Allison M74 transmission growing in popularity

 

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Tech Tip: Allison M74 transmission growing in popularity

Since 2001 General Motors released its medium duty truck chassis equipped with the much needed Allison automatic transmission. Since that time the torque converter has been a target for failure and has received mixed reviews from owners and rebuilders alike. The much stronger and better performing Allison provided five forward speeds along with a lock-up clutch-equipped torque converter compared to its counterpart, 4L80E transmission that supplied four speeds and a lock-up torque converter. Although there seems to be a debate among rebuilders on the number of different GM configurations this converter may have, it is known to have at lease 19 or more, including stall speed, mounting pad design, torque multiplication and bearing design. Early diesel converters had multiple weaknesses. Bearing failure and front covers cracking around the mounting pads are very common. It is suggested not to reuse the roller bearings or front cover on these early units (see Photo A). If the converter is a 2003 or newer design the bearing issue has been resolved. Only you can decide to replace the bearings or not. General opinion on this subject that this is a $3000.00+ transmission: Replace the bearings. Also, it is suggested to offer the Raybestos high energy Power Torque clutch liner and a billet front cover as an upgrade to prevent premature failures or comebacks. Your customers will pay for quality. If you do decide to reuse the front cover, a very careful inspection of the clutch surface will need to be performed. Be extremely careful. The smallest cracks can go undetected.

Most the problems have disappeared after 2005 with the introduction of the LUK designed and manufactured torque converter for 2007. This converter has been completely redesigned and has improved clutch, bearings and front cover mounting pads (see Photo B). One part number is 29543003. Notice the machined register ring on the top of the impeller (see Photo C). This converter will not interchange with earlier converters due to an increase in the assembled height of about .190 of an inch. Early units measure 7.630-7.645 and later models, 2007, are 7.825-7.840 inches. Do not allow indicated run out to exceed .008. Balancing needs to be within 10 grams and requires proper internal alignment tooling. If you do not have it, buy it. You have to have it. Internal balancing is suggested before assembly if the clutch liner has been replaced or any machining performed on the piston. Balancing the piston and turbine together as an assembly works the best and allows for rotating the two components to achieve a balanced condition with out adding or subtracting weight. Zero grams is the target we aim to achieve although 5 grams is acceptable. Perfection can be accomplished with a little effort and some patience.

Dennis Sneath

Midwest Converters, Inc.

– Found in December 2008 Newsletter