AT545 shifting issues

[deezlugnutz]

Got a '99 International 4700 with a DT466E and an Allison AT545.


When transmission is cold it shifts fine through all gears. Once it warms up you can be driving along in 4th gear and suddenly it'll downshift into either 3rd or 2nd (swear I saw 3300 RPM on the ScanGauge display). Stays in that gear and won't shift any further, although if it's in second it will downshift to 1st if you slow down enough.



Fluid is nice and clean and nothing looks or smells burnt.



We removed the electronic modulator and tested it, it's working good.


Tested voltage supply at the modulator, works good when throttle is past 60ish %.


Relay tests fine.


Wiring looks fine.


Someone told us the modulator could be to blame. I don't think this is the case but we figured we'd check it out anyway. After the transmission warms up and starts to act up, the modulator is still being energized when throttle is above 60%.



We hooked up a test light to the modulator while driving around and it's definitely being energized even while bouncing down the road above 60% throttle.


Removed governor and it looks great.


At one point I did notice the circuit breaker that energizes the circuit for the modulator had popped. Not sure if any relation here. Reset it, no issues. Looped it out through a multimeter and the relay draws about 2.3 amps are being pulled through the 15a fuse to energize the relay.


From my understanding though the modulator issues would show up more when it comes to not shifting at part throttle, soft shifts, etc.



Any recommended tests we can do to further narrow this down? My experience has always been with Eaton manual transmissions in big trucks, haven't touched these little Allisons much.


Thanks

 

[Booyah45828]

Is this truck new to you? When did the problem start to occur and what have you done to try and resolve it?

Do a main pressure check when warm, in both N and R, and let me know what the numbers are.

A modulator wouldn't cause your issues.

Most likely you're losing governor pressure when the fluid is warm which is causing the transmission to downshift. Most of the time I see the governor port or governor itself worn out causing these issues. You could have an issue with the valve body or governor tube leaking if they weren't installed correctly. But if this just started and nobody has messed with it, I'd say governor valve. It might look fine, but you can't see where it's worn unless it's broken/taken apart.

You could also throw thicker fluid in it, like a 15w40 motor oil. But that would be just kicking the can down the road. There's a hard fault present causing your issues.

FYI clean trans fluid on a rig that's new to you might show immaculate maintenance, or that it has issues and someone tried changing the fluid hoping that would fix it.......

 

[deezlugnutz]

Is this truck new to you? When did the problem start to occur and what have you done to try and resolve it?

Do a main pressure check when warm, in both N and R, and let me know what the numbers are.

A modulator wouldn't cause your issues.

Most likely you're losing governor pressure when the fluid is warm which is causing the transmission to downshift. Most of the time I see the governor port or governor itself worn out causing these issues. You could have an issue with the valve body or governor tube leaking if they weren't installed correctly. But if this just started and nobody has messed with it, I'd say governor valve. It might look fine, but you can't see where it's worn unless it's broken/taken apart.

You could also throw thicker fluid in it, like a 15w40 motor oil. But that would be just kicking the can down the road. There's a hard fault present causing your issues.

FYI clean trans fluid on a rig that's new to you might show immaculate maintenance, or that it has issues and someone tried changing the fluid hoping that would fix it.......

We've had this truck for years. Issue just showed up last week. Short of removing and inspecting the modulator/relay/wiring and governor that's all we've done.

I changed the fluid and external transmission filter around a year ago. Refilled with Dex 3 and it's been running fine without issue until recently. It only gets around 5-10k miles per year put on it.

Pretty consistently I can get the truck to go through all 4 gears when the trans fluid is cold. As soon as it heats up it seems to drop down to second, and then will only shift 1 - 2 and that's it.


Idle pressure in neutral is about 145 PSI.

Foot on the floor accelerating it'll rise up to 150 PSI, then drop down to 145 PSI, shifts to second gear and then back up to 150 PSI again and will hold at 150 until it hits the governed engine speed in second gear, and not shift any further.

Reverse is around 180-190 PSI at idle.

If I let the fluid cool down I can try to watch the pressure in the higher gears if it will shift into 3rd and 4th until it warms up.

There is only a test port for main pressure on these 545s, no governor pressure test port correct?

