first off, hello, i am knew and just purchased this truck. it is a 90 f-250 xtra cab, with a 351, and 3 sp auto with od and a push button to dissengage od.(its on the dash)
now my problem, the truck runs good starting off, but once it goes into third, it does not want to downshift when i push the gas pedal. i have the gas pedal to the floor and the truck struggles to gain speed in 3rd. it does the same in od. it will get up to speed , i traveled six hours with this, and it would get to 75, but in situations where i pushed the gas, it would not shift down. i believe that when i slowed below 30 mph it would then shift back down.
now, if i was on a steep grade an lost speed, if i shut the od off it would then shift out of od, and i could gain speed. it is flashing a code, but the mechanic was unable to get it, he said the computer was not "talking" with his, and that he had to check my computer out first.
now when its trying to gain speed in the current gear( instead of downshifting) it feels as thought the truck is missing and it starts to shudder, usually i turned off the od, and it would recover.
one last thing, if i shift thru the gears it does fine, including downshifting, it just won't do it on its own.

thanks for reading, and any possible suggestions.

one more thing, i went thru full tanks of fuel, and added lucas additive to the gas, and this did not seem to do anything.

 

Some of the transmission experts here may be able to point you in the right direction, however I would pull the codes both for the engine and transmission. You can do this yourself and you don`t need a code reader. Go to fordfuelinjection.com and follow the instructions under Self Test.
To get the transmission codes I think with key on engine off you need to floor the gas pedal while counting the light blinks. Give it a try.

 

thank you i will try that and post results.

 

i actually have a actron scantool and i ran tests with it, just to double check my mechanic, and it worked, so i don't know why his very expensive tool would not work. anyway, with key on and engine off, it had 2 codes, 26-maf sensor out of range and 34-evp sensor voltage above closed limit.
then in continuous memory- 22-bp map sensor out of range
34-evp sensor voltage above closed limit
41-heated o2, signal lean right side
67- park-neutral switch open
under keor- 26- maf out of range
41- again
34 again
73 tp sensor, too low during dynamic test- showed as user
error

now, i don't see any that refer to trans, do you think that all or one of these could be my problem, and if so, any suggestions on where ot start. thanks

 

There are two codes there that may cause problems for the transmission.
Code 67 is showing a fault with the Manual Lever Position Sensor (MLPS). The MLPS is important for the proper operation of the trans. If you get under the truck you should see it on the drivers side of the trans. It might be worth checking and cleaning the electrical connections.
Next you have code 73, this points to a defective Throttle Position Sensor (TPS). Both the engine and transmission rely on this to tell the computer how far down you have pressed the gas pedal.
If you go to fordfuelinjection.com it should tell you how to test the TPS.
All I can say is that if it was me I think I would replace both sensors, however wait for some more opinions.
Hope this helps.

 

I am having a similar problem with my 94 f-150. Once I get past second gear the truck is not downshifting and has the same shudder that perhaps is talking about. I will get out and try to check out the codes on mine.

 

i did some checks today and have a couple of questions. first, it appeared as though the accelerator cable is possible to long causing me to have less pedal. there is an inch or less in pedal till it hits the floor. when i push the pedal towards the fire wall, there is approx. 1/2 in of cable that comes thru.
second thing is, when i hooked hand pump up to egr, i can't get it to pressure up. i checked the pump and it would build pressure when i held finger over the end, but not at all on egr. it appears as though it is open if i'm looking in the right place. so i'm thinking that its stuck in place. i checked the plug and it tested fine according to haynes. do you think this could have caused the evp code. thanks

 

So I went out to buy a code reader and these are the codes I have, that concern me...
628 Converter clutch Lock-Up error (E4OD)
634 Transmission Manual Lever Position Sensor circuit out of self test.

I also had this to go along with those...
179 Right side still rich, at leanest adaptive limit, during part throttle.

Those are the 3- digit code definitions from fordfuelinjection.com. The definitions from the book that came with the code reader are the same just worded differantly.