The M939 Truck is a 5-ton 6×6 U.S. military heavy truck. Designed in the late 1970s to replace the M39 and M809 series of trucks, it has been in service ever since. The M939 evolved into its own family of cargo trucks, prime movers, and recovery vehicles, with about 32,000 in all produced.
- Engine 1.1
- Tires 1.2
- Wheelbases 1.3
- Cargo Truck M923-M928 2.1
- Dump Truck M929/M930 2.2
- Tractor Truck M931/M932 2.3
- Expansible Van M934/M935 2.4
- Wrecker, Medium M936 2.5
- Chassis-cabs 2.6
- Dimensions 3
- Gallery 4
- Operators 5
- Safety 6
- See also 7
- References 8
- External links 9
All models of the M939 share a common basic chassis, cab, and hood/fenders. The basic truck is a 6×6 (three axles, six wheels, all of which are powered) medium truck. Early M939s were rebuilds of M809 vehicle chassis by AM General, with a new automatic transmission, cab, and hood/fender. Suffix –A2 are new production by Bowen-McLaughlin-York/BMY with later model Cummins engine. The vehicles have a wide variety of configurations and weights.
Note that the motor and tire specifications, along with other improvements, apply to the A2 versions (and A1 versions) of each base model listed below. There is an M928, an M928A1, and M928A2.
The M939 and M939A1 models use a Cummins NHC 250, a 855 cu in (14.0 L) naturally aspirated inline 6 cylinder diesel engine developing 240 hp (180 kW) at 2100 rpm and 685 lbf·ft (929 N·m) of torque at 1,500 rpm. This was the standard engine of the M809 series. The M939A2 models use a newer and smaller Cummins 6CTA8.3 504 cu in (8.3 L) turbocharged inline 6 cylinder diesel engine developing 240 hp (180 kW) at 2100 rpm and 745 lbf·ft (1,010 N·m) torque at 1,500 rpm.
All models have an Allison 5 speed automatic transmission with a two speed transfer case. The front axle was engaged in low range.
The M939 uses 11:00 R20 tires with two tires per side per axle in the rear (rear tandem duals). The M939A1 and M939A2 series use oversized 14:00 R20 tires and rear tandem singles. M939A2 series vehicles use a central tire inflation system (CTIS).
There are three wheelbases (measurements are from the centerline of the front axle to the centerline of the rear tandem). The short, used for tractors and dumps, is 13 feet 11 inches (4.24 m), the long ("standard"), used for cargo and wreckers, is 14 feet 11 inches (4.55 m), and the extra long, used for long cargo and expansible vans, is 17 feet 11 inches (5.46 m).
Cargo Truck M923-M928
The M924 (M926 w/winch) was the standard cargo version of the series. It had a 7 ft (2.1 m) x 14 ft (4.3 m) low sided box with a bottom hinged tailgate. Side racks, troop seats, and overhead bows with a canvas cover were standard.
The M923 (M925 w/winch) had a body with drop sides so it could be loaded from the side by forklifts.
The M927 (M928 w/winch), with an extra long wheelbase, had a 20 ft (6.1 m) long box. There was no drop side version.
Dump Truck M929/M930
The M929 (M930 w/winch) was a dump truck used to haul sand, gravel, dirt, rubble, scrap, and other bulk materials. It had a dump body with cab protector and a tailgate that could hinge at either the top or bottom. Side racks, troop seats, bows, and a canvas cover could be fitted.
Tractor Truck M931/M932
Semi tractor-trailers have limited off road performance, having to stay on relatively flat ground, so loads are not rated for full off road use. The fifth wheel load rating is 15,000 lb (6,800 kg).
Expansible Van M934/M935
The M934 (M935 with a hydraulic lift-gate) had a 17 ft (5.2 m) van body with a slide out section on each side. When the sections are extended the working floor was almost 14 ft (4.3 m) wide. The body could support 5,000 lb (2,300 kg) of communications equipment. 
Wrecker, Medium M936
The M936 was a wrecker used to recover disabled or stuck trucks and lift large components. A rotating, telescoping, and elevating hydraulic boom could lift a maximum of 20,000 lb (9,100 kg). Although the truck was not meant to carry a load, the boom could support 7,000 lb (3,200 kg) when towing. They had 20,000 lb (9,100 kg) front and 45,000 lb (20,000 kg) rear winches, outriggers, boom braces, chocks, block and tackle, oxygen-acetylene torches, and other automotive tools.
