2006 GS300 transmission

[billphriffe1199]

Hello, Hoping to get some advice on my 2006 GS300 transmission issue.  When I'm about to stop the transmission has a hard down shift from second to first.  Has this happened to anyone?

[Bykfixer]

Could be as easy as a clogged filter. Or it could be the famous Toyota 1st to 2nd shift solenoid has begun to fail. 

If the tranny fluid is brown or darker I'd opt for a fluid change and see if that solves the problem. But first make sure the fluid level is correct by starting the car, set the parking brake, put the tranny in neutral and check the dipstick for proper level of fluid. 

[dcfish]

Drop and fill 2 litres, Drive it, Drop and fill 2 litres...Easy to do and may save you some $ T4 is inexpensive  😉

[Bykfixer]

Moved to 06-012 GS section in case GS owners of that era have issues specific to that genre and can share their know how. 

[Sdigginz]

I found the solution to this. There's a more detailed write-up on a different forum but the short story is:

If you have symptoms that match TSB L-SB-0126-12 for a Gen3 GS300 or GS430 with the A760E (and possibly the A960E) trans, then the problem is likely to be the solder joints to certain SMD components on the PCB of the ECM. Using a soldering iron, touch up solder joints the following components:

• All through-board pins to wiring harness connector "H" for ECT (2nd plug from-the-left on top-side of PCB) including pins:
  • 2
  • 10-13
  • 16-19
  • 24-27
  • 34-35
  • (see pages 8 and 11 of attached for header pinout)
• Transistors T900, T901, T728
• Diodes D903, D901, D905, D906
• Integrated circuits IC901, IC903
• Resistors R910, R912 (see page 8 of attached PDF).
• On back side of PCB, touch up 6 fly-back diodes (not numbered), all resistors related to ECT (see page 9 of attached PDF) and all integrated circuits with part numbers SE690 and 851A (see diagram).
• Inspect for damage and replace capacitors C724, C901, C771, and C103. (Not likely the root cause, but good practice given the PCB's age)
• Inspect and touch-up solder on diode D711 connected to the gate of T728 through resistor R839.

If the above is not successful in resolving the issue, touch up ALL resistor connections on both sides of PCB.

Attached is a set of notes that I generated while tracing and diagnosing issues. These are working notes so expect that some errors and typos will be found.

Lexus GS ECU ECT Wiring Rev04 - forum.pdf

Edited August 16 by Sdigginz