I just came back from a camping trip and when I was 60 miles from home towing my 30 ft fifth wheel and 12' flat bed trailer with a side X side on it, I stopped for fuel and when I pulled out of the pump I could hear a noise under the truck so I stopped and the noise quit soon after kinda sounded like exhaust spit to me. When I got back on the highway and got back up to speed everything was fine till I went for 5th and it just ground I backed off and slowed down then got back up to speed and same thing it ground in 5th. I had one shop familiar with the fifth gear nut problem say they usually make not noise at all. Does this sound like a fifth gear nut issue or worse. And for those that have had the nut issue has anyone tried one of these to help keep the nut on.

 

https://puredieselpower.com/dodge-products/dodge-4wd-nv4500-transmission-5th-gear-repair-solution.html

Your going to need the upgraded mainshaft and then have the rebuilder torque it to the proper torque. Like the stock shaft and nut is like 520 foot pounds then the upgraded shaft is 660 foot pounds. Don't quote me on the torque values but I know they are quite a bit. You most likely got the splines beat up now from the gear being loose.

Could it be a synchro. 

If its grinding it's still there?

It sounds like the gear walked off the shaft splines.  Towing heavy in 5th gear and/or driving with a habit of abrupt ON and OFF throttle in 5th gear will place a lot of pressure on the gear nut and eventually allow the gear to walk off the splines.  It must be hanging right off the splines too if it made a grinding sound when you tried to put it into 5th gear.

You'll want to pull off the transmission end and pull the nut and gear off.  Inspect the shaft for wear on the splines.  If they're damaged then you'll need to replace the shaft, but its likely they're fine.

The part to fix is supposed to be really great but it only works on 4wd trucks.  Also, make sure its from the people who invented the product too because there's copy cat versions which are not the same quality.  http://fifthgearrepair.com/nv4500repair.html

All the years of owning a NV4500 I've to this day can say I've never lost a 5th gear nut. Even though my current transmission is out and 5th and reverse are damaged but not from a 5th gear nut problem. The current transmission does have the updated mainshaft. 

 

The biggest problem is the 5th gear nut has a huge torque number for tightening. Like 550 foot pounds on the stock shaft and like 660 foot pounds on a upgraded shaft. Don't quote me on the numbers. The torque value is very high. Something that REQUIRES a proper spline socket and a 5th gear socket to tighten properly. Please don't do the redneck fix with a hammer and chisel this will never tighten the nut enough to keep it from coming off. 

The FSM says 280#, I go to 450#. I use the 3/4 drive nut wrench with my 150# and a 3' pipe.

 

https://torqueking.com/product/882036/qt2036-3-4-drive-mainshaft-nut-wrench-quadtool/

Ok my numbers are off from memory but I know it not something you can just beat the nut on with a hammer. Seen way too man of those redneck video of guys using a hammer a chisel. Your going to need a spline socket as well to hold the output shaft. 

Dropped the truck at the shop this morning it whines in first and second now loudly, they drained part of the fluid it was full of metal shavings, so time to get a rebuilt trans and I was thinking of upgrading to a South Bend or Valair 13" clutch, I do tow a lot at the 13,000 lb weight and was figuring on pushing the horsepower up closer to 400 hp.

On 9/2/2019 at 6:14 AM, Mopar1973Man said:

 Your going to need a spline socket as well to hold the output shaft. 

 

Just slide it into two gears, 2nd and 4th and that locks the output shaft.