The M109 used the same turret and hull as the 105mm SPH M108. The muzzle brake and bore evacuator on the short M126 or M126A1 howitzer are obvious. The howitzer travel lock is folded down on "Bouncer's" front plate, and the .50cal MG is mounted on the turret roof. The two hatch doors on the sloping front plate above the drive sprocket were for battery access, and the travel lock is resting on the transmission access doors. The turret left side door is open, and the mushroom-like ballistic cover for the M117 panoramic telescope is elevated on the turret roof. (Picture by Jon W. Madzelan.)
The XTG-411-2A or -4 power train is shown here. The XTG-411-2A's overall dry weight was 3745lb (1699kg), its maximum input torque was 880 lb-ft, maximum net input horsepower was 360, and the maximum input speed was 2300rpm. The later XTG-411-4 weighed 3805lb (1726kg), and its maximum input torque, input speed, and net input horsepower were 1200 lb-ft, 2500rpm, and 575, respectively. The -2A's maximum torque converter ratio at stall was 3.3:1, compared to 3.2:1 for the -4. (Picture from TM 9-2520-234-35.)
A schematic of the gearing and clutches in the transmission is provided here. (Picture from TM 9-2520-234-35.)
The longer ordnance on this machine is strikingly different from the picture above. These marines were taking part in Operation CAX 1-2-82. (Picture taken 1 Nov 1981 by Cpl J. A. Daniels; available from the National Archives.)
Details of the front of the vehicle are shown here. 1. Cannon and howitzer assembly. 2. Travel lock assembly. 3. Headlight. 4. Final drive drain plug. 5. Engine oil level check access door. 6. Front lifting eye. 7. Front towing eye. 8. Bilge pump outlet. 9. Drive sprocket and hub. 10. Road wheels. 11. Track assembly. 12. Hull. 13. Cab. 14. Cab side door. 15. Commander's cupola. 16. M2 caliber .50 machine gun. 17. Panoramic telescope ballistic cover. 18. Cab weather cover. (Picture from TM 9-2350-311-10.)
The rear of the howitzer is sketched here. 19. Projectile access door. 20. Cab lifting eye. 21. Bustle door. 22. Rear lifting eye. 23. Exhaust deflector. 24. External power receptacle (M109A4/M109A5). 25. Idler wheels. 26. Stoplight and blackout light. 27. Towing pintle assembly. 28. Rear hull door. 29. M13 decontamination apparatus. 30. Reel bracket. 31. Spade. 32. Stowage box. (Picture from TM 9-2350-311-10.)
The top of the howitzer is sketched here. 33. Air intake grille. 34. Radiator fan access door. 35. Exhaust grille. 36. Radiator cap access door. 37. Personnel heater exhaust deflector. 38. Fuel tank access door. (Picture from TM 9-2350-311-10.)
39. Cab access cover. 40. Gunner's escape hatch. 41. M140 alignment device mount. 42. Fire extinguisher handle. 43. Personnel air vent ventilator. 44. Driver's hatch cover. 45. Battery compartment access doors. 46. Transmission access doors. (Picture from TM 9-2350-311-10.)
Major internal hull components are detailed in this picture. 1. Coolant surge tank. 2. Fuel tank and pumps. 3. Air cleaner. 4. Personnel heater. 5. Fixed fire extinguisher. 6. Canister stowage. 7. Projectile racks. 8. Accessory control box. 9. Driver's seat assembly. 10. Portable and driver's instrument panels. 11. Driver's controls. 12. M2A2 air purifier (M109A4/M109A5). 13. Battery. (Picture from TM 9-2350-311-10.)
The left front of the cab is drawn here. 1. Operating handle (breech mechanism). 2. Dome light switch. 3. Rammer actuating valve control handle. 4. Firing mechanism manual control lever. 5. Rammer handle. 6. Turret lock. 7. M3 electrical air heater (M109A4/M109A5). 8. Accessory control box. 9. Manual traverse handwheel. 10. Gunner's control assembly handle. 11. Gunner's selector switch box assembly. 12. Equilibration handpump assembly. 13. M145/M145A1 telescope mount. 14. M117/M117A2 panoramic telescope. (Picture from TM 9-2350-311-10.)
The rear of the cab is drawn here. 8. Power pack assembly. 9. External filter (M109A4/M109A5). 10. Air line filter and hygroscopic breather (M109A4/M109A5). 11. Cab ammunition rack assembly. 12. Canister rack propellant. (Picture from TM 9-2350-311-10.)
