Delivering the Goods - May - 2020

Old Shifty Toyotas! Diagnosing Shift Quality Issues in Toyota’s Six-Speed Units

Toyota has always maintained a unique approach for managing shift quality. Ports lined up in a case like missile silos loaded with accumulator pistons and specifically calibrated springs were their trademark weapon for ensuring good shifts. Since the introduction of pulse width modulated solenoids, shift quality management duties have slowly drifted away from accumulators and orifices in hydraulic circuits towards computer programs. Adaptive shift quality is used to constantly monitor and adjust the shift feel based on conditions. This change placed higher demands on the performance and integrity of hydraulic components that affect shift quality.

The six-speed applications were the first Toyota units to take full advantage of the higher processing speeds of computers and combine them with newly designed solenoids to provide excellent shift quality under most driving conditions. The front-wheel drive U660 and U760, and the rear-wheel drive A760, A960 and AB60 units share this technology. Let’s explore the reality of problems that cause issues as these units age and come to our shops.

A General Overview

Whenever looking for shift concerns, we know that we must look at the electrical, mechanical and hydraulic systems to fully assess the root-cause of our issues. Even with newer technology, the same rigor must be followed. Unfortunately, with normal aging, wear and tear taken into consideration, many unknowns are introduced as possible causes. These six-speed units are so well mannered that most times when they come into a shop with a shift or torque converter clutch complaint, a typical sump inspection shows no evidence of trauma or wear.

Problems with these units usually occur at around 100,000 miles (more or less depending on maintenance). Toyota Tundras, Camrys and Tacomas will show up at your shop with these common complaints:

  • Shudder felt from 3rd to 5th gear; around 30-50 mph.
  • A “Check Engine” light on in the dash. Typical DTC P2714 (Pressure Control Solenoid D Performance, Shift Solenoid Valve SLT).
  • Various solenoid electrical performance and/or circuit codes.

Note that ANY Toyota or Lexus equipped with the aforementioned units may exhibit these problems. It is important at this point to take into consideration the mileage and age of the vehicle before listing parts and prices for a target repair. Here is where adaptive shift quality can mask underlying issues that only an overhaul can fix.

Electrical Control Issues

The pulse width modulated solenoids are the stars of the show for the six-speed units. Toyota redesigned their hydraulics utilizing them as full fl ow, direct control for the clutches and brakes (figure 1). This strategy allows them to use precise computer control programming to tailor shift timing and quality.

With this in mind, solenoid performance is critical. With time, debris contaminates the fluid, the protective properties of the transmission fluid break down and the solenoid plunger and valves begin to wear and/or stick (figure 2). Here is where “solenoid performance” codes come into play. Even though the electrical function of the solenoid is good, the solenoid valve movement cannot respond as accurately as it did when it was new.

Think of it like an old sports car. After thousands of miles of driving through hard turns and over rough roads and hard acceleration. The suspension system wears and gets loose and engine performance begins to lag.

With the SLT Solenoid in control of the main line pressure, it works the hardest. It has the responsibility of supplying the shift solenoids with modulated line pressure. When the SLT Solenoid has performance issues, it can cause the shift solenoids and the lock-up solenoid to have issues too. The common shudder complaint that occurs from 3rd to 5th gears under moderate acceleration is, in part, attributed to this.

The SLU Solenoid (TCC Solenoid) begins to modulate the torque converter clutch on from 3rd to 5th gear. It allows a certain amount of slip determined by the computer strategy to increase fuel efficiency. Toyota calls this “Flex Lockup”. Since the SLU Solenoid is a linear solenoid as well, it can have the same mechanical performance issues as the SLT Solenoid. Therefore, the shudder complaint can be caused by the SLT and/or SLU solenoid performance. Of course, we must include the possibility of torque converter and valve body related concerns.

Use a pressure gauge to assess the ability of the SLT Solenoid to manage line pressure relative to demand. Perform a stall test cold and hot. Vary the rate of throttle apply and observe the SLT Solenoid command relative to the actual line pressure. Both relative response rates should be the same.

If you are using the Techstream Lite (Toyota’s OEM scan tool) or equivalent aftermarket, you can use the bidirectional control function to control the current to the SLT Solenoid to ramp the pressure up and down.

Another area of concern is corrosion in the transmission case connector. Inadequate current due to corroded contacts can cause performance and/or electrical codes. Unplug the harness and use a bore scope to inspect the transmission connector. Also, check the vehicle side harness. Repair kits are available through Toyota and aftermarket resources.

Hydraulic and Mechanical Concerns

As mentioned, a sump check can lead you to a conclusion that a target repair can most likely fix any of these issues. While this may be true, keep in mind that you are dealing with a high mileage unit that may harbor more problems than just what seems obvious.

One very important area of concern are the piston seals. These units utilize conventional aluminum pistons with seals as well as molded rubber seal pistons. They also have reaction pistons (also known as lube dams). At 100k miles, these seals have seen their best days already and may show their bad side when introduced to fresh ATF.

On the U660 and U760 units, the C1/C2 clutch pistons are common to failure (figure 3). Flared shift into 4th gear or flare on a 6-4 or 5-4 forced downshift are a common result. When rebuilding these units DO NOT REUSE MOLDED PISTONS. Always replace all molded pistons!

On the A960, A760 and AB60 units, the molded piston rule holds true as well; REPLACE THEM ALL. No exceptions. On these units, DTC P2714 is more common and may be caused by bad piston seals. An additional complaint of neutralizing coming to a stop may be observed. With these units, it may be a situation caused by hardened Low/Reverse piston seals. There are a total of (5) seals in this area; all of which must seal correctly.

Scan Tool Diagnostic Information

Keep in mind that when you use an aftermarket tool, there may be some data or areas that you cannot access just because it is not a factory tool. This seems to be the case when trying to clear the transmission adaptive settings. This is a critical procedure to perform after major repairs on the transmission are performed. Even if your scan tool allows you to get into the “Reset Memory” area and it gives an indication that the process was performed, it MAY NOT have been performed.

The Techstream Lite navigation looks like this: SELECT Powertrain > Engine and ECT > Utility > Reset Memory. Aftermarket scan tool navigation should be similar. If you are unsure if your scan tool actually performed a “Reset Memory”, you can get a subscription from the OEM Toyota website and download Techstream Lite onto a laptop computer, and use a J2534 pass thru device to perform this function.

Another item of concern is that most aftermarket scan tools do not identify shift solenoid codes using the Toyota OEM names. You will often see names like “Shift Solenoid #3” or “Shift Solenoid E”. It is always recommended to use the diagnostic trouble code generated and cross-reference it with the OEM listed definition to ensure you identify the correct solenoid/circuit.

For example, diagnostic trouble code P0974 is shown on several aftermarket scan tools as “Shift Solenoid ‘A’ electrical fault” or something similar. When working on a 2015 Toyota Tundra, the factory information readily converts to Shift Solenoid Valve S1 for identification. You can use the Toyota factory manual, Alldata, Mitchel on Demand or equivalent resource to get this information.

Engineers design vehicles to last through their warranty period. What happens after that is our opportunity. Taking advantage of what we know “good” looks like can help us to solve the challenges new technology brings our way. Mapping patterned failures helps us to diagnose and deliver real solutions that customers bring our way with confidence!