Causes of Manual Transmission Failures

Causes of Catastrophic Manual Transmission Failures

By Dan the Gear Man™, 4x4 Tech, Inc.

Catastrophic manual transmission failures are an expensive headache you can happily do without.  Blowing your transmission apart while towing a heavy trailer at 70 mph is dangerous and very, very inconvenient.  As you walk home or wait for the tow truck, you can ponder the sad fact that most catastrophic failures are easily preventable.

The worst killer of manual transmissions we see on Dodge Turbo-Diesel trucks is a small, often overlooked, $10 part that is not even part of the transmission.  The pilot bearing or bushing is pressed into the flywheel.  It’s function is to support the forward end of the transmission input shaft, keep the clutch disc centered on the flywheel, and allow the input shaft and flywheel to rotate independently of each other when the clutch is disengaged.  Dodge used a thin bronze pilot bushing on 1989-1993 trucks and a small, needle type pilot bearing from 1994 on. 
 

When a pilot bushing/bearing gets worn oversize or disintegrates, you may experience erratic clutch performance and/or shifting problems because the front of the transmission input shaft is essentially free floating.   With a failed pilot, your transmission input shaft is no longer held in rigid alignment with the mainshaft and counter shaft.  The gear on the input shaft transmits full engine power to the countershaft in all gears except direct (4th on a 5speed, and 5th on a 6 speed) so any misalignment will cause that power to be transferred from gear to gear with inadequate tooth contact.  Given time, metal fatigue will weaken the gears until a catastrophic failure occurs.

Over the years we have tried various types of metal bushings, Kevlar bushings, needle bearings, and ball bearings in an effort to find the “permanent fix”.  None of the metal or Kevlar bushings we tried consistently performed satisfactorily in Montana and Wyoming due, I believe, to dust, dirt, and grit getting between the bushing and shaft.   The original, single seal pilot bearing Dodge introduced for the 1994’s quickly lost it’s lubricant and many failed within 50,000 miles.  About 1998, Dodge started using a wider bearing with a seal on either side.  This bearing appears to have at least twice the life of the original single seal bearing, but it is still less than the life of most clutches.  We then tried boring the flywheels to accept standard 3/4” ID ball bearings.  Much to our surprise, the thin inner race of standard series ball bearings quickly broke apart.  Finally, when we were about ready to admit defeat, one of our suppliers sent us some ball bearings with thicker races.  These special bearings (p/n QU51010) have performed flawlessly in every truck we installed them in.
 

(L)QU51010 HD Ball Bearing (R) Standard ball bearing

Lubrication related failures are often so severe that the transmission must be replaced with a new unit.  Most lubrication related failures are totally preventable.  Common causes include using the wrong oil, operating with insufficient lubricant, or going too long between oil changes.  If you have a first generation truck with the Getrag G360, your transmission has inherent lubrication problems that can be partially alleviated by both overfilling the transmission with an extra quart of oil, and using only synthetic lubricant of the viscosity recommended in your owners manual. 

Refilling the transmission with the wrong lubricant is becoming more common all the time.  Part of the problem is the plethora of manual transmission lube choices.  The G360, NV4500HD, and NV5600 all use radically different lubricants.  Fortunately, using the wrong lubricant usually causes shifting problems (particularly with the NV4500HD) long before total failure occurs.  Regular automatic transmission fluid may work great in your buddy’s Ford Ranger 5 speed, but it will probably make your Dodge, Ford fullsize, or GM truck transmission self destruct. 

Over the years, we have worked on many transmissions that were run out of oil when the transmission oil level wasn’t checked periodically.  The G360 and NV4500HD have a common problem with leaking rear seals, while the NV5600 seems to have an affinity for losing oil from the input seal.  These types of leaks are usually rather minor, but the fluid loss can become very significant over time.  A loss of a couple of quarts or more will stop you about as quick as a landmine because no 5 or 6 speed manual transmission is very forgiving of high loads and low oil levels.
 

