P0770 transmission trouble code and loss of forward progress

cartercd

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This is in reference to my 1997 FZJ80 with A343F transmission and 185K miles. What happened:
  • My UltraGauge signaled a pending trouble code P0770, which per FSM is a “Shift Solenoid E Malfunction (Shift Solenoid Valve SL)”. The check engine light never came on.
  • I drove about 8 miles on surface streets (less than 45mph) with no issues
  • I got onto I-10 in Tucson and after a few miles I lost forward progress. Pressing the gas pedal increased the rpm but no acceleration. I coasted across 4 lanes of traffic and made it onto the shoulder, although it was a precarious spot. At that point the transmission was not working at all.
  • I shut the vehicle down while contemplating my next move. After a few minutes I restarted the truck and was able to progress another hundred yards to a safer spot. I did have to rev the engine higher than normal to make forward progress. Transmission fluid level was good, and I did not smell any burning oil.
  • I got my money’s worth from my AAA Plus membership, with a 90-mile tow to Phoenix with my 100-mile allowance. The tow truck driver drove on and off of the flatbed with no issues and I drove into my garage with no problem. The next day I drove around the neighborhood with zero issues. I have not taken it on the highway for higher-speed testing to see if I could replicate the problem.
Per the FSM I know the first thing I need to do for P0770 is drop the transmission pan and test the “shift solenoid valve SL”, which entails blowing compressed air through the valve with and without 12V applied to test for proper function. So let my questions begin:
  • Why a pending trouble code on my UltraGauge, but no check engine light? It seems like there was adequate time for the computer to trigger the trouble code.
  • Is the function of the shift solenoid valve SL solely to lock the torque converter, or does it have another function? My problem referenced above seems to be beyond a torque converter lockup issue (which I believe only happens at around 50mph)
  • Is the solenoid in question the one highlighted below? Solenoid Valve SL = Lockup Solenoid?
  • If it is not the solenoid, but a torque converter or clutch issue, would I still be able to gleefully drive around my neighborhood now as if there was no problem?
Also for reference, my transmission has led a pretty good life. She had complete fluid exchanges at 60K intervals, no towing, and my cooling system is always in good shape. This is the first time my truck has ever left me stranded in 17 years of ownership, so I want to fix this right to restore reliability and confidence. Thanks for your inputs.

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You are correct about low speed torque converter lockup; that doesn't happen. The purpose of the torque converter lockup is to "lock" the engine to the transmission; the torque converter is a clutch to keep relative differential in speeds between the engine and the transmission copacetic. So it doesn't sound like you have a torque converter related problem.

If you had a clutch(es) failures, you wouldn't see it change overnight; to improve, I mean. They either work or they don't; and IME, they always give you warning of impending doom. I haven't seen them fail catastrophically without warning. Not saying it couldn't happen, but if it did, I would expect it to happen while towing or under some other heavy load.

I don't have the FSM open in front of me, but I don't think the TCU fault for a solenoid would necessarily generate an ECU fault. I wouldn't read anything into that.

Yes, the lockup solenoid is the SL (Solenoid, Lockup). The A343F is completely controlled by the signals to the solenoids, so flaky behavior is reasonably attributable to a solenoid.

To answer the function question - I can't. I haven't had the inclination to chase down the function trees for the various solenoid operating conditions. It's in the transmission FSM, though. That's a separate volume from the engine or body/chassis manual, so you have to get a copy of that to troubleshoot the whole unit properly. I think I put a copy in the Resources section. If you can't find it there, let me know and I'll put a complete PDF there for everyone's use.
 
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I have had a similar but not exact experience. I bought a 96 FZJ80 that would not drive forward or backwards. It would try, but would not go. Like it had the e brake on. It flashed codes for transmission solenoids. I cycled it in and out of every function and added T-case and trans fluid. Finally, it just started working again??? Never gave me another problem. I chalked it up to a solenoid or something that had gone faulty. It was very strange. It's why the Previous owner sold me the vehicle...They thought the trans was done.
 

cartercd

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I took it for a test drive this weekend. It started off fairly normal. When I was going around 40mph I punched the pedal to the metal to see what would happen. The rpm jumped up, but the transmission didn't respond. It then started slipping. I pulled over and couldn't progress (in forward or reverse). I waited a few minutes and was able to limp it back home. I dropped the pan today and there were no metal fragments and only a light dusting of particles on the filter screen and pan magnets. The fluid was red with no visible signs of overheating. In addition, a "buzzing" sound that I thought was my heater core seems to be coming from the front of the transmission. This is increasingly leading me to believe there is a torque converter issue that surfaces once the fluid heats up. This doesn't seem to be a shift solenoid issue as the transmission appears to shift okay when cold and the lockup solenoid should not have come into play at these low speeds. I looked at the A343 manual and don't seem to be able to find what is causing this. Any suggestions before I seek the input of a transmission specialist?
 

cartercd

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I pulled the lockup solenoid and it tested bad. I am still not convinced this is my only problem, but will replace it and see what happens. I am saving $150 by ordering through Partsouq, but I need to wait for their weeklong holiday - uggh.
 
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That's much less expensive than a complete rebuilt kit.

Did you look at the solenoids for other trucks? IIRC, Isuzu uses the same solenoids and last time I looked they sold for about $40.
 

cartercd

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Problem solved! I replaced the lockup solenoid and all is well. It is clear that the lockup solenoid does more than locking the torque converter, it impacts the overall function of the transmission. This $150 part (from Partsouq, would be $260+ domestic U.S.) #3525050030 will indeed leave you stranded if it fails. Replacing it is easy, but does require dropping the transmission pan.

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