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At540 problem karl 9-10-05  
Have a 1977 IH bus with a at540 trans. when it shifts to 3rd and drive it jump's between the 2 gear's. and jerks the crap out of us. any ideas?? thank's
karl
Re: at540 problem Phil Feinstein 9-12-05  
Start with the basics: check fluid condition and level. Any shiny bits in the fluid? Are the cooler lines kinked? Any noises when this happens?

If the fluid is iffy, you may be looking at a rebuild. My first suspicion is that something is causing the line pressure to drop once 3rd is engaged and that could be the front pump or the pressure regulator.

Hope that helps!
Re: at540 problem karl 9-12-05  
Fluid look's ok. don't see any kink's. if i manually put it in 3rd and take the rpms higher than where it usually shift's into 4th, let off the gas and manually shift to 4th it stay's in gear fine.
Re: at540 problem Phil Feinstein 9-13-05  
Yeah, that's also an indicator of a line pressure problem.

The real test is to hook up sopem pressure gauges at the 3rd gear clutch port and see if you have the right pressure in 3rd gear, but I think you may have found something important: when you set the shift valve manually, you are bypassing the accumulator valve in the valve body. If this sticks (which is common in *ALL* transmissions) then you'd have an issue like this.

From here, I'd say contact Allison and get some manuals on your tranny and see what controls line pressure and how to check it. I'm not familiar enough with this trans to give you specific troubleshooting tips from here.
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