Jason O Posted October 26, 2015 Share Posted October 26, 2015 Hi Guys-I've put my search for an old Mack on hold for now, so I figured I'd post a few photos of the trucks I currently own. I bought these at auction a few years ago for no reason other than they looked fun. Turns out they are! The trucks are great off road, easy to drive, and will travel 55 + on the highway safely. Although they look similar to the classic M35A2 Deuce and a half, they are an entirely different vehicle. The M939 trucks were produced from 1984-1992 in various configurations (cargo, road-tractor, wrecker, etc)A few features:Cummins power (NHC-250 on early trucks, 6CTA on the later A2's)Allison 5 speed transmissions (MT 654 CR)Full air brakes (wedge type)ABS added on most trucksCTIS (central tire inflation) on A2 trucksHere are a few photos of the 2 I own. If anyone is interested, I'll add more. Have a good day and stay safe on the road!Semi tractor - 1991 M931 A2Cargo truck - 1991 M923 A2 Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/ Share on other sites More sharing options...
68-f-model Posted October 26, 2015 Share Posted October 26, 2015 Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-308864 Share on other sites More sharing options...
alex g Posted October 26, 2015 Share Posted October 26, 2015 They certainly look funSent from my SM-G386W using Tapatalk Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-308865 Share on other sites More sharing options...
41chevy Posted October 26, 2015 Share Posted October 26, 2015 Cool!! Paul Quote "OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK" Thomas Edison “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’” P.T.CHESHIRE Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-308866 Share on other sites More sharing options...
ranchhopper Posted October 27, 2015 Share Posted October 27, 2015 I had a bunch of those through the years they are fun alright until it comes to the price of replacement parts and they are a PITA to work on too but they are cool. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-308897 Share on other sites More sharing options...
Jason O Posted October 27, 2015 Author Share Posted October 27, 2015 Thanks for flipping the pics. Couldn't get them to upload correctly from the I phone. No major repairs - yet... But I do have a transfer case to rebuild on one of them. I'm planning on streching the cab for room for a back seat at some point. A manual trans would be a nice improvement too, but I'm not sure if any twin-countershaft box would allow room for the front driveshaft to run beside it. I'm open to suggestions on a new paint scheme too. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-308911 Share on other sites More sharing options...
ranchhopper Posted October 27, 2015 Share Posted October 27, 2015 Went with a digital pattern on the last deuce I bobbed. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-308945 Share on other sites More sharing options...
m16ty Posted November 2, 2015 Share Posted November 2, 2015 Just don't run those M939 trucks in low range/reverse or you'll be replacing a bunch of transfer cases. I've had a couple of M939 tractors (M931 and M932) and a M936 wrecker that I took the wrecker bed off of and installed a lime spreader bed.I currently have a M35A2C that I put a dump bed on to use around the farm. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-309759 Share on other sites More sharing options...
gearhead204 Posted November 3, 2015 Share Posted November 3, 2015 what is it that kills the transfer case in low rev.? the m 925 that I use as a brush truck has that warning tag on the dash, but I haven't been able to picture in my mind what would damage the case. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-309884 Share on other sites More sharing options...
41chevy Posted November 5, 2015 Share Posted November 5, 2015 what is it that kills the transfer case in low rev.? the m 925 that I use as a brush truck has that warning tag on the dash, but I haven't been able to picture in my mind what would damage the case.The few that were being used here as stump jumpers 2 cracked the transfer cases in low range reverse. I guess that is the problem? Quote "OPERTUNITY IS MISSED BY MOST PEOPLE BECAUSE IT IS DRESSED IN OVERALLS AND LOOKS LIKE WORK" Thomas Edison “Life’s journey is not to arrive at the grave safely, in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy shit, what a ride!’” P.T.CHESHIRE Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310090 Share on other sites More sharing options...
Jason O Posted November 5, 2015 Author Share Posted November 5, 2015 The reverse ratio on the Allison MT654 is 10.76 to 1. It's just too much for the transfer case when its in low, front drive engaged, and torque loaded. 1st gear is only 4.71 to 1 1 Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310098 Share on other sites More sharing options...
Quickfarms Posted November 5, 2015 Share Posted November 5, 2015 The reverse ratio on the Allison MT654 is 10.76 to 1. It's just too much for the transfer case when its in low, front drive engaged, and torque loaded. 1st gear is only 4.71 to 1On top of this the MT654 is considered an obsolete transmission by allison.I wonder how they would work with a MT643 in there? Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310116 Share on other sites More sharing options...
ranchhopper Posted November 6, 2015 Share Posted November 6, 2015 I took the big washer off the linkage of my trucks I safely could use low range in reverse that way. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310203 Share on other sites More sharing options...
gearhead204 Posted November 6, 2015 Share Posted November 6, 2015 now does any one know of transfer cases failing if the front axle was not "engaged" but transfer case in low range, and used in reverse? Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310291 Share on other sites More sharing options...
ranchhopper Posted November 6, 2015 Share Posted November 6, 2015 Transfer case will not grenade unless the axle is engaged that's why I took off the washer that automatically engages it. 1 Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310301 Share on other sites More sharing options...
Jason O Posted November 7, 2015 Author Share Posted November 7, 2015 I was thinking about using that trip washer/ air switch, adding a valve and relay to build an air curcuit that would disengage the front axle when the truck is shifted into reverse while in low range. I know better, but my trucks may be used by someone unfamiliar with them on the trail. Last time out, my Allison skipped a beat and slipped just a little between 2-3 upshifts. Maybe its time for a Fuller transmission. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310324 Share on other sites More sharing options...
m16ty Posted November 7, 2015 Share Posted November 7, 2015 While the Allison in question is deemed obsolete by the company, there are thousands of good transmissions and parts on the surplus market. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310332 Share on other sites More sharing options...
gearhead204 Posted November 7, 2015 Share Posted November 7, 2015 i'm missing something here, the m925 has a air switch on the dash to engage the front end. or so it's labeled at least, I haven't chased out the air lines to confirm. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310342 Share on other sites More sharing options...
ranchhopper Posted November 7, 2015 Share Posted November 7, 2015 They had a real problem with the rebuilt trucks too lots of them had the airlines switched so they were being driven in all wheel drive on the hard roads some people didn't know enough to check them when the driveline would bind up on the road. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-310354 Share on other sites More sharing options...
High Binder Posted November 30, 2015 Share Posted November 30, 2015 Who is the expert on the forum for the MT654cr transmission? We just got an M939 in the shop from a volunteer fire dept. with a transmission leak. It is leaking in the rear between the main case and the spacer(?) that connects the rear plate to the transmission. We are wondering if we can repair it without pulling the transmission? We don't have a parts diagram or service manual for the transmission. This will be a first for us and the students. Any help would be appreciated. Thanks, Dan Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-313647 Share on other sites More sharing options...
Jason O Posted December 1, 2015 Author Share Posted December 1, 2015 You may want to check that the transmission case is properly vented if fluid is leaking. Some trucks are equipped with a water fording kit that pressurizes the trans and gear cases with a pound or so of air. If it remains switched on, you will likely have fluid leaks. Make sure the trans isn't way over filled (had one come out of service this way)I know little about these transmissions, good luck on the repair. Quote Link to comment https://www.bigmacktrucks.com/topic/42356-m939-5-ton-military-trucks/#findComment-313713 Share on other sites More sharing options...
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