The M939 to M945 were chassis with cabs, meant to carry specialized bodies. They were available in long and extra long wheelbases, and with or without front winches.
|M923 Cargo||long||25 feet 7 inches (7.80 m)||8 feet 2 inches (2.49 m)||9 feet 8 inches (2.95 m)||21,600 lb (9,800 kg)|
|M924 Cargo||long||25 feet 7 inches (7.80 m)||8 feet 2 inches (2.49 m)||9 feet 8 inches (2.95 m)||23,337 lb (10,585 kg)|
|M925 Cargo||long||27 feet 5 inches (8.36 m)||8 feet 2 inches (2.49 m)||9 feet 8 inches (2.95 m)||22,360 lb (10,140 kg)|
|M926 Cargo||long||27 feet 4 inches (8.33 m)||8 feet 2 inches (2.49 m)||9 feet 8 inches (2.95 m)||24,060 lb (10,910 kg)|
|M927 Cargo||extra long||31 feet 11 inches (9.73 m)||8 feet 2 inches (2.49 m)||9 feet 8 inches (2.95 m)||27,749 lb (12,587 kg)|
|M928 Cargo||extra long||33 feet 8 inches (10.26 m)||8 feet 2 inches (2.49 m)||9 feet 8 inches (2.95 m)||27,811 lb (12,615 kg)|
|M929 Dump||short||22 feet 9 inches (6.93 m)||8 feet 2 inches (2.49 m)||9 feet 9 inches (2.97 m)||25,880 lb (11,740 kg)|
|M930 Dump||short||24 feet 6 inches (7.47 m)||8 feet 2 inches (2.49 m)||9 feet 9 inches (2.97 m)||23,990 lb (10,880 kg)|
|M931 Tractor||short||22 feet 9 inches (6.93 m)||8 feet 2 inches (2.49 m)||9 feet 4 inches (2.84 m)||22,089 lb (10,019 kg)|
|M932 Tractor||short||23 feet 10 inches (7.26 m)||8 feet 2 inches (2.49 m)||112 in (280 cm)||22,841 lb (10,361 kg)|
|M934 Expansible van||extra long||30 feet 5 inches (9.27 m)||8 feet 2 inches (2.49 m)||11 feet 4 inches (3.45 m)||29,946 lb (13,583 kg)|
|M935 Expansible van||extra long||30 feet 11 inches (9.42 m)||8 feet 2 inches (2.49 m)||11 feet 4 inches (3.45 m)||31,851 lb (14,447 kg)|
|M936 Wrecker||long||30 feet 2 inches (9.19 m)||8 feet 2 inches (2.49 m)||8 feet 10 inches (2.69 m)||39,334 lb (17,842 kg)|
- Argentina 
- Saudi Arabia
- Republic of China
- United States
The safety of the M939 series of trucks has been criticized, especially braking performance and stability when loaded. In 1999 the U.S. Army began refitting anti-lock brake systems to the M939 trucks. Until the trucks were modified, they were limited to a 40 mph (65 km/h) top speed by an Army-wide safety order.
Prior to that improvement, 26% of all Army vehicle accidents and 53% of all Army vehicle accident fatalities were in M939 series trucks. From 1987 to 1998 the series made up 9% of the total U.S. Army vehicle inventory, but accounted for 34% of all fatal accidents.
The problem seemed to be that the torque converter would "lock up" in 2nd gear, and would not unlock easily. When the driver attempted to brake hard, often in a sudden or 'panic' stop, and accidentally locked the brakes (no wheel movement, tires skidding), this would kill the engine; this also killed the power steering, and the driver would suddenly be unable to steer. Too often, the truck would veer sideways and either hit something or roll over.
- Doyle, David (2003). Standard catalog of U.S. Military Vehicles. Kraus Publications. pp. 202–203, 205.
- "TM9-2320-272-10 Operator’s manual for Truck 5 ton, 6X6, M939 series". US Depts. of the Army and the Air Force. 25 Jul 1984. Retrieved 3 Sep 2015.
- Ejercito Argentino REO M931A2 03 Octubre 2014
- M939 series information page at globalsecurity.org, Accessed 5 Dec 2014
- "Military Safety: Army M939 5-Ton Truck Accident History and Planned Modifications". GlobalSecurity.org. US General Accounting Office. 4 Sep 1999. Retrieved 23 May 2014.
- M939 Technical Manuals M939 Technical Library
- M939 series info page at globalsecurity.org
- Military Safety: Army M939 5-Ton Truck Accident History and Planned Modifications (Letter Report, 04/09/99, GAO/NSIAD-99-82).
- M939 series Technical Manuals at jatonkam35s.com
- eMilitary Manuals-M939 5-Ton, 6X6 Series Trucks Repair and Maintenance Manuals
- M939 manual on scribd.com