This is the driver's position. 1. M42 driver's periscope. 2. Hatch locking lever. 3. Fuel shut off control assembly handle. 4. Master warning light. 5. Fire extinguisher handle. 6. Transmission shift control lever. 7. Manual control handle [used to engage parking brake]. 8. Hand throttle control lever. 9. Accelerator pedal. 10. Driver's heater outlet duct. 11. Driver's seat vertical adjusting lever. 12. Brake pedal. 13. Steering wheel. 14. Driver's fixed instrument panel. 15. M3 electrical air heater (M109A4/M109A5). 16. Dome light switch. 17. Portable instrument panel [used when driver is in raised position]. (Picture from TM 9-2350-311-10.)
Suspension components are diagrammed here. 1. Torsion bar anchor. 2. Torsion bar. 3. Road wheel arm. 4. Road wheel. 5. Shock absorber. 6. Bump stop. 7. Idler arm assembly. 8. Idler wheel. 9. Track adjuster. (Picture from TM 9-2350-311-10.)
The mounted crew positions are shown here. HD: howitzer driver. AG: assistant gunner. G: gunner. 1-3: cannoneers. CS: chief of section. AVD: ammo vehicle driver. ATC: ammo team chief. (Picture from TM 9-2350-311-10.)
The large howitzer and imposing muzzle brake of this M109A2 stand out. The travel lock mounting point was moved down to the extreme front with the installation of the longer howitzers. The mount for the .50cal M2HB machine gun is visible on the turret's right side, and the new protective ballistic hood for the panoramic telescope M145 is on the turret's left, facing to the right. (Picture courtesy Armor Foto.)
This rear view shows the lengthened turret bustle, which is flanked on either side by stowage racks. The back of the turret can be contrasted to the M109A1 above, where there is no bustle visible between the rear stowage baskets. The stabilization spades are folded up on either side of the hull rear. (Photo by Richard S. Eshleman.)
The turret stowage bustle and commander's cupola hatch are open on this vehicle, and the stabilization spades are lowered. (Photo by Richard S. Eshleman.)
The locations for the 13 drains added to the M109A4 are marked as 1. Cover plates for hull access holes are labeled as 2. (Picture from TM 9-2350-311-10.)
From this angle and distance, the M109A5 is difficult to distinguish from the M109A4, or indeed earlier howitzers, whence it was derived.
Here the vehicle can be positively identified. The forward-facing optic with the closed, hinged cover is the periscope M42. Enclosing the howitzer tube to its top left and bottom right is the variable recoil mechanism, attached to the upper and lower recoil cylinders.
Zooming in on the base of the howitzer tube in the previous image, we can see the torque key that fits into the keyway on the cradle mount to prevent the tube from rotating. On the howitzer mount M178 this was held in place by ten cap screws. On this M109A5's mount M182, only four cap screws are present on each side.
The recuperator can be seen to the howitzer mount's bottom left, unconnected to the cover of the variable recoil mechanism. A dust shield is attached to the base of the howitzer tube and the variable recoil mechanism cover.
A closeup of the cover for the panoramic telescope is provided here, and it is pointing to the vehicle's front left. The commander's machine gun mount can be seen in the background. In front of the turret lifting eye can be seen the mount for the M140 or M140A1 alignment device, which ensures the panoramic telescope and the howitzer tube are aligned.
The driver's hatch is highlighted in this image. Three periscopes M42 ring his position, and a handle for external activation of the vehicle's fire extinguishers is enclosed in a guard on the hull roof. Between the driver's hatch and the turret is the grille for the personnel air vent ventilator, and to the driver's right is the large air intake grille. A mounting bracket for the antenna of the radar chronograph M90 is attached to the upper recoil cylinder.
Two access doors for the battery compartment are on the front hull slope directly behind the left headlight, and inboard of the lower battery compartment door are access doors for the transmission. On the more vertical slope of the front of the engine compartment can be seen the engine oil level check access door and, outboard of this, the bilge pump outlet. Beside the headlight cluster are a lifting eye above and a towing eye below.
Driving with the howitzer unsecured could lead to serious damage, so the there were locks for both the turret and howitzer tube. To release the tube, the locking pin was removed (already done on this vehicle: it can be seen hanging on a string between the struts of the travel lock), then the handle was raised and the tube elevated until it cleared the travel lock cradle rest. The travel lock was folded to the rear to be stowed on the engine compartment deck.
The exhaust grille is in the center of this picture, with the exhaust deflector just visible to the upper left of the image. Just inboard of the exhaust grille is the radiator fan access door; the hinge near the turret shows the location of the radiator cap access door.
The exhaust deflector was mounted at the rear of the exhaust grille. Behind it is a fuel tank access door.
The location of the right rear shock absorber can be gleaned from this picture.
This vehicle was converted from an earlier-production M109A4, shown by the double doors on the hull rear. The locking handle is pointed up towards the bracket for communication wire, and a mount for a liquid can is on the right hull door. Recoil spades flank the hull doors, and a towing pintle is below them. The turret bustle can be opened either by the two large doors or by the smaller tunnel-shaped door in the bottom center, which allowed projectiles to be passed into the cab with greater security for the turret crew compared to the large doors. Stowage boxes flank the turret bustle.
