I have a odd gearbox issue

[dorsetdave]

Hi all, I am new to this forum so hello and here is hoping someone here can help me.

I have a 2007, IS250 auto

I have a "good understanding" of cars, engines, electronics, etc. so I am looking for someone who may have actually stripped one of these auto gearboxes and maybe give some advise, not just the usual buy a new gearbox etc.

My problem started with the car only being able to go up to 4th gear, it went up and down through the gears without any issues, but if the car was on a slope it wouldn't reverse. It would try, but seemed to slip and the engine wouldn't rev, on level ground reverse worked fine. No engine or any warning lights ever came on. I found if I pulled the 2 normal fuses in the engine bay and blanked the ecu memory,  forward gears drove perfectly up to 6th, as if it was a brand new box, yet the reverse stayed the same. The car would stay like this between a week and a month then all of a sudden the gears seem to get confused jumping between gears and then it would lock me out and back to only having 1 to 4 again.  

At this point I pulled the fault codes and it showed up as shift solenoid c sticking on. I also found through Google that low fluid could also cause this fault code, which, in my head, would also cause the reverse issue as the fluid level would change if on a slope. My next step was to drain and pull the sump off, remove, test and clean every solenoid, then fit a new filter and oil. (All refilled etc. to Lexus specs).

Now reverse works 100% percent always, but I only have 1st to 3rd going forward. If I pull the fuses, the second it tries to go into 4th it flashes up 4th, slight jolt, then locks the fault code and I am back to only having 3 gears.

OK, sorry for the long winded start but want to try and give as clear as possible understanding of whats happening. The "experts" simply say it needs a transmission rebuild, but I would rather know what exactly the fault is and then decide if a very costly rebuild is the only solution. (not many IS250s in scrap yards here in SA)

When I removed one of the solenoids, the small filter they have had some blockage from a very small amount of material from something, but the magnets were fairly clean and no other signs of contaminants in the oil. My thinking is when the fluids all got stirred up whist the stripping and re filling something has got blocked somewhere. 

What do the solenoids actually do? I understand they alter fluid movement, but do they cause an actual physical valve to open and close? If so, could it simply be a case of this being stuck so I can strip and clean it up.

I will say again that when the fuses were first pulled (so it lost its memory) the gearbox drove perfectly from 1st to 6th so the clutch's plates etc., bearings must still be OK for now. The fact it used to still shift up to 4th and not just 3rd means it wasn't going into limp home mode before the flush, yet it maybe is now as it only lets me go to 3rd. All solenoids are within tolerance and activating on and off perfectly when connected directly to a voltage. Has anybody taken the valve bodies off, cleaned them up and replaced with any success, or even taken the bodies apart and found the fault instead of just rebuilding a whole box each time?

On a side note, does anybody have the pin outs for the ecu that controls the solenoids so then I can simply test the resistance and activate the solenoids from above, along with being able to put the tester on and see if the ecu is telling the box to activate the solenoids as it should.

Well thanks for sticking with me (hoping you did) and hopefully someone here can help me out with some advice or answers. 

Dave