Alright, this is a bit of a long one, but I wanna give ya all the details upfront, so please bear with me here:

The first time this happend I was coasting down a slight grade, foot off the throttle, converter unlocked, OD not cancelled. No load in the truck, if you don't count like ten 2-liter bottles of coke rolling around in the bed. Almost at the bottom I caught a glimpse of a slight spike in the tach reading, this didn't make sense since the vehicle speed didn't jump to match it, and converter wasn't locked to begin with so you can't say it was that unlocking for whatever reason. A minute later now on level ground I get on the throttle, and surprise-surprise, OD is not there! Alright, figured she went into some sort of a failure management mode and moved the upshifts higher in the engine rpms - pick up some speed, and sure enough OD comes on, but I can't lock the converter, locks fine in 3rd gear tho. Now, get this - my converter is on a full-manual switch, meaning it has no connection to the PCM whatsoever, so if I flip the switch it should lock, no matter what issues the PCM may or may not have. No blinking OD light either. Got home, measured temps at the pan, no overheating. Read codes, found code 62, converter slip - now that's interesting, seeing how computer is fed dummy signal through a resistor, so at least in theory it should thinks the converter is doing what it tells it to (remember we're talking a 1990 PCM here, not the smartest thing out there). Measure resistance of the TCC solenoid, come up with 20 ohms - turns out my dummy-load resistor was way too high for that, but it's been worked just fine for years, why act up now all of sudden? Cleared codes, took truck out for a quick test drive, it was still sifting very late at first but then all of sudden everything went back to normal, shifting like she used to, with converter locking and unlocking instantly whenever I tell it to do so. Pulled codes just to make sure all is good, and sure enough code 62 is back, but this time the PCM behaves as if that code ain't even there. Alright, swap out the resistor for another one that was much closer in value to what the TCC solenoid measures at, then drop pan and inspect fluid and deposits on magnet - fluid is dark, but not brown, no burnt smell either, and deposits of magnet aren't excessive in amount and don't contain metal flakes. As a good measure I put a new filter in the trans, fill her up with Mercon-V (an upgrade from the Mercon/Dextron-III that was there initially), proceed to flushing the converter and coolers with Merc-V. End up with the entore system purged of the old fluid, and full of nice fresh and clean Merc-V. Took truck out for another test-drive, all is good, no code 62 either - victory is mine, or so I thought.

Fast forward two weeks and maybe 100 miles, to yesterday - coasting down another slight grade, again off throttle and with converter unlocked, and no load in truck - hit the bottom, stop for an intersection, get on the throttle (light-moderate throttle, diesel fuel ain't cheap) and the moment tachometer went right past 2200 rpms without the trans getting into 2nd I knew my problem is back. Same issue as before - all gears are there, this time I even get converter lockup in OD, but upshifts are delayed big time. No noticeable increase in line pressure tho, so definitely not a full-blown limp-home mode with pressure maxed out and just two useable forward gears... This time I'm good 20-miles from home tho, so being very easy on the throttle I nurse her back without an incident - all gears are still there, lockup works like it should, it's just that the whole upshift schedule is delayed by several hundred RPMs, and downshifts happen way easier. At home I find code 62 again, but that's all there was. TPS measures good (idle setting, WOT setting, and the sweep in-between). Figiured the barometric sensor may be going nuts and thinking maybe the truck is a submarine or an airplane, but after a little research it seems like when that happens there are codes stored for it, and I have none of those... Fluid is still nice and clean and smells as fresh as the day I put it in.

So I'm quite stumped now, as to what the heck is going on. Someone mentioned to me the trans may be overfull, but even if that were true right now (can't quite read the dipstick too good with that nearly-transparent Merc-V, will modify it tomorrow for that purpose), it was definitely not the case the first time the issue presented itself. I've checked the TPS again, and it stills behaves like it should (this is also the upgraded grey TPS from Ford, and not the problematic black one the truck originally came with). Not sure how to test the baro sensor, but like the TPS, wouldn't the PCM throw a code for it if the baro sensor signal was not up its liking? And I know for a fact the converter still holds good (kinda hard to miss it if it were slipping), and it's an aftermarket converter to begin with, not the factory Ford unit that likes to self-destruct randomly - so I'm pretty sure code 62 is probably still a dummy-load resistor issue, and today I finally managed to locate a 20-ohm resistor to directly match the TCC solenoid's resistance so hopefully that takes care of that reoccuring code (haven't driven the truck yet to verify it tho). Even so, the manual switch setup has been in this truck for several years now, and I've thousands and thousands of trouble-free miles on it - why start throwing a fit now all of sudden? By the nature of this issue I'm thinking it's electronic in nature, as in something causes the PCM to alter its shifting schedule dramatically but without entering an all-out limp-home mode, but with no usable codes recorded where do I begin to diagnose? I can throw parts at it all day long, but in my experience that rarely accomplishes anything useful...

So what says the FTE collective?

 

The resistor can't set a code 62. That code means that the computer has detected more than 50 RPM difference from engine speed to transmission output shaft speed, calculated for the 0.71:1 ratio in overdrive. The code can only set when the transmission is in overdrive.

I think you are having intermittent issues with the vehicle speed signal. It's been way too long for me to remember what that system looks like on a '90.

 

Mark, you maybe onto something there - I did replace gauge clusters not too long ago, (speedometer went nuts) and in the process of disconnecting the cable from the transmission the speed sensor fell apart on me, so I stuck a used one from an AOD transmission I had laying around in there as a temporary measure and then completely forgot about it. I did however notice that after the gauge and VSS swap the gauge reads good 5mph slower than it should, (GPS confirmed), while the old cluster/VSS combo was spot-on accurate.

Also, if what you're saying is true about how the code 62 being set, then it wouldn't matter at all if I have the resistor with the dummy-signal at all in the first place, since all that accomplishes is simulate the voltage drop that occurs across the TCC solenoid when it's energized - meaning I can have whatever resistor I want in there, but the moment I flip the switch and unlock the converter while the computer wants it locked I'll get code 62 cause of the rpm difference between input and output shafts, correct? So in a sense, I've been kinda chasing my tail there...

So apart from code 62, can a goofy speed sensor move my whole upshift shedule up in the rpm band, without setting off a code of its own?

 

Yes, the wrong speed sensor can offset the shift schedule without setting a code.

 

Wouldn't it cause the offfset to be permament tho, as in shouldn't I have noticed the new shift schedule immediately? But I did follow your advice and bought a new speed sensor and installed it, gauge still reads 5mph slower but transmission is back to its old self. One thing I noticed on the VSS that I swapped out today, the hollow end of its rotor (for a lack of better terminology) where the speedometer cable enters it had some radial play - do you think it's possible the sensor was wore out internally, and thus in certain situations gave the PCM wrong readings?

Also, drove the truck for about 15 miles today, locked and unlocked the converter like I always do, and you're 100% correct on the code 62 thing - even with a resistor that perfectly matches the solenoid coil (20 ohms) the code is set immediately upon me unlocking the converter in OD. However that does not alter the way the transmission behaves in any way, so my guess would be the PCM records but otherwise totally ignores code 62.

So overall it seems like the issue is fixed, tho I'm not completely sure if it was just the VSS, or maybe the PCM had some grounding issues as well (I moved the ground to the battery directly). Now I can only hope the fix is permanent