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Sticky 4 speed transmission

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I am trying to figure out what is going on with my non-synchronous 4 speed truck transmission that sort of "sticks" ~ not letting me shift into any gear slot.

I have a 1940 International / Fabco fire engine with the chassis being a International D35 truck powered by the International 241.54 cu. in. displacement inline 6 cylinder gas engine.

The transmission is the stock International 4 speed forward, 1 reverse, non-synchronous, mounted to the engine, with a “fire” modification (lock capability ~ 4th gear ~ to allow use of a PTO operated fire pump) ~ Northern pump. Single speed rear end. This truck is rated at 1 ½ to 2 tons.

What’s happening is, while I understand “double clutching” is a requirement for the non-synchronous transmission many times I can’t get to stick to slip into any gear. This is especially true when the truck is stopped, as at a red light. It seems like there is a physical block keeping the stick from slipping into any gear.

I’ve changed the transmission fluid and am using standard 90-140 weight gear oil (off the shelf from the local auto parts store). The transmission doesn’t shift sluggish, so I’m not sure if it’s a sludge issue. I also sprayed BP Blaster brand penetrating oil into the gap between the shifter stick and the top of the transmission, where it doesn’t get oil trying to lube the apparatus at the top.

With the engine turned off, the stick goes into all gears easily and smoothly. The clutch seems to be working normally, with no slip. The gear blocking seems to happen with the engine running and it doesn’t matter if the truck is rolling or at a complete stop. I push in the clutch, try to slip the shifter into any gear, and it's like something mechanical is blocking the shifter from dropping into a gear.

Would any of you have thoughts on what could be going on? I don’t mind a little gear grinding on normal shifting (operator error on my part), but sitting at a changed traffic light trying to slip the rig into gear is a total pain! Thank you for taking time to think about what might be happening.

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When you depress the clutch to change gears. the shaft stops turning so the change can be made. If it does not stop perfectly aligned, then the shifter can not slide to the next gear. Try easing the clutch out gently to just barely move the shaft while putting some pressure on the shifter. When aligned, it should fall into place. once you get the feel for your particular tranny, it will become easier. Hope this makes sense.

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Thanks for the tips Fleek! I'm thinking this might be the solution, slipping the clutch a bit. A lot of the problem is operator error. This rig doesn't get driven very much, so I lose the finesse of feeling the gears. I'll try your idea! Thanks again!

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  • 1 month later...

On manual transmissions there are detent balls, springs and spacing pins to give the locked in feeling when shifting into gear thus also holding it in gear. These also are used to keep the transmission from shifting into two gears at one time. You mention a 4th gear lock system for the pump pto. If this has any devices used to keep the truck from being shifted into gear to be driven I would look into this feature closely. If a transmission does go into two gears at once it is impossible to move until one gear is shifted out or the weakest link breaks. Check your linkages for movement while the engine is running versus engine static.  

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