The C6 Transmission: GSE’s Driving Force

Feb. 9, 2007

The C6 transmission has been the driving force behind ground support equipment for many years. Although other transmission types have been used, the C6 has proven to be the workhorse of choice, mainly because of its adaptability to be used behind a number of different tractor applications and power plant configurations. Another reason for the success of this transmission is that it can be modified from the original design—the three speed transmission—into a one or two speed depending on how the OE manufacturer wants the tractor to perform. This has made the C6 a very versatile transmission for the OEM ground support equipment industry.

I’m not going to bore you with C6 transmission 101, but you need to know a few things about this transmission to fully understand the success of this unit. Ford Motor started producing this transmission back in the mid 1960’s. It was designed to meet the demands of higher horsepower engines and heavier vehicles that were being produced at that time. Starting in the early 1980’s, Ford started using diesel engines for the power plant in some pickup truck applications, the first of which was the 6.9L and later on the 7.3L. The C6 provided an adequate torque rating behind these powerful engines.

From its original design in the 1960’s, the mechanical parts and hydraulics have stayed pretty much the same. The thing to remember here is the transmission (from its original design) ran behind gasoline engine applications, which use manifold vacuum. This vacuum is used by the transmission to control shift patterns and also a load-sensing device which controls line pressure up and down. The problem is that when it is put behind a diesel application, the diesel engine does not develop any type of vacuum. To simulate the vacuum that a gasoline engine creates, they had to install a vacuum pump and a regulator to control the vacuum up and down with throttle and load change. The reason I bring this up is that most of the industrial applications are using diesel engines like Deutz, Perkins, Cummins etc. which are usually not equipped with a vacuum pump or regulator system. This is why you see—on most applications—a mechanical cable that runs down from the throttle linkage to the transmission. Although this cable does work very well as a replacement for manifold vacuum, it can cause many issues, especially if it is not set up properly or adjusted to follow throttle travel.

Equipment manufacturers have really shown how much this transmission can withstand. This transmission is very capable of pushing, pulling and towing vehicles in the light truck world at weights exceeding 25,000 pounds GVW (gross vehicle weight) with no problem. Manufacturers really put this transmission to the test when they make it push, pull and tow vehicles weighing as much as 75,000 pounds GVW.

One of the main reasons they can get away with this is that most of the vehicles are geared very low, sometimes having as much as a 25 to 1 rear axle. Of course, this would give the vehicle a top speed of around 15 miles per hour. With these types of rear axle ratios, you can easily see how these vehicles can push, pull, haul and tow very heavy weights, plus the fact that you wouldn’t want to see a tow tractor, belt loader, etc. at an airport running around the tarmac at the speeds of a normal vehicle. I’m not sure about you, but I wouldn’t want to see a tow tractor or belt loader running around the tarmac at 60 mph.

You can see why tractor manufacturers put this transmission to use behind a number of different applications by using a one, two or three speed transmission as well as changing the rear differential ratio and engine configuration. The same transmission that is used to carry you back and forth to the grocery store is the same transmission being used to push and pull the commercial jet across the airport. Now I can already hear many of you are saying that the C6 transmission isn’t the only transmission being used in industrial equipment and you’re right, but it’s still the most widely used and has proven to withstand the demands of this industry.

Like most things in this world of technological advances, products have to change to meet newer standards and to be more efficient while still providing the same performance as their predecessors. The use of computer-controlled technology has been no stranger to automotive industry, but has only recently began to makes its way into the ground support equipment industry.

This has also changed the way we have to diagnose and repair these vehicles when a problem arises and also the special tools that will be needed to do so. We will talk more about this in the next issue.

Jody Carnahan is the Ground Support C6 Technical and Warranty Management Administrator of Certified Transmission. Certified Transmission remanufactures and converts C6 transmissions for off-road industrial usage in new industrial OEM vehicles. Certified Transmission will have a booth at the Aviation Industry Expo or you can visit us online at