The right rear of the hull is detailed here. Above the rear lifting eye is the connection for external telephone communication, and below the lifting eye is an external power receptacle common to the M109A4 and M109A5. Below the power receptacle is a stoplight and blackout light. A stowage box is hidden by the spade, and the turret stowage boxes are mounted on stowage baskets.
The new turret with a full-width bustle can be seen on this vehicle from C Battery, 41st Field Artillery Regiment, 1st Armored Brigade Combat Team, 3d Infantry Division. The remote-control travel lock can be contrasted to the vehicles above. A blank adapter is on the .50cal MG, and the crew are wearing NBC protective gear. (Picture taken 1 Aug 2017 by SGT Joseph Truckley; available from the Defense Video & Imagery Distribution System.)
The enlarged bustle is more apparent in this rear view of a howitzer from the 155th Armored Brigade Combat Team, and the stowage baskets attached to the bustle are swung back. This can help decrease the vehicle's width. (Picture taken 2 Jun 2017 by SGT Edward Lee; available from the Defense Video & Imagery Distribution System.)
The locations of major internal components on the right side of the cab are illustrated here. 1. VRC radio (voice). 2. Hydraulic control box. 3. AFCS Display Unit (DU)/PDFCS Display Unit (DU). 4. Keypad (PDFCS). 5. Chief of section control handle. 7. Full Function Crew Station (FFCS). 8. M3 heaters. 9. PDIU (AFCS). 10. Automatic fire control system/Paladin digital fire control system batteries. 11. AFCS Power Conditioner Unit (PCU)/PDFCS Power Conditioner Unit-2 (PCU-2). 12. Commander's seat assembly (stowed). 13. AFCS Azimuth tachometer/PDFCS azimuth cover plate and shorting plug. 14. Master Control Station (MCS). 15. Footrest (stowed). 16. VRC radio (digital). 17. Vehicle Motion Sensor (VMS) modem. 18. AFCS Computer Unit (ACU)/PDFCS Computer Unit (PDCU). 20. Boot dongle (PDFCS) (Will be removed from vehicles equipped with EPIAFS [Enhanced Portable Inductive Artillery Fuze Setter, which can initialize the global positioning system for guided projectiles and provides automated fire mission transfer]). 21. Paladin Serial Interface Adapter Device (PSIAD) (PDFCS). 22. Paladin Serial Interface Adapter Device (PSIAD) (For vehicles equipped with PDFCS, DAGR [Defense Advanced GPS Receiver], and EPIAFS). 23. Platform Interface Adapter (PIA) (For vehicles equipped with PDFCS, DAGR, and EPIAFS). 24. PIK [Platform Integration Kit for the EPIAFS, which is a data processor that manages data streams from multiple sources in the system] (For vehicles equipped with PDFCS, DAGR, and EPIAFS). (Picture from TM 9-2350-314-34-2.)
The locations of major internal components on the left side of the cab are illustrated here. Text in brackets has been added. 1. Heater (M3). 2. Driver's hose. 3. MCS [Microclimate Conditioning System] control box. 4. Full function crew station (FFCS). 5. Equilibration manifold. 6. Elevation selector valve. 7. Azimuth selector valve. 8. Pantel [panoramic telescope] linkage. 9. Elevation hand pump. 10. Return manifold. 11. Hydraulic interior compartment panel. 12. Gunner's control handle. 13. Fuse manifold. 14. Handwheel assembly. 15. Turret traverse mechanism. 16. Traverse limit valve. 17. Equilibration hand pump. 18. Turret lock. 19. Crew seat. 20. Periscope box. (Picture from TM 9-2350-314-34-2.)
The mount M182A1 is illustrated here. 1. Cradle. 2. Breech operating handle cam. 3. Trunnion bearing. 4. Recuperator cylinder. 5. Upper rotor shield. 6. Lower rotor shield. 7. Variable recoil cylinders (2). 8. Buffer cylinder. 9. Replenisher accumulator. 10. Actuator and linkage.
The variable recoil system shortens the recoil length for higher elevations. The recuperator returns the howitzer to battery, and the buffer governs the recuperator's slamming action and works to ease the ordnance into battery. (Picture from TM 9-2350-314-34-2.)
The breech mechanism of the howitzer M284 is shown here. 1. Breechblock. 2. Carrier. 3. Operating crank. 4. Obturator. 5. Firing mechanism. 6. Operating handle. (Picture from TM 9-2350-314-34-2.)
Parts of the commander's cupola are detailed in this image. 1. Hatch cover. 2. Machine gun support assembly. 3. Torsion bar. 4. Spring-loaded latch. 5. Cupola body assembly. 6. Race rings. 7. Spring-loaded latch (rotation). 8. Periscope slot. (Picture from TM 9-2350-314-34-2.)