A750f Transmission Issues..

[connor2ten]

Hate that this has to be my first post on on here but here we go...

I have a 2007 Lexus GX470 with about 245k on the ODO. Not too long ago, the transmission cooler in my radiator busted and leaked into the transmission, requiring a replacement due to a couple codes I received on my dash (P0776 and P2714). Which sent me on a huge rabbit hole where I dug through countless transmission threads on here, finding that the answer is replacing the transmission was what I came up with. So, $1800 dollars later, the same issue still occurs. With the same two codes. I will post the Techstream picture of the codes from my reading today. But I have topped of the ATF at least 4 times, reset different things inside the car, still the same codes. Planning on checking the PCM connectors tomorrow, but wanted to make a post asking for help/ideas before I dump $600 into 2 shift solenoids. Thanks a ton, any help is appreciated and I will try to be as active as possible to respond to questions if any arrive.

 

[Rednexus]

The transmission wire harness inside the A750F is somewhat of a wear item as the GX ages, considering it's consistently sitting in hot transmission fluid for ~20 years. Mine started throwing codes last year for the TCC lockup solenoid, though they were related to the resistance being out of spec as opposed to the solenoid being stuck on or off. The wire harness is a dealer part that is only around $60 and is not hard to replace. In terms of the actual issue....I have not expeirnced those codes before, so I'm not sure if it is harness-related or due to something else inside the transmission.

The FSM has a detailed troubleshooting procedure for dealing with trans codes like this. For my TCC lockup solenoid code I used the FSM to isolate the resistance issue to the transmission harness (checking resistance across the solenoid from the plug on the outside of the transmission). I'd follow the FSM here and see what it tells you for these codes - you may be able to remove the solenoids and test them on the bench prior to replacing them (which is also a good time to proactively replace the transmission wire harness).

 

[connor2ten]

The transmission wire harness inside the A750F is somewhat of a wear item as the GX ages, considering it's consistently sitting in hot transmission fluid for ~20 years. Mine started throwing codes last year for the TCC lockup solenoid, though they were related to the resistance being out of spec as opposed to the solenoid being stuck on or off. The wire harness is a dealer part that is only around $60 and is not hard to replace. In terms of the actual issue....I have not expeirnced those codes before, so I'm not sure if it is harness-related or due to something else inside the transmission.

The FSM has a detailed troubleshooting procedure for dealing with trans codes like this. For my TCC lockup solenoid code I used the FSM to isolate the resistance issue to the transmission harness (checking resistance across the solenoid from the plug on the outside of the transmission). I'd follow the FSM here and see what it tells you for these codes - you may be able to remove the solenoids and test them on the bench prior to replacing them (which is also a good time to proactively replace the transmission wire harness).

Do you know where to find that FSM? I don't currently have access to it.

 

[Rednexus]

Do you know where to find that FSM? I don't currently have access to it.

Manuals

Local download of all owner’s manual, brochures, quick reference guides, navigation, warranty service guides, etc for GX 460/470 Download HERE Lexus/Toyota ele…

lexusgxor.com

 

[connor2ten]

Manuals

Local download of all owner’s manual, brochures, quick reference guides, navigation, warranty service guides, etc for GX 460/470 Download HERE Lexus/Toyota ele…

lexusgxor.com

Welp... After trying to trace the electrical components back, I don't have any power at the main transmission plug on any of the connectors. I feel like the more I try to figure out the problem with this car, the further I go backwards. Any help would be appreciated, cause I'm majorly lost.

 

[Jstawgn]

A750F TCU is combined with ECU and no solenoid power would be sent out when engine not running and ECU think it needs to power one of the solenoid.

 

[Rednexus]

Welp... After trying to trace the electrical components back, I don't have any power at the main transmission plug on any of the connectors. I feel like the more I try to figure out the problem with this car, the further I go backwards. Any help would be appreciated, cause I'm majorly lost.

Might be time to take it to a transmission shop, although most of them will likely just say "you need a replacement". You could also try a Toyota dealer....the A750F was used in 4Runners up to 2023 so they should be able to work on it.

 

[connor2ten]

Might be time to take it to a transmission shop, although most of them will likely just say "you need a replacement". You could also try a Toyota dealer....the A750F was used in 4Runners up to 2023 so they should be able to work on it.

I wanna try to prevent that. Anybody able to find the OEM part number for the A750f internal wiring harness?

 

[Rednexus]

I wanna try to prevent that. Anybody able to find the OEM part number for the A750f internal wiring harness?

Here is what I purchased in 2024:
8212560651

You can also check it in the Toyota/Lexus parts websites. Usually it's "wire transmission".

If you are going to try the harness, while the valve body is out you should check resistance across the solenoids per the procedure in the FSM (page 05-1007 for SL2, page 05-1034 for SLT). Looking at the FSM there are some pretty detailed diagnostic procedures for both of these codes that uses a "hand held tester" - I'm honestly not sure if that is Techstream or some other kind of tester.

