Allison 754 shifting

[Hi yo silver]

Thought my jake quit working, but found that regardless of gear selected on my 754, the transmission just upshifts rather than holding in the lower gear and allowing the jake to do its job. Of course, I'm hoping its something simple like an external modulator valve, air line or something. Any expertise or experience? Thanks guys.
Dennis

[eagle19952]

I may misunderstand....not unusual...
But i think you have to pre-select a position on the gear shift that won't up-shift...
unless you have a world Trans...then I don't know,unless it needs re-programmed.
are the Jakes newly installed ?

[buswarrior]

A little more specific description of the conditions in which the transmission upshifts please?

An Allison is a wickedly smart machine, even the old mechanical ones: if you are approaching over rev, it will upshift to protect the engine.

Were you below the speed for that gear that the engine can push to?

It won't withstand being spun much higher than you can push it with the engine when attempting to retard the speed with a set of Jakes or a retarder.

happy coaching!
buswarrior

[luvrbus]

4 bolts on the rear of the transmission passengers side remove those and check the governor, the weights maybe stuck you won't even get oily If that is not the problem pull the valve body you broke a spring

[Hi yo silver]

Thanks for your responses, guys. Jakes are not newly installed; been working like a charm. Also, I'm within the appropriate rpm range, not where a downshift would prompt an over rev condition. For example, I can be driving at low speed (15-20 mph?) with the selector in position "3", expecting the jake to hold when I let off the accellerator, but the transmission just upshifts as if it's in a higher gear position. (My selector is marked 1-2-3-4-D-N-R)

Oh, yeah. It's an old one. My bus is an '82, and I believe it's the original transmission. Around 300k coach miles on a Custom Coach conversion. I know how it used to behave and it "ain't doin' it".

Luvrbus, thanks for the headsup on the governor and valve body. When I can get it over a pit, I'll know to check into that first. This means there's hope that I can afford the repair! Haha! Thanks, I'll let you know. Marilyn would beg to differ with you regarding not getting oily!
Dennis

[luvrbus]

The torque converter is unlocking at that low of speed doesn't matter what gear you have selected unless yours is equipped with a lockup solenoid what rpm are turning at 15 to 20 mph ? and what is the oil pressure sounds more like a Jake problem

good luck

[robertglines1]

Dennis: coming down this winter?  Transmission; when air is up shut engine off go to back around right rear tag and listen for a bubble type sound from inside trans oil pan. If you hear it that is prob the mod valve on side of trans. Mine went out and caused erratic shifting. Less than $50 from local Allison shop. easy fix/external.   air was traveling thru it into trans sump-pan.   Everyone else is probably rite but this is a simple check and doesn't cost $ Bob

[luvrbus]

Does Dennis have a air throttle ? with a air modulator if he does those are supposed to be replaced every years it's been a while since you bought one Bob inflation don't you love it 95 bucks from DD now lol

good luck

[Hi yo silver]

Thanks, Bob. I'll do the check. Hoping to come down this winter. Luvrbus, No air throttle. I'll have to take it out again and note more specifics re. speed, rpm, etc. But I can tell you it does not engage and hold the jake like it should, in the appropriate rpm range. I know how it should behave and now the jake will engage momentarily then the transmission upshifts, almost imperceptively, rather than locking up in the selected gear. ...Did you notice my use of that big word? Probably improper usage, but impressive, don't ya think?

Disclaimer: These tests are being conducted on an enclosed track with a professional driver. Do not attempt this at home.

Dennis  

[luvrbus]

You won't have a air modulator then it will be the mechanical type with a cable working off the throttle let me know on the rpms, I still think it is a Jake problem like a cross over tube broke or blew a cross over seal 

good luck