Quick question about driving M939 truck without rear prop shaft

[LCA078]

I gambled and purchased a non-working M929 and had it towed to the house. The tow company removed both the middle prop shaft (between transfer case and middle axle) and rear prop shaft (between the middle and rear axle) , hoisted the front end off the ground, and used tow truck air to release and operate the brakes for the tow. I reinstalled the middle prop shaft but haven't installed the rear shaft since I need Soldier B and don't have one at the moment. I got the truck running by replacing the rubber fuel lines but need to park it out back for a bit until I have more time for other things like upgrading alternator, rebuilding PCB, etc.

So here's the two part question:
-Can I drive the truck without the rear prop shaft? Mainly for short distances on the road to verify all is working. I think the answer is yes but wanted to verify as it may be a while before I can get a capable Soldier B to help reinstall the rear shaft.

-Can I engage the front axles to make it a 6x4 in high only (not low) so I don't get stuck (or at least get unstuck from where I'm sitting. LOL!) Again, I think the answer is yes but wanted to verify with the expert since I understand there are limitations with being in low gear and backing up. Not sure if there is anything else to be concerned about.

 

[cbrTodd]

I gambled and purchased a non-working M929 and had it towed to the house. The tow company removed both the middle prop shaft (between transfer case and middle axle) and rear prop shaft (between the middle and rear axle) , hoisted the front end off the ground, and used tow truck air to release and operate the brakes for the tow. I reinstalled the middle prop shaft but haven't installed the rear shaft since I need Soldier B and don't have one at the moment. I got the truck running by replacing the rubber fuel lines but need to park it out back for a bit until I have more time for other things like upgrading alternator, rebuilding PCB, etc.

So here's the two part question:
-Can I drive the truck without the rear prop shaft? Mainly for short distances on the road to verify all is working. I think the answer is yes but wanted to verify as it may be a while before I can get a capable Soldier B to help reinstall the rear shaft.

-Can I engage the front axles to make it a 6x4 in high only (not low) so I don't get stuck (or at least get unstuck from where I'm sitting. LOL!) Again, I think the answer is yes but wanted to verify with the expert since I understand there are limitations with being in low gear and backing up. Not sure if there is anything else to be concerned about.

I don't see any reason why you couldn't, assuming the brake lines are all still hooked up like normal so the spring brakes release. You're basically making a bobber without actually removing the back axle. Just be aware that since the two rear axles share load, it's going to be more prone to getting stuck like that. You might need the front axle engaged more than you think.

 

[msgjd]

although I don't own any M939-series trucks, you shouldn't have any issue doing what you want on-road other than the obvious "tractive" issues which others have mentioned..

I have for several years operated a couple of my M809-series trucks without the rear/rear driveshaft as well as without front driveshaft on extended (250-mile) road trips ..

One of my M818's had been run a couple months without the rear/rear driveshaft as "a 4x4 with tag axle", for lack of better way to describe.. ... It did just fine as long as operator stayed away from bad ground.. The front axle kicked in occasionally to assist the one "slippery" rear axle... But on the blacktop, the transfer case sprag sure made it known that it did not like running without a rear/rear shaft, only because of how the M39/M809-series sprag works on slippery ground or by wheel hop generated on rough surfaces.. The sprag situation was worse while bobtailing. I thusly removed its front driveshaft to experiment with one-axle drive OTR (6x2) .. Let's just say it was very slippery when wet and not inducive to any noticeable fuel economy, not in my kind of operation anyway.. But I had to try the experiment to verify my pre-assumption it wasn't going to work well for any of my operational situations. Curious cat here.

Even if you're running radials you might notice some wheel slippage at bridge joints, lateral road cracks, and on rough road whenever your tandem set starts to bounce.. In my case I still run NDCC bias tires and they hop pretty good on rough road no matter what.. I don't believe you will have any serious problem with your M939-series, but you will probably notice how slippery your truck can be if it's propelled by one rear axle.

Unrelated to your situation but included here for the curious, my other M818 has been without its front driveshaft for almost 16 years.. Up until a couple years ago when I obtained a FLD120, the "de-fronted" M818 had been performing a few 250-mile road trips flawlessly every year in that configuration as long as an operator treats it as a commercial rig with two live rear axles, in relation to the ground its operating on

 

[LCA078]

I don't see any reason why you couldn't, assuming the brake lines are all still hooked up like normal so the spring brakes release. You're basically making a bobber without actually removing the back axle. Just be aware that since the two rear axles share load, it's going to be more prone to getting stuck like that. You might need the front axle engaged more than you think.

I didn't even consider this was effectively a bobbed truck drivetrain but yes, that makes complete sense. These trucks have so many design quirks that don't pass the common sense test, I just had to verify there was no issue with running without a rear axle being powered. And yes, I was surprised how quickly the middle axle started to dig in when I turned on soft ground behind our shop. Everything was fine until I started the hard turn and she started digging. But by that time I was committed in that tight area so I just stopped and let her sit there while I worked on my M923 (that's another thread...)

although I don't own any M939-series trucks, you shouldn't have any issue doing what you want on-road other than the obvious "tractive" issues which others have mentioned..
I have for several years operated a couple of my M809-series trucks without the rear/rear driveshaft as well as without front driveshaft on extended (250-mile) road trips ..

Lots of good info there. My dump has 14.00R20 Michelin radials which I can't remember were standard issue or not. I thought the 395/85R20 were standard but either I think these are effectively the same size. But regardless, it's good to know that whether 6x2, 6x4, or 6x6 axle configs don't really impact mileage. As soon as I get this M929 road ready, I'll only run her as a 6x6 as I bought her to help put in a new road on our land and will need every wheel pulling.

 

[msgjd]

But regardless, it's good to know that whether 6x2, 6x4, or 6x6 axle configs don't really impact mileage.

please keep in mind that was my conclusion when the trucks were doing construction work and OTR heavy hauls .. Results may be different for those who only bomb around town or run empty/light loads on occasional road trips ...

As the saying goes, "your results may vary"