cool stuff on the cables!! something for me to think about when I finally get into restoring my fishbowl.. the shifter isnt horrible but definitely has slop in it for sure..
GMC products ALL used the same rods and bell-cranks to transmit motion from the front to the transmissions. They varied in the actual number and routing of the rods and locations of bell-cranks due to the actual vehicle in question and body style and underfloor layout.
For instance the single level 4104, 4106 were fairly simple with a minimum of bell-cranks needed to change direction of the motion, and those are at the rear mounted to the firewall and take the motion from the centrally running rods and sending it to the two transmission shift levers. The two shift levers on the transmission each only control one of the internal shift rails. One is 1st-2nd gear and the other is 3rd-4th gear. The rods are connected to their respective shift lever and the motion is carried to the front.
Under the front shift tower below the floor is the mechanism for moving the two shift rods fore and aft. It's internal design is of two square blocks that move independently and attached to their respective rear headed shift rods. These blocks each have a square notch facing each other where the bottom of the shift lever has a heavy duty lug that moves from one to the other in neutral and once engaged in a block can move it either fore or aft. This is why the shift lever will feel very easy and tight to move in neutral yet can be very sloppy with lots of lost motion obvious when actually shifting into or out of a gear.
One area that can really cause a lot of lost motion is the shift tower itself and where it is bolted into the floor. If the bolts loosen up or the floor is rotted the tower will move and the critical pivot fulcrum point at the top will be moving around and not transferring the motion into the rods below the floor. I've experienced this first hand through the years with horribly maintained GMCs and the result can be to make shifting almost impossible. The trick is to flick the wrist when shifting and impart a large amount of initial high speed motion and let inertia take over to move the motion of the rods all the way to the rear.
Reverse is internally controlled by a third shift rail and inaccessible from the normal 1/2, 3/4 shift levers. So GMC devised the electric solenoid to pull the internal lug over into the 3rd reverse rail. This is accomplished with the driver putting it in 1st gear and then hitting the reverse switch/button and pulling it back into 2nd gear. This is the typical GMC manual transmission operation from the very earliest days.
You will be interested to know that to the maximum extent possible GMC designed their buses to use as many as possible common parts and similar overall engineering designs to keep manufacturing costs and bloated parts inventories down. From roughly the 4104/4106 designs they created the city transit models which were almost exactly the same below the floors as the highway buses. You'll find many interchangeable parts for your Fishbowl used in the 4104/4106 models. Main differences are the inline 6 vs the two V-6/8 engines and the design differences needed to operate them. This commonality of design philosophy was one of the many genius innovations GMC did. Many operators liked the overall designs and ease of maintaining the buses to keep them on the road and making money. Simple, easy to repair, and very reliable are all important to a for-profit operator.
You could replace the rods with cables for sure, but it won't be easy since you'll need to gain access to the underfloor tunnel the rods and other lines run through. That's not a normally accessible area and may promote some creative new expletives... Try to secure the front shift tower assembly if possible to the floor and continue to use the below floor shift block system. It's better to use this since it's real easy to attach any new cables to the blocks themselves in place of the current rods' clevis's. That's what I did on the Scenicruiser and it was an extremely simple process since the blocks already have mounting holes to accept the clevis pins.
Instead of going to all the trouble of installing cables you may find it fairly easy to locate the needed parts, bell-cranks, clevises, pins, possibly new or from another donor bus where they aren't so worn. Remember to look at highway buses since they will be very similar if not identical parts. Then make the effort to repair and tighten the front shift tower to the floor so it doesn't move at all. This will make a huge difference in the whole system response. It's an easily overlooked problem area.
Sorry for the length but I do get carried away sometimes trying to make myself clear on what I'm describing. Good luck and happy hunting.
