Mitchell OD In MY AA - Update

[emf]

Sorry for the long post...

I thought I should bring everyone up to speed on my Mitchell Overdrive installation in my early 28 AA Firetruck. This is a 36% overdrive unit with a 3 speed transmission and a high speed rear.

The 110 pound box arrived by UPS and I was ready to dive in. The front torque tube with the shaft installed was attached to the gear box, along with the rear torque tube. The rear shaft and its coupler were not installed. This needs to be installed by the user. The unit was shipped with a cable shift linkage that was intended to be clamped to the standard shift lever. I thought this looked too modern, so a call was placed to Mitchell. They agreed to replace the cable linkage with a shift lever type. It was explained to me that hard linkage using clevises and a rod on AA trucks may not be desirable, since the box sometimes jumps to neutral when hitting bumps or pot holes with these heavily sprung trucks, so they have gone to a shift lever with cable. Since they have never shipped a kit for a firetruck, they had no way to determine the length of the cable needed to get around the large firetruck pump that sits between the transmission and the second universal joint. They shipped an ‘off the shelf’ length of cable and suggested that I try it. If modifications were necessary, we could do so once the unit was installed.

The parts went together without issue and I was surprised how quickly it was all assembled. I assembled the unit on a spare rear end assembly so as not to lay the truck up for an indefinite amount of time. This is parade season… I would say that the total time necessary to remove the old drive shaft, torque tube, and radius rods from the old assembly was 1 ½ hours. The assembly of the new parts to replace those removed was about the same. Working on a lift was a tremendous advantage.

The hardest part was connecting the cable linkage between the box and the handle. After much consternation, and since they did not have an off the shelf front cable bracket for the AA, I decided that the easiest method would be to fabricate my own hard linkage using rod and clevis. This forced me to extend the bell crank at the transmission end by about 4 inches to get below the large frame cross member just behind the pump. Once I made that decision, the rest followed without issue.

Results. I am amazed at the workmanship of this unit. It is quite well made and my guess is that I’ll get many years of great service. It shifts very smoothly and is almost effortless. I can guess at my top speed in overdrive is about 40-45 mph. I need to have someone follow me and clock my speed. I’d do it again in a heartbeat.