Records seem to indicate this engine was not available in 2001 or 2002.
-Notes: This engine is the most common gas engine ordered. All initial production trucks had this engine as others were not yet available.
-We have reports that some of these engines found their way into some early 3500hd's before standard production use ended in 1993.
-Naturally aspirated (non-turbocharged).
-Indirect fuel injection.
-Cast iron block and heads.
-160hp at 3,600 RPM.
-285 ft-lbs. at 2,000 RPM.
-Not available in initial production. Records indicate availablity from 1992 to 2002 (production for other trucks stopped in 2000).
-Cast iron block and heads.
-180hp at 3,400 RPM.
-360 ft-lbs. at 1,700 RPM.
-Production started with Stanadyne DB-2 mechanical fuel injection pumps, 1994 onward used DS-4 electronic direct injection pumps.
-Only available in later years (2001 to 2002), seem to be more rare in 2001.
-330hp at 4,200 RPM.
-450 ft-lb. at 3,200 RPM.
All transmissions have prop-shaft mounted parking brake.
4L80E 4-speed automatic with overdrive:
-Not available with the 5.7L engine.
-Gear ratios: 2.482 (1st gear), 1.482 (2nd gear), 1.000 (3rd gear), 0.750 (4th gear), 2.077 (rev).
-7 position shifter quadrant: P, R, N, OD, D, 2, 1.
-Column mounted shifter only.
NV4500 5-speed manual with overdrive:
-(part numbers, order codes, and gearsets taken from the Novak Guide for the NV4500)
-Not available in initial production as this unit was not yet ready. Only the 4L80E was initially available. Availability started in 1993 from production records, but may vary due to actual installations on the assembly line.
-PTO access from both driver and passenger side.
-6.34 (1st gear), 3.44 (2nd gear), 1.71 (3rd gear), 1.00 (4th gear), 0.73 (5th gear)
-1995 and onward geat ratos: 5.61 (1st gear), 3.04 (2nd gear), 1.67 (3rd gear), 1.00 (4th gear), 0.73 (5th gear)
-Floor mounted shifter only.
-1993/1994 used GM# 15660762, GXD order code, 6.34 gearset.
-1995 used GM# 15691903, GAKH order code, 5.61 gearset.
-1996 used GM# 15970012, GAML order code, 5.61 gearset.
-1997 used GM# 15716057A, GARF order code, 5.61 gearset.
-1998 used GM# 15716057, GARF order code, 5.61 gearset.
-1999 used GM# 15020887, GBBK order code, 5.61 gearset.
-2000/2001/2002 used GM# 15747131, GBFU order code, 5.61 gearset.
-Approx. dry weight: 195 lbs. without prop-shaft brake.
-Oil capacity: 8 pints.
-Main case cast iron; aluminum extension housing and top cover.
-All use electronic speedometer output, and backup switch.
-Output shaft: 35 spline.
Allison 5-speed automatic:
-later option (about 2001 to 2002), rare, seems to have been with 8.1L gas engine only.
-'I' beam style with 5000 lb. FGAWR.
-Based on P model (box trucks, motorhomes, and other forward-control truck chassis) front axle in use from 1985 onward.
-Offset of 114 mm.
-Rockwell International 11,000 lb unit.
-12-1/8" diamter ring gear.
-4.63 gears or 5.13 ratio.
-Notes: all initial units used the Rockwell axle. Dana units were phased in later.
-Dana 80 (85.8" wide with 11" ring gear).
-4.63 gears or 5.13 ratio.
-All units had the front 4" lower grille filler piece to allow more engine compartment vertical space.
-All 3500HD trucks had plastic front fender flares.
-Upper cab marker/clearance lights were standard equipment.
-GMT-455 units used the same cab sheet metal as medium duty (Topkick/Kodiak lines), but the dashboard, steering wheel, gage cluster, etc. was from the GMT-400 line.
-Either 'west coast' or 'camper' style mirrors were offered. No regular mirrors.
-Mid-mount fuel tank was standard (22 gal.), rear tank optional (30 gal.). At least in the beginning no option to connect tanks was offered, meaning one or the other had to be specified not both.
-1994 and later cabs used the medium duty door handles.
-All units were fitted with hydroboost brakes, run off the power steering pump. The brakes were equipped with a height-sensing load proportioning system to allow automatic front/rear pressure split based on load.
-All trucks had power steering with 14:1 ratio packaged in same housing as full-size pickup trucks. Pump output was increased by 6% for lighter steering effort when loaded.
-All units had the same 4 wheel disc brakes, with ABS added in later years.
-All units were sold in a cab/chassis configuration with rear straight and flat 34" frame rails for upfitter body mounting, and an extended fuel filler hose for the same reason.
-All units had a transmission tailshaft mounted parking brake, due to the use of the rear disc brakes. This brake was pedal operated rather than the lever used on other GM prop-shaft mounted brakes.
-All trucks regardless of powertrain had 15,000 lb. GVWR, and 10,000 lb. tow ratings, and 19,000 lb. GCVWR.
-All trucks single axle dual rear wheel, regardless of frame length.
-Cab interiors followed along with the 1995 GMT-400 style changeover, unlike the medium duty trucks.
-All exhaust systems were single-pipe 3" diameter stainless steel, exiting on the passenger side behind the rear wheels.
-19" x 32" x 2-1/8" radiator in all units (larger than full-size pickups).
-No 4x4 offered from factory, but aftermarket conversion companies offered 4x4 with dana 60 or 70 front axle. Monroe Truck offered ship-through upfit 4WD using RPO code VCB.
-Spare tire is an option, to be strapped to rear frame rails at factory when ordered.
-Goodyear 225/70 R19.5 "F" tires.
3500HD design and development
1999 Chevrolet Chassis Upfitter Guide
Article originally written Oct. 11, 1990, in the Journal Times (Janesville):
Article from the Janesville Gazette: https://www.GazetteXtra.com/gmtimeline/