HMV Transmission Basics

As Snoopy might have written, “It was a dark and stormy night,” and I was driving a 1947 Ford ex-Railway Express Agency 2-ton truck on a lonely mountain road. While attempting a double-clutch from fourth to third to spare the brakes on a steep downgrade, the shift lever came out of the transmission! I found myself holding the lever while rapidly gaining speed in neutral. I did what seemed logical: I stuck the lever back in the hole and completed my down-shift. As soon as I was able to stop, I found the shift lever retainer cap had come loose, so I screwed it down tight and finished the trip without further adventure.

I share that story as an illustration of just one thing that can happen if you don’t regularly inspect and service your vehicle’s transmission.

THE BASICS

Motor vehicle transmissions may be divided into two basic types: manual and automatic. With the exceptions of some tanks and other types of tracked vehicles and the GMC M211, most common U.S. military vehicles from the turn of the 19th century through WWI, WWII, and into the 1970s, had manual transmissions — vehicles such as jeeps, MUTTS, Dodge WCs and M37s, GMC CCKWs and DUKWs, and Reo M35s. Late model deuces, most CUCVs, and HMMWVs have automatics.

Beginning with the technical basics, the purpose of a motor vehicle transmission is to provide the driver with a selection of gear ratios between the engine and driving wheels so a vehicle can be operated at best efficiency in a variety of terrains, speeds and loads.

There are three basic types of manual transmissions: sliding gear, planetary, and friction disk (though the friction-disk is mostly obsolete).

The planetary (as was fitted to Model T Fords) is also no longer in general use, though its design is incorporated in many automatic transmissions using fluid couplings or torque converters, as well as in overdrives and in some dual-ratio rear axles.

The sliding-gear type is the most common today, and is what most people think of when manual transmissions are mentioned.

There are two basic types of sliding gear transmissions. The first and earliest type is commonly called a (or “crash-box”) in which the gears themselves actually slide. The second is the in which the gears are fixed in