By Dan Guarino
Owners of diesel trucks equipped with NV4500 5 Speed Transmissions have been looking for a cure for 5th Gear failures from as early as 1993. The problem has been experienced with both General Motors trucks using the 6.5l V8 Diesel and Dodge trucks with the 5.9l inline 6 cylinder Cummins Diesel engine. So, exactly what is the problem and what is being done about it?
“I Lost 5th Gear!”
Almost all cases of Mainshaft 5ifth Gear failure can be directly attributed to insufficient support for 5th Gear. In the classic case, the nut retaining the Mainshaft Fifth Gear backs off allowing the Gear to slide back in the transmission extension housing and out of contact with countershaft 5th gear. This failure usually occurs without warning or noise. You are driving happily along in 5th gear, either you (or your cruise control) lets off on the accelerator for a moment. You step on the accelerator to resume speed and the engine races, but your transmission is no longer transmitting power! White faced and shocked, you mutter “what the #%!!” as you check to make sure the shifter is still in fifth, then downshift to fourth and let the clutch out. Your truck responds instantly as power is returned to the rear axle. Your blood pressure lowers a bit and you try to upshift to 5th. Again, no power and no noise! You mutter “Oh no, I lost fifth gear!” Back into fourth and away you go wondering if you are going to make it home and HOW MUCH IS THIS GOING TO COST ME TO FIX!
Typical 5th Gear failure on a stock NV4500HD Transmission. Note the reddish/orange sludge discoloring the shaft. This is a sign of wear or “fretting”. What you see is actually oxidized metal particles worn from the gear and shaft. Vibration and cyclic loads are the main causes of fretting

Every time someone tells me “I lost 5th gear”, I am reminded of an occurrence in my youth. I was riding to school on a Brockway school bus. We had a lady bus driver who was a chronic gear grinder. One morning, when she was unable to force the transmission into fifth gear, she radioed the school that “I lost 5th Gear”. The reply was something to the effect “Wh’ad ya do drop it on the ground?”. I presume she had little solace with that reply, but she bravely continued on in 4th gear and we all eventually arrived at school. Like my bus driver, you too will probably also make it to your destination after your 4500 turns itself into a 4 speed because this type of failure normally does not cause incapacitating damage in the short term. Chances are, what occurred inside your transmission was simply the 5th gear wore into the mainshaft. Once the gear is loose, it will work the nut off until the gear slides back and your day is ruined.
Owners of diesel trucks equipped with NV4500 5 Speed Transmissions have been looking for a cure for 5th Gear failures from as early as 1993. The problem has been experienced with both General Motors trucks using the 6.5l V8 Diesel and Dodge trucks with the 5.9l inline 6 cylinder Cummins Diesel engine. So, exactly what is the problem and what is being done about it?
“I Lost 5th Gear!”
Almost all cases of Mainshaft 5ifth Gear failure can be directly attributed to insufficient support for 5th Gear. In the classic case, the nut retaining the Mainshaft Fifth Gear backs off allowing the Gear to slide back in the transmission extension housing and out of contact with countershaft 5th gear. This failure usually occurs without warning or noise. You are driving happily along in 5th gear, either you (or your cruise control) lets off on the accelerator for a moment. You step on the accelerator to resume speed and the engine races, but your transmission is no longer transmitting power! White faced and shocked, you mutter “what the #%!!” as you check to make sure the shifter is still in fifth, then downshift to fourth and let the clutch out. Your truck responds instantly as power is returned to the rear axle. Your blood pressure lowers a bit and you try to upshift to 5th. Again, no power and no noise! You mutter “Oh no, I lost fifth gear!” Back into fourth and away you go wondering if you are going to make it home and HOW MUCH IS THIS GOING TO COST ME TO FIX!
Typical 5th Gear failure on a stock NV4500HD Transmission. Note the reddish/orange sludge discoloring the shaft. This is a sign of wear or “fretting”. What you see is actually oxidized metal particles worn from the gear and shaft. Vibration and cyclic loads are the main causes of fretting

Every time someone tells me “I lost 5th gear”, I am reminded of an occurrence in my youth. I was riding to school on a Brockway school bus. We had a lady bus driver who was a chronic gear grinder. One morning, when she was unable to force the transmission into fifth gear, she radioed the school that “I lost 5th Gear”. The reply was something to the effect “Wh’ad ya do drop it on the ground?”. I presume she had little solace with that reply, but she bravely continued on in 4th gear and we all eventually arrived at school. Like my bus driver, you too will probably also make it to your destination after your 4500 turns itself into a 4 speed because this type of failure normally does not cause incapacitating damage in the short term. Chances are, what occurred inside your transmission was simply the 5th gear wore into the mainshaft. Once the gear is loose, it will work the nut off until the gear slides back and your day is ruined.