91 f350 2wd shifting issues.

So it's been awhile since my last post regarding the old 91 f350 dually. I was laid off temporarily and havent really been driving it until recently I started traveling up north to a buddies house for some side work. So the issue has slowly gotten worse and with the route to this new destination being on the upside of a large grade the problem seems to have become worse.
So it all started a few months back when I posted on here about my transmission not wanting to shift on it's own and being in a limp mode. That turned out to be the power wire to the trans computer not being hooked up after I rewired the glow plug power wire. The truck drove great for a few weeks and I noticed when it was cold it would occasionally not want to shift out of 2nd gear the truck would rev up until I let off the throttle a bit and then it would shift. I ignored it for awhile since it was only a minor inconvenience but now lately especially after traveling over the grade I've noticed the trans will regularly not want to shift out of 2nd sometimes even if I let off I have to rev and let off several times before I can get it to shift into third...
Another new issue I've noticed is on a particularly warm day after traveling up the grade crawling at 50mph when I was then traveling in flat road the rpms seemed too be higher than normal typically with my 4:10 gears I managed about 2200 rpm at 65 on flat ground on that day it was more like 2700 -3000 rpm and I felt as if I had almost no power. I got to my destination and let the truck sit for probably four hours and checked the fluid . Level looked good(checked while running )and the fluid looked clean and smelled normal (not burnt or black) and then on the return trip it continually would not shift out of 2nd when coming out of a stop unless I manually shifted into second and revved the motor a bit then shifted back into drive I would get third... and I still ahd the lack of power issue also noticed down the grade if I let the truck roll down the hill with no input from brakes or pedal the truck would rev up and would not go over 60mph just rolling down the grade.
So when i got home suspecting a computer issue I unhooked the batteries for the night and the next morning tried again.. the truck shifts great no problems at all. Even after going up the grade again several times but after about 3 days of driving around it began to act up again and once again in hooking the battery to reset the memory fixed it.
Has anyone else had this issue before? Is it right to belive this is a computer issue or could it be a failing component that is making the computer learn bad habits over time to compensate? Any info is much appreciated thanks in advance.

@Nick_Rego

Take a picture and post it on here of the trans (driver side ) of your MLPS , if you have the older style they tend to go bad/fail intermittently causing trans to go into limp mode (OD light flashing)
, replace it , seems to me as if yours is starting to fail ,

I included a picture of an older style and new style MLPS looks like

I'll have to crawl under there tonight to get a pic. Another thing I've noticed is the battery light flickers on and off now as well.. the light did the same thing a few months back and turned out to be the vacuum switch or whatever I un hooked it. Checked the battery connection and if it was charging all that looks good. Also havent noticed the o/d light doing anything.

So I got some pictures and I think I may have found atleast a starting point. On the passanger side there is what looks to be a controller of some kind mounted to the frame and I noticed the black quote coming out of there that I assume is a ground has been cut stripped and wrapped around one of the pan bolts for ground.. I feel like this is probably a good starting point.

Without having seen one for about 20 years, it sounds like you have a Banks TransCommand. I would for sure give that wire a better ground.

@Nick_Rego

looks like you have the older switch, going bad if your having days where it shifts find then all of sudden shifts terrible. Also make sure your speedo sensor in the top of the motor is good.

but the battery light coming on/off screams alternator to me, does the dummy battery indicator flicker? either your wires are not making a good connection at the alternator or the pulley itself is starting to seize. Take your belt off and turn it by hand for resistance.

Start with that wire you found and replace it , also looks like your leaking oil from your engine oil pan like me lol

judging by the exhaust piping your running a bank turbo kit? Also I would get rid of that crap that is around the exhaust and wrap it with some heat exhaust wrap off Amazon. 50ft

Slicknik said:

@Nick_Rego

looks like you have the older switch, going bad if your having days where it shifts find then all of sudden shifts terrible. Also make sure your speedo sensor in the top of the motor is good.

but the battery light coming on/off screams alternator to me, does the dummy battery indicator flicker? either your wires are not making a good connection at the alternator or the pulley itself is starting to seize. Take your belt off and turn it by hand for resistance.

Start with that wire you found and replace it , also looks like your leaking oil from your engine oil pan like me lol

judging by the exhaust piping your running a bank turbo kit? Also I would get rid of that crap that is around the exhaust and wrap it with some heat exhaust wrap off Amazon. 50ft

Click to expand...

Yes it is the dummy light flickering and I've tested the output of the alternator is still good and pulley spins freely. Also yes this truck has a very old first gen banks turbo kit on it. I havent had a chance to re locate the ground wire yet but I'm thinking that should be atleast part of my issue. Thanks for the tips I will let you guys know how it goes.

Sorry it took so long it's been the craziest time of my life recently and I havent really had much time to work on anything. But I did manage to repair the ground wire that was in question and it seemed as if It had worked until about a week later the trans began to not shift properly again. It dosent really do it as bad however it dosent feel like its struggling on the freeway or that its revving higher than normal at freeway speeds. Lately the issue I've had is it not wanting to shift from 2nd to third properly which I believe was a symptom before. It is especially noticable on a large hill I have to drive up to get to my parents house. It usually acts up when it's cold but has done it once warm. After leaving a stop it shifts from first to second normally but then as I increase rpm it wont shift to third without either revving it to the red line (the truck really dosent accelerate very much when doing this either seems lile possibly slipping?) Or manual downshifting to 2nd and then revving the engine while shifting back into drive usually gets it into third. It will have this issue until its parked and I disconnect the batteries for a bit and then it will shift perfectly or I have had one time where I simply parked it for nearly a week and the issue went away for awhile. When it works it works great and it seems to me like this is a computer issue still. Any thoughts?

How long has it been since the trans fluid and filter were changed? Electrical sensors that give a variable input to the computer are the MLPS as mentioned earlier, the FIPL that's on the side of the injection pump, Tach Sensor on the inj pump gear cover, Speed signal from the sensor at the rear of the trans, and a MAP sensor. There are a couple digital inputs as well but they aren't as important to shift quality as the variable voltage signals are. The MLPS and FIPL need to be calibrated for a specific voltage input to the computer, the other signals need to be clean signals otherwise the controller will change how the shifts work not based on reality. Tach sensor is a common culprit of bad shifts if the tach is acting weird. Double check all the connections to the Banks Transcommand and make sure there are good connections where they spliced into the trans wiring. Simple butt connectors and tape or worse just twisted together with tape might be causing problems this many years later.