Don't Use Low Range Reverse with 939's? Here's Why !!

[rivercreek]

So, I thought I would share the result of using low range reverse on a 900 series truck, for those of you who are not clear as to exactly why this is a bad idea...

I'll give a short background on exactly what caused this catastrophic failure: We use a number of 900 series trucks for snow removal - they are contracted with VDOT for the winter season. Since they sit all summer, inevitably some of the brakes are seized up by October, requiring a few strategic smacks on the brake drums with a 4lb hammer to release. I had a buddy over helping me with pre-season inspection/maintenance a few days ago. Assuming he was checking fluids, tires, etc., I didn't worry too much about what exactly he was doing outside in the back yard until I heard a huge BANG. Long story short, he had fired up the truck, pulled the t-case down into low range and apparently was attempting to break the shoes loose by rocking it back and forth. Physics 101: When the wheels won't spin, the power must go SOMEWHERE! This pain in the @$$ wasn't completely bad, however - there is a bright side. We've discovered a valuable time-saving maintenance tip that I am happy to share with everyone: This is by FAR the quickest and easiest way I've ever seen to drain the transfer case. Much quicker, easier and faster than having to climb up under the truck with a wrench and a shop rag and a drain pan and all that hassle. This method I will call the "Mark Technique" in honor of my buddy Mark (the guy who did it).

 

[therooster2001]

Ouch!! I just thought it would have bound some gears. That's a complete blow up... The Mark technique...

 

[bchauvette]

Accepting Engineering that do this is in the same category as not being able to put it in gear when self extracting winching.

 

[simp5782]

Maybe it just needs a little JB Weld and Gorilla Tape to cover that hole!

 

[Chief_919]

Yep. In Afghanistan the trucks we had working doing mine clearance at Bagram had to work in low range, and the drivers would "forget" to shift the transfer case when backing up and we replaced several cases every month. I put forget in quotes because it was more a case of they didn't want to shift the t-case all the time so thought they could be easy on it and not break it.

When the MWO came out for the kit that blocks reverse in low range we didn't see any more broken cases, but we did see the number of round trips the drivers made drop about 15% a day because now they were actually having to shift instead of just saying they were and breaking cases.

 

[1 Patriot-of-many]

Ever notice everybody, it's always the "other" guy...... Thanks for the photos, just reinforces the point. DON'T REVERSE in low range!

 

[Archytas]

That's pretty bad. My M923A2 had a lockout. I couldn't put the truck in reverse if the transmission was in low-range.

 

[acme66]

I did the modification to prevent the engagement of the front axle automatically when put in low range, only works now with the selector switch on the dash. I would be curious why this little tweak wasn't done in the field if case failure was so high. I assume it would it would fail inspection and get the mechanic in hot water? Seems so simple the drivers would sneak under and do it themselves just to avoid the high/low hassle. I know I would have been tempted...

Ken

 

[Floridianson]

The other thought would be also not to let something sit for a year. With the salt a sand used this can be a bad thing on the equipment. I also plowed for VDOT and would clean the trucks the best I could but that salt is a killer.

 

[Chief_919]

Tracking down these kits would probably be a good idea for thsoe using these trucks in fleet service.

 

[Katavic918]

Anyone know if this applies to an m35a2 also?

 

[Scar59]

Only applies to the M939 series of trucks.

 

[red]

Anyone know if this applies to an m35a2 also?

Does not apply to the deuce.

 

[rmgill]

Yeah, this is a very specific thing to the M939s.

Is it because the reverse is already a very low range that low reverse makes for far more torque than should be used? Seems like a major failure of design and engineering.

 

[74M35A2]

Without checking the TM or Allison resources, I believe I remember the Allison MT654CR transmission reverse gear ratio is in the range of 10:1, as where for a comparison, first forward gear is 4:1 (or 4.xx:1, but just rounding here). Another simple way to see the reverse ratio is to simply drive in reverse (T case H range), it doesn't go all that quick. Even pedal to the floor on the governor, and you are not backing up all that fast. Bad guys could still run and jump on kind of speed.

 

[JohnnyBM931A2]

 

[bchauvette]

Yeah, this is a very specific thing to the M939s.

Is it because the reverse is already a very low range that low reverse makes for far more torque than should be used? Seems like a major failure of design and engineering.

Not a failure in engineering. Anybody involved in the acceptance process knows or should know this is not acceptable. Its a matter of accepting the unacceptable. Makes you wonder whats going on behind the scenes. Why did the lock out WMO come out way after the fact?

 

[Katavic918]

Yup. Seems like someone dropped the ball. Or realized it in the 11th hour and kept quiet.

 

[dmetalmiki]

Yeah, this is a very specific thing to the M939s.

Is it because the reverse is already a very low range that low reverse makes for far more torque than should be used? Seems like a major failure of design and engineering.

Indeed, to keep company with some handbrakes easily knocked off exiting trucks..no locking diffs..wiz kid (un-hardy electronics)..and winch and roll has always been a very usefull feature.

 

[The HUlk]

We've discovered a valuable time-saving maintenance tip that I am happy to share with everyone: This is by FAR the quickest and easiest way I've ever seen to drain the transfer case. Much quicker, easier and faster than having to climb up under the truck with a wrench and a shop rag and a drain pan and all that hassle.

I believe this method also speeds up the process of "splitting" the cases!