my 1991 F250 has been having unknown transmission issues for a while and i just cannot seem to figure it out. the biggest current issue is that when driving at any speeds at all loads it feels like someone is pushing in an imaginary clutch, the engine will rev, feels like it disconnects from the transmission, grab again a second later, and then you can continue driving. this happens constantly sometimes within seconds of happening and never gets better. the only codes i have are 626 (coast clutch solenoid) and 328 (EGR valve). i recently replaced the MLPS, shift solenoid pack, filter, transmission harness connector, throttle position sensor, ECM, and speed sensor with no improvement. the EGR valve is newer and shouldnt be failing. the curveball is that this doesnt happen when i shift into manual 2nd. the truck will pull and drive perfectly fine in 2nd gear. which while some glimmer of hope, is still not very useful as a truck.
ive checked all the harnesses and connectors at the transmission and engine bay, theyre all clean and covered in electrical grease. im thinking this is some kind of electrical issue since it works fine in 2nd but if anyone has any better ideas id love to hear them. transmission shop quoted me no less than $1200 for a rebuild so if possible id like to avoid that bill

 

sounds like you have a lot of wear to me but im not expert

 

its only got 120k odd miles. when i changed the filter the fluid didnt look alarming, a bit dark and some fine material in the bottom but ive definitely seen far worse on perfectly fine transmissions

 

you should check your transmission cooler lines to the radiator
My f150 I had a severe blockage that was causing poor flow so i had overheating, but, I didnt have slips. I would also wonder if your MLPS is properly adjusted and would want to know if the manual valve in the pan but that thing that spins "does" the shifting I think when you move the linkage(I could be mistaken) is proper too. I think its worth looking at your shifter mechanism (not cable right?) going into the transmission for any out of ordinary situations, send pics if you think you see any. Did you drain your torque converter?

I have a broken low one way clutch and im at 126k , and my truck is a f150, and its also a 96 which is much more reliable version of the E4OD
so dont be too sure its not bad.. but dont think its all lost., if you can get it to act normal in 2 then maybe there is hope. does activating the coast clutch when in drive do anything for braking? and does activating it in manual 2 does it change any of the behavior?

@Mark Kovalsky we need you

 

I expect that if you have it rebuilt the new trans will have the same problem. The problem is most likely electronic, not mechanical.

I have seen these exact symptoms plenty of times. I suspect that the PCM is getting a false signal that you have moved the shifter to N. That causes the PCM commands the trans to shift to 1. Since there is no engine braking in first gear when in drive, the trans appears to be in neutral. I see you've replace the MLPOS. Either that one is bad, or there is a bad wire between the PCM and the MLPS.

I can't see any way this could cause his symptoms. How do you figure it?
If the coast clutch is activated in fourth gear things will get interesting REALLY quickly. The output shaft will be locked to the case. Either the rear wheels stop rotating RIGHT NOW, or a shaft breaks. One of those two will happen, I just don't know which one.

The coast clutch is hydraulically active when in manual 2. Electrically activating it won't change anything.

 

on the fluid line thing, mark, I was thinking if there was a fluid restriction in flow that it would prevent it from feeling consistent. BUt its good to know it sounds electrical, I was worried but I know these earlier years had some real bugs and flws to be woked on. iirc I recommended the OP (I could be mistaking the O with someone else) to adjust the MLPS because there is a tool for it and that each gear has a specified voltage spec ramge

 

the speedometer is bouncing around pretty badly and usually doesnt go above 25mph, tried to remove the dash today but theres something hanging up. i also noticed a small computer set vertically along the firewall directly behind where the OD switch is. does anyone know what that is for?

 

I haven't read the entire thread but check for codes? Bouncing speedometer could be anything from a VSS to a PSOM and/or a bad connection between the two.

(Edit) nevermind my post it's not the same as the 92-96 so I believe those are mechanical and I got a chance to read it

 

bouncing speedometer is traditionallym most likely a speedo cable. However your speedo cable runs into a small box (I believe its called a DRAC module?) that then drives your speedometer off that I think...? That converts your speedo cable to a signal for your computer

 

328 and 626 are the only codes