# allison at 542 shifting issues



## lazer man

Hi im new here. Hello folks! Im an rv/camper guy and getting ready to start thread concerning my oachman catalina (furnace problem).but was just searching for help on my truck and the net sent me here so i joined! So here goes,i have a 93 ford f600 7.0litre (429) with at 542 tranny.had the truck 5 years runs good,75000 miles driven seasonal from may to Oct.short trips. (Asphalt sealing) the only issues are intermittent! Sometimes going from neutral to d it idles up high.transmission shifts great except at about steady 45 it will d ownshiftt u pshift downshift u pshift etc.let off throttle or accelerate its fine.im not sure if vacuum is a factor? Or what would control that function?fluids/filter have not been changed.lucas grilling assisted brakes i think from trans fluid?i know this thread isn't a rv stuff but im a camper man lol.so if anyone helps i gladly appreciate it.and will watchthreads for any camper related problems in case i can help thank you!


----------



## lazer man

Hello me again! I posted my problem for allisonman! I hope he helps. But ill still take any advice here! Thanks lance.


----------



## dbarton291

The engine increasing in RPM as you go from neutral to drive should not have anything to do with the transmission.  The AT542 is  hydromechanically controlled, so there's no computer or any connection to the engine controls.

The 429 gasoline engine will connect to the transmission modulator circuit via either a mechanical modulator that is a linkage connected to the throttle, or a vacuum modulator that will be mounted on the transmission and be connected to a vacuum line that runs to the engine intake somewhere.

I could be wrong, but since you have an engine RPM increase going from neutral to drive, along with a shift cycling issue, I suspect there is something wrong in the fuel delivery/intake/emission controls area somewhere on the engine.  I honestly don't think it's a transmission problem.


----------



## dbarton291

I thought about this some more last night.  Sounds like a vacuum fluctuation.  Likely a vacuum leak somewhere or malfunctioning emission hardware.


----------



## lazer man

Thank you for replying!ill try to check for leaks around manifold with propane.still not sure why it would rev/idle up putting in gear.and i think the tranny issue is kickdown linkage out of adjustment.havent found link yet to see if its right or how to.


----------



## dbarton291

The AT542 does not have a kickdown linkage.


----------



## lazer man

Well could the shift position be out of sync?maybe intermittent shifting from 4-3 3-4 via shifter cable/lnkage?


----------



## dbarton291

The AT542 is real simple.  It shifts based on three input:
1. Output speed via the governor that is driven by the output shaft and makes governor pressure.
2. Operator selection that is an input via the selector cable and positions the selector valve.
3. Throttle position that is an input via the modulator circuit. On your vehicle, probably a vacuum modulator.

The shift cable is almost never the cause of shift cycling.  It either puts you in drive or it does not.  To adjust it, put the selector in neutral, and check for free pin fit of the clevis down at the transmission.  I usually also check for free pin fit at the extremes of the selector's travel.

Shift cycling is normally caused by either a valve body calibration that does not have enough hysteresis between two shift points or a fluctuating modulator signal.  If the truck has always done this, you can drop the pan and lower the 2-3 shift point and raise the 3-4 to spread them out using the adjusting collars.  
If this is a phenomena that has developed over time, it is more likely to be something causing the engine's manifold vacuum to fluctuate, making the valve body think you are moving the throttle around.


----------



## lazer man

Ok db,ill check the cable clevis at tranny anyway.i drove the truck yesterday at 45 and it was fine.like i said in earlier posts it downshifts,upshifts.4-3 3-4 etc most times at that speed but not always.like i said ill go over vacuum again.and possibly replace modulator.when i repaired the end of it it was rusty/smashed so i cut off and checked diaphram,it was working then.i didnt replace the entire hose to engine so thats a good idea also.thank you for your help and dont thinkyour advice is falling on deaf ears lol.im a darn good mechanic just not allison knowledgable so im trying to eliminate simple issues first.you see the one thing that throws me is with the lucas girling assisted brakes.from what i understand are assisted with transmission fluid/power steering?.applying brakes will idle up,putting in gear idles up.thats why i questioned if the 4-3 3-4 is vacuum related or modulator?if not i still have a system issue with vacuum.


----------



## dbarton291

I agree.  Do the simple stuff first, that's always a good idea.  I'm not familiar with the brake system you are referring to but if it is vacuum boosted, it may be a contributor to fluctuations in manifold vacuum under certain conditions.  Also the fact that the system is present may exacerbate a decrease, or fluctuation, in manifold vacuum caused by another source.  Good luck!


----------



## lazer man

Thank you for your help! Ill keep ya posted.


----------