 

[Booyah45828]

Reverse seems low, but if that was done at idle, I think spec is at 1500 rpm or something. IDK off the top of my head anymore.

No governor port, otherwise it'd be a cut and dry test.

Governors are cheap, throw another in it and see what you get. Inspect the inside of the bore and see if it's egg shaped.

Other then that, it's valve body leakage, which isn't common unless it's been pulled before, and is another can of worms to tackle.

 

[s2mikon]

I can't remember but is there a governor filter screen on that? Or am I thinking about the MT series. Also check for loose valve body bolts. Seen that more than once. Use a torque wrench to spec.

 

[cadillackid]

this is the same thing the 545 in my red bus did... lost line pressure when hot..



fluid is clean, these transmissions have 2 filters... there are the external spin-on filters which are in the cooler lines... there is also a pan filter on these as well, have you dropped the pan to make sure that filter is also clean and seated properly?

 

[Booyah45828]

I can't remember but is there a governor filter screen on that? Or am I thinking about the MT series. Also check for loose valve body bolts. Seen that more than once. Use a torque wrench to spec.

There is one in both models. With the at545 I think you have to remove the valve body to change it. Only time I've seen those plugged is when the trans was abused or junk and the pump sent trash throughout the valve body. The screen in the pan should catch it before going into the pump and then the valve body.

 

[deezlugnutz]

Reverse seems low, but if that was done at idle, I think spec is at 1500 rpm or something. IDK off the top of my head anymore.

No governor port, otherwise it'd be a cut and dry test.

Governors are cheap, throw another in it and see what you get. Inspect the inside of the bore and see if it's egg shaped.

Other then that, it's valve body leakage, which isn't common unless it's been pulled before, and is another can of worms to tackle.

Thanks, that's good info. Guess we'll pull the governor again and have another look.

I can't remember but is there a governor filter screen on that? Or am I thinking about the MT series. Also check for loose valve body bolts. Seen that more than once. Use a torque wrench to spec.

Will check the valve body bolts if everything looks good with the governor and it's bore.

this is the same thing the 545 in my red bus did... lost line pressure when hot..



fluid is clean, these transmissions have 2 filters... there are the external spin-on filters which are in the cooler lines... there is also a pan filter on these as well, have you dropped the pan to make sure that filter is also clean and seated properly?

I have not dropped the pan yet. I have read from a couple different places that the internal filter is more of a "screen" and is usually only replaced during overhaul, so all I've ever done on this transmission is drain the fluid and replace the spin-on filter when it's due for service. Sounds like I'll need to have a look after dropping the pan then.


Thanks all

 

[Booyah45828]

Originally the internal filter was no more then a screen. The cooler line filter did the actual filtering and is the one you should change yearly.

They've obsoleted the screen and made a better pickup that actually has a filter in it and a retrofit kit to install it. I used to have the numbers on a sticky note, but that's been thrown out. Allison should still be able to get that.

 

[cadillackid]

I have one of the filters for the shallow pan but I dont have a part number for the deep pan version... they are still available.. when i changed the one in my DEV bus rigth afyer getting that nus it was a filter in it vs a screen so am guessing it had been updated.. I never knew they didnt have an internal filter originally.. I havent dropped the pan in my 1978 yet to see what it has..

 

[deezlugnutz]

Dropped the pan. Appears to be a fair amount of metal in the bottom, although I'm not sure what is "normal" for these transmissions. I did find pieces of what looks like some kind of a spring? Hoping someone might recognize what they could be from. Pulled the governor out again and took some pictures.

 

[Booyah45828]

You won't see anything looking at the governor, the wear part is internal.

The sediment in the pan isn't alarming, but the metal pieces are, and that does look like a valve body spring.

At one time weller, or was it jasper?, offered reman valve bodies. Might be worth a call before going too far into it.

 

[INTL470028FT]

Following. My 4700 with the AT545 also does the same thing. I can get to 4 when its cold but once warmed up its intermittent. My cluster also does not work 100% of the time but I can tell its revving high and not in the top gear.

I got mine this way.. so never bothered to change it although I would love too as it eats away at gas.

Anything I can do to help I am all ears