R model Mack

[Don Wilson jr]

I am restoring a R model that had a five speed. I have installed an Allison HT 740 auto and have been trying to figure out what to do with the modular. Trans did not come with kick down linkage. I do have the shifter. A Allison dealer recommended going with a plug kit to bypass it. What are your thoughts. The truck is a toy and will only be used for shows and drive around town. He did say it would shift a little harder.

Thanks Don

[mowerman]

Holy cow that’s way over my head. I can’t help you at all. Good luck….  Bob

[mrsmackpaul]

Don I have no idea sorry

Just try it with out the kick down connected I guess and see what happens 

 

Paul 

[BOBWhite]

It might hang gears and be a slug without a modulator. Is it vacuum modulated or is it supposed to have a linkage to the throttle? 

Edited February 4 by BOBWhite

[JoeH]

Id take Allison Dealers advice. My understanding of autos, as it shifts out of one gear into another there's a momentary overlap period where multiple clutches are working against eachother. Harder shifts are because it makes the shift happen quicker, which reduces the clutch slippage durations. In your application i don't see any concerns. Easy life ahead for that Dog.

[Don Wilson jr]

Thanks to all. Just going to give it a try.

[Vladislav]

Unfortunately I'm not familiar with Allison. And also any other truck automatic transmission. But I have extensive experience with Mercedes cars hydromechanical transmissions, fixed many of those. 

My guess the principals are the same. Those trannies I can tell about have 3 sources of info they use to figure out when and how to shift. Ok, there's 4th one either. So..

The 1st is a position of throttle. Or gas pedal. Organized by a link or a cable connected to the pedal. The more you press the gas the later the tranny shifts. Or even shifts one step lower if you step on the pedal full way. There's a control valve inside the tranny which opens or closes depending on the position of the pedal and supplys a certain level of control pressure. It's called a throttle valve. 

The 2nd is speed of the output or propeller shaft. By other words that's actual speed of the vehicle. The faster you go the sooner moment for an upshift. That is also provided by a valve inside the tranny which reacts to the revs of the output shaft. Some way. Its pressure is called a speed regulator pressure.

There's a kind of check valve in the hydraulic system (actually 2 or 3 of them depending on the amount of gears in a particular transmission). It has a plunger in the middle. The throttle valve pressue is applied to one side of it and the speed regulator pressure to the other. Depending on which pressure is higher (you press the go pedal or vehicle speed is high enough) the plunger goes to a side making signal to shift. Say upshift to the right or downshift to the left. Or keep the current gear if difference of the pressures isn't sufficient for the action.

Let's go further. The 3rd control. Cars have a modulator. Usually it's operated by vacuum in the intake manifold (my guess a truck would have some kind of control connected to the throttle linkage, the reason will be clean further). The function of the modulator is check of the load applied to the drivetrain. When you pull or acccelerate shifts must go fast to not allow multidisc clutches to spin. So you (the tranny hyd system) applys high pressure to servocylinders. But when you drive smooth with no load or on a flat road you prefere smooth shifts with no kicks or hits. So the operational pressure is set lower. This also allows to lower down load on the main oil pump in the transmission and save a bit of fuel burned by engine. So the modulator corresponds its control pressure in the hyd system (called a modulator pressure) to the grade of load applied to the tranny.

And the 4th point of control I promised to explain is kick-down. Usually that's an electric switch put under the gas pedal. It brings voltage to an electric valve in the tranny to open it and drain the speed regulator pressure to the oil pan. So the main shift valve(s) (the plunger(s)) go to the position for downshift. 

Hope the above is clear and of any help.

Vlad