Oil from leaking NV5600 input seal collected at bottom of bellhousing

Dodge 4x4 trucks equipped with performance enhanced engines, NV4500HD transmissions, and auxiliary two speeds, such as the Gear Venders™ or U.S. Gear Dual Drive™  units, may have problems with the aluminum transmission tailshaft housing cracking.  The cracks may or may not cause oil leaks, but if they do, oil loss and subsequent bearing seizure is a certainty if not caught quickly.  We recommend installing an optional, cast iron tailshaft housing on the NV4500HD whenever an auxiliary transmission is installed in a 4x4 truck.
 

  
Mark Highlights Cracked NV4500 Tailhousing QK1039 Cast Iron Tailshaft Housing Kit

One of the laws of vehicle preventative maintenance states “changing oil is cheaper than changing iron”.  Over the years, I have found few Dodge owners violate that rule with their Cummins engine, but many never apply it to the transmission.  How often should you change the transmission oil?  For most years, Dodge does not even have a recommended manual transmission oil change interval.  However, back in 1993, Dodge did recommend changing the Syntorq LT™ transmission oil in NV4500 equipped  trucks at 18,000 miles for severe service and 36,000 miles for normal service.  Those intervals are what we recommend for all manual transmissions bolted behind a Cummins engine.  If you use a manual transmission filter kit like our QK1000, which has powerful magnets combined with a simple, but astonishingly effective filter to cleanse the oil of both magnetic and non-magnetic particles, the oil change interval can be extended by 6,000 miles for severe duty, or 12,000 miles for normal service.  Filtering the oil is especially important on NV5600 transmissions to remove bronze particles worn from the synchro rings.
 

Debris caught by QK1000 manual transmission oil filter on NV4500HD.  Transmission was run with low oil after stock tailshaft housing shown above cracked from extra weight of auxiliary transmission.

The second most embarrassing way to wreck a transmission is by towing your truck with another vehicle (the MOST embarrassing way is to drain your tranny oil and then forget to refill it!).  We have seen some spectacular examples of seized 4x4 transmissions that came out of towed trucks.  The problem seems to be oil starvation because the mainshaft bearings do not get adequate lubrication when the counter shaft is not flinging oil throughout the transmission.  The Getrag G360 seems to be particularly susceptible to damage of this type.  The 1993 Dodge Full Size Truck service manual allows short distance (15 mile), low speed (30 mph max.) vehicle towing if the transmission is in neutral.  1994 and newer Dodge service manuals recommend 4x4 vehicles only be towed if all the wheels can be lifted off the ground using tow dollies.
 

Twisted off mainshaft splines from G360 that was flat towed 45 miles with transfer case in gear and transmission in neutral.

There are of course, other, even less common causes of catastrophic transmission failures.  The infamous NV4500 fifth gear problem for instance, almost never causes a total failure the first time the gear backs the mainshaft nut off.  However, the temporary fix of putting on a new nut and 5th gear (instead of replacing the mainshaft) often does lead to a major failure of either the transmission or transfer case.  As a rule, the old 5th gear wore into the shaft.  Once the shaft is worn, it will be impossible to keep the gear tight and concentric on the shaft.  Since the gear is loose, it can’t keep the rear mainshaft bearing tight against the reverse gear thrust washer.  The bearing wears into the shaft and/or thrust washer which increases mainshaft endplay.  When this happens, the transfer case input ball bearing is also supporting the transmission mainshaft which it can’t do for long without failing itself.  At that point, anything can happen and it won’t be cheap to fix.  If you are not very lucky, your wobbling fifth gear is going to break the fifth gear fork, pieces of which will get ground between the gears, which in turn will stop your transmission in a heart beat. 

 

Since they are mechanical devices,  manual transmissions do occasionally have a catastrophic failure that is caused by a single defective factory part.  Back in 1997, we had a brand new NV4500HD main case break after 200 miles from a casting flaw.  However, such failures of original equipment parts are quite rare.  The record on aftermarket parts for the NV4500 is not quite so good since there is a profusion of aftermarket parts available.  Some are quite good and some are not.  Still, most problems associated with certain aftermarket parts do not lead to true catastrophic failure unless ignored.

By practicing good preventative maintenance and paying close attention to subtle changes in the way your truck drives, you will probably never have first hand experience of a catastrophic transmission failure because you will detect the problem before it gets worse.  If, however, you drive with the pedal to the metal, oblivious to everything except the road ahead and the tunes blasting from your stereo, you might want to keep the phone number of your favorite transmission repair shop close at hand.