FWIW, the two solenoids are ~$220 each from my local dealer. If you are going to go through the trouble of dropping the pan and swapping the harness, it might be worth the additional $420 to just replace both of them while you are in there.

 

[white_lx]

If I understand your OP correctly: You had two AT codes and decided to replace the transmission. Now the same two AT codes are back? It would be quite a coincidence if the new-used transmission resulted in the exact two AT codes as the one that was replaced. This indeed suggests electrical, but maybe outside the transmission?

Maybe start with the basics first: Were you able to clear the codes? Did the shop set the fluid level correctly? What ATF did they use? Did you end up flushing the system to get the coolant/ATF mix out of the lines? I assume they replaced the torque converter as well. If not, I hope it was drained properly?

 

[fozzir]

I agree with checking what stayed with the vehicle, but the same two codes don’t necessarily prove an electrical problem. P0776 (SL2) and P2714 (SLT) are performance codes, not simply open/short circuit codes. Low line pressure from an SLT/pressure-control problem can also trigger P0776, so it doesn’t automatically mean both solenoids are bad.

Before spending another $400–$600, I’d want to clarify:

Was the replacement transmission used or rebuilt, and did it come with its own torque converter, valve body, solenoids, and internal harness?

Was the failed radiator replaced, and were the cooler lines cleaned of the coolant/ATF mixture before installing the replacement?

You mentioned topping off the ATF four times. Was it actually losing fluid, or are you still establishing the correct level? Was the final level checked using the factory temperature/overflow procedure?

After clearing the codes, do they return immediately, or only after driving? What does the transmission actually do slip, flare between gears, engage slowly, or shift hard?

I wouldn’t condemn the ECM or harness based only on finding no voltage at an unplugged transmission connector; that needs to be checked under the conditions specified in the wiring diagram/FSM.

My next step would be verifying the fluid level and contamination cleanup, then following the FSM electrical checks and getting a line-pressure test rather than buying both solenoids on speculation. A good resistance reading can help rule out an electrical fault, but it doesn’t prove a solenoid or valve body is working hydraulically.

 

[connor2ten]

If I understand your OP correctly: You had two AT codes and decided to replace the transmission. Now the same two AT codes are back? It would be quite a coincidence if the new-used transmission resulted in the exact two AT codes as the one that was replaced. This indeed suggests electrical, but maybe outside the transmission?

Maybe start with the basics first: Were you able to clear the codes? Did the shop set the fluid level correctly? What ATF did they use? Did you end up flushing the system to get the coolant/ATF mix out of the lines? I assume they replaced the torque converter as well. If not, I hope it was drained properly?

1. Yes
2-3, 5. I have no idea; purchased on eBay (which was probably my biggest mistake)
4. Yes, fully drained and washed out

 

[connor2ten]

I agree with checking what stayed with the vehicle, but the same two codes don’t necessarily prove an electrical problem. P0776 (SL2) and P2714 (SLT) are performance codes, not simply open/short circuit codes. Low line pressure from an SLT/pressure-control problem can also trigger P0776, so it doesn’t automatically mean both solenoids are bad.

Before spending another $400–$600, I’d want to clarify:

Was the replacement transmission used or rebuilt, and did it come with its own torque converter, valve body, solenoids, and internal harness?

Was the failed radiator replaced, and were the cooler lines cleaned of the coolant/ATF mixture before installing the replacement?

You mentioned topping off the ATF four times. Was it actually losing fluid, or are you still establishing the correct level? Was the final level checked using the factory temperature/overflow procedure?

After clearing the codes, do they return immediately, or only after driving? What does the transmission actually do slip, flare between gears, engage slowly, or shift hard?

I wouldn’t condemn the ECM or harness based only on finding no voltage at an unplugged transmission connector; that needs to be checked under the conditions specified in the wiring diagram/FSM.

My next step would be verifying the fluid level and contamination cleanup, then following the FSM electrical checks and getting a line-pressure test rather than buying both solenoids on speculation. A good resistance reading can help rule out an electrical fault, but it doesn’t prove a solenoid or valve body is working hydraulically.

1. Used, came with its own everything.
2. Yes, both things are true
3. Establishing correct level, but could not find factory temp/overflow procedure instructions
4. Used to be that I would have to drive for 40 minutes, now they return immediately, no warning lights on the dash, but I can see the codes come back in Techstream.
5. It does all of the above as far as shifting goes, almost seems like its trying to protect itself from grinding gears so it shifts a lot harder.

UPDATE: I replaced the old internal harness, no changes to that. Every solenoid has been tested and works fine. Don't know how to do a line pressure test so I think that would be my next step. Do I do the line-pressure test myself or does a trans shop do it for me?

Kinda feel like im at a dead end. Don't really wanna work on this thing anymore, but I don't have a choice due to my current financial position. Any continued help/support is greatly appreciated.

 

[white_lx]

After replacing the transmission, did you re-set the adaptive values (Reset Memory on scan tool / Techstream)? (I see you posted a Techstream screenshot. Should be under 'Utility').