Looking for a pressure test procedure along with a hydraulic schematic. Dropped the pan, oil was clean, governor seems good, modulator linkage and actuation seems good, shifts late into second and will not shift into fifth, all shift points erratic.
What do you mean shifts late into 2nd as these transmissions usually started out in 2nd unless it is that special model that used all 5 IIRC
I see this issue all the time. Have you checked the shift linkage to make sure the pin in the transmission shift arm is free in every selector position? If it pulls or pushes on the arm against the detent in the slightest it can lead to low clutch pressures and other odd behaviors. A worn cable or cab shifter is usually the culprit.
I think I've run into these, they'll only shift down into low if you're on a hard pull with the throttle wide open? Always took me by surprise, lol.
All of the 5 speeds MT series I have seen have a low reduction planetary scabbed on the back of the case and it is not engaged unless you manually put it in 1st. The other ranges are 2-5, 2-4 and 2 usually allowing to the lack of positions on the shifter although they could be ordered with other shift positions but in no case have I seen one that automatically shifts from 1-2 although I have been told they existed.
I thought I counted 4 shift points obviously not, checked the shift linkage and there is no tension on the linkage in all ranges. Another odd behavior is it seemed like the convertor was locked up, while braking I heard and felt the engine being driven. No matter how long i wind it out it never hits fifth gear, engine rpm 2300, approximately 40 mph.
Tell me if I'm telling you what you already know but normal operation is [C = converter L = lockup] : 2C-3C-4C-4L-5L and the converter lockup feels just like another definite shift. If you are in a lockup gear and you let off the throttle it will stay locked up just like a manual transmission. On some of them if you let off the throttle completely in the lower gears it would run in converter for a second and then go to lockup causing you to kiss the windshield at slower speeds. Anyway you are not getting 5th gear and IIRC there is a set of springs on the spool valve that makes it go to 5th that is adjustable with a little t-pin thingy. But I don't have a manual so I can't really help much more. There is an outside chance it has something to do with the throttle valve or modulator. Do the other upshifts occur at a lower RPM if driven at part throttle?
Still too many variables here to troubleshoot by post. Is this a "new to you" truck? A takeout replacement trans?, Or a truck you know the history of, from several years back? Did the problem just crop up or did it build up slowly? Also, have you made any changes to the drivetrain? larger tires, gear ratios, etc? Those will affect the governor function.
This truck belongs to my neighbor , the problem started out of the blue, no mods, no working on it in any way, it is an old Ford truck maybe 1979. 2-3 shift is very late and may not happen at all until i let off the throttle. 3-4 shift seems normal, 4-5 shift never happens no matter how long I wind it out.
http://allisonparts.net/Allison-Transmission-Parts-Tips.html
Sounds like modulator pressure is way too high but that is just a general diagnosis. I don't know if there are pressure taps for that but it is time for a service manual I'm sure. Or governor pressure too low.