Automatic transmission lost first gear

[Mr Independent Wheels]

2001 ford ranger 3.0L 2wd automatic (5r44e ???)

My friend bought this truck and when he was driving it home, it wouldn't accelerate as he got onto a ramp. It just revved but wouldn't move. Tried manually shifting to first which didn't work. Tried manually shifting to second and it would work to get moving, then he could shift back to drive and it would run fine until the next stop sign, when he would have to start in second again.
When you shift to first or drive (from a stop) you can feel it engage, and the engine bogs slightly, but there's no hint of moving forward, even with a push to get it rolling. It has diagnostic code P0731.

So we dropped the transmission pan to check for debris, which there was little of. The fluid seems good; not black and burnt. I took the valve body off, which looks quite clean, and the gasket was either torn a little already, or I tore it getting it out, but no noticeable blowouts that would keep it from building pressure. All the solenoids moved when I applied 12V to them, and I think their resistances were in range. The wiring harness was "crispy," and the insulation was breaking off as I bent it around, but otherwise, nothing visibly wrong.

Having seen a YouTube video of transmission disassembly but never having had one apart ourselves, we went ahead and did the right thing and tore it to pieces.
The first one-way clutch(sprag) to come out seems fine. It only turns one way. The bands were coming apart, but nothing crazy. The clutches look new, with no rounded teeth as far as I can tell. There is some noticeable wear on the overdrive planet sun gear(??) in the overdrive/coast-clutch drum. I'll post pics.

Can someone tell me what to do next? I was really hoping it was that sprag but it appears to be good, and the clutches seem good so I'm about at my wits end. Could it just be the wiring harness? Maybe the pump is going out (haven't taken it apart yet)??
Help?

 

[pentode]

I'll put my 2 cents in here, and I'm hesitant to share my thoughts after you've already torn it most of the way down, but my first instinct would be wiring/electrical related as I've seen a couple weird glitches with the 5R44 due to wiring. Not exactly what you described mind you.

I'm sure someone who knows these inside and out will offer some wisdom soon, and it seems you've ruled out most of the mechanical things anyway. But I'd be looking at wiring, inside and out, switches etc. I don't know the electricals of the shift mechanism off hand, but I wonder if it thinks it's in neutral when it's in drive (or first). That's my gut feeling for all that's worth. Good luck!

Edit for specifics: wiring to the PCM/TCM and to the 1st gear solenoid would be my first suspects

Edit for thoroughness: I'm sure you know that you don't want pieces of wire insulation floating around in the system once you put it back together, nor do you want bare wires, so you'll have to address that carefully.

 

[Mr Independent Wheels]

I'll put my 2 cents in here, and I'm hesitant to share my thoughts after you've already torn it most of the way down, but my first instinct would be wiring/electrical related as I've seen a couple weird glitches with the 5R44 due to wiring. Not exactly what you described mind you.

I'm sure someone who knows these inside and out will offer some wisdom soon, and it seems you've ruled out most of the mechanical things anyway. But I'd be looking at wiring, inside and out, switches etc. I don't know the electricals of the shift mechanism off hand, but I wonder if it thinks it's in neutral when it's in drive (or first). That's my gut feeling for all that's worth. Good luck!

Edit for specifics: wiring to the PCM/TCM and to the 1st gear solenoid would be my first suspects

Thank you for your thoughts! Yes, I plan to replace the crumbly internal wiring harness and any sensors. Maybe I can check for continuity to the control module from the exterior wire connectors. I just wonder what else I should also replace while I'm here so I don't have to do it again. If I can't find a sure cause, I guess I'll have to replace possible causes and hope it works...

 

[pentode]

Thank you for your thoughts! Yes, I plan to replace the crumbly internal wiring harness and any sensors. Maybe I can check for continuity to the control module from the exterior wire connectors. I just wonder what else I should also replace while I'm here so I don't have to do it again. If I can't find a sure cause, I guess I'll have to replace possible causes and hope it works...

If you replace the internal harness, double checking continuity from inside to the exterior of the trans would be a good check so you don't have to go back in. Re: mechanical checks, I'd wait for one of the pro techs to offer thoughts. That sun gear does look like it's been banging around but that might be normal wear - I doubt it's going to fail anytime soon but if I could lay my hands on one for $20 I'd probably swap it out. Clutches, bands and solenoids would be what I'd be focused on with it that far apart and if they all look good I'd focus back on electronics. Again, good luck, and kudos on the ambition and bravery levels

 

[Mr Independent Wheels]

Update:
Managed to get the second one-way clutch/sprag out of the housing; it was at the very back and didn't want to come easily and the pictures below show why. I gather that it's called the ''low-reverse sprag'' and shouldn't affect first gear, but I don't know. It needed replacing anyway.
So we bought a new reverse sprag, turbine speed sensor, wiring harness, and a kit with clutches, seals, bands, gaskets, filter, etc. We then installed everything and stuck it back under the truck. It had some rough shifting at first, and the overdrive light started blinking, but apparently it was just relearning the new parts, because it got better and better and runs quite nice and smooth now with no light. Such a relief!
I appreciate all the helpful suggestions and comments from you guys! I'm falling in love with these trucks and I'm tempted to get one of my own (with a manual transmission of course), so I'm not leaving this great forum just yet!

 

[bobbywalter]

LOW-reverse sprag.


low is 1st gear.



thank you for reporting back.

 

[Mr Independent Wheels]

Alright then that was it! Makes total sense, I was just led by the guy on YouTube to believe it had to be the first sprag.