Problems with transmission or brakes when slowing down

[GR8Santini]

There is a lot going on in the LC hybrid driveline, & to me it is much more complicated than the normal RWD or P/T 4WD systems. Been digging into the why these make heat & what I cn do to better control it. For less cold markets, the PPEPower Thermal Bypass (Thank You @Bad1975) just might be my ticket for towing w/ the LC AT, along with/ holding gears in Manual mode.


Below is my summary from playing in AI collecting info on how the heat is being made.

The LC hybrid driveline layout order is:

• T24A Engine
• K0 Clutch
• Hybrid Electric Motor
• Torque Converter
• 8spd AT
• Transfer Case
• F/R Driveshafts
• F/R Differentials

K0 clutch: This is a wet multi-plate clutch that couples or decouples the engine from the e-motor & the rest of the driveline.

• Closed: engine & e-motor both drive the transmission.
• Open: the engine is disconnected, so the vehicle runs on the e-motor alone (EV mode) & engine doesn't drag during regen braking.
• Engine start: the hybrid e-motor spins the engine by slipping K0 closed, so K0 sees heat during starts.

Hybrid Electric Motor: The e-motor's rotor is always mechanically connected to the converter impeller, so it spins whenever the driveline is moving, but it isn't always powered. It has three states:

• Driving: current flows and it adds torque (EV launch, engine assist).
• Generating: it's loaded to charge the battery, such as during regen braking or when the engine is producing extra power.
• Zero torque: the rotor spins along with everything else but the inverter commands no torque, so it's essentially a passenger. The only cost is small losses like bearing drag and some magnetic drag.

When the vehicle is stopped with the engine off, the rotor isn't turning at all. And with K0 closed and the engine running, the engine turns the rotor, so the e-motor is spinning even if it isn't contributing.

AT Torque Converter: The e-motor's rotor drives the torque converter impeller, so the converter sits after the e-motor & smooths launch & multiplies torque from a stop. It has a 3-disc friction system & lockup clutch that ties input to output mechanically once speeds match. The 3-disc converter has more friction discs to give much higher clutch capacity in a smaller compact package, which matters with the combined engine + e-motor torque. It can lock earlier & hold, or can run a controlled slip to damp vibration, supposedly without overheating the clutch.

Electronics matter. Converter slip is one of the biggest heat sources in a AT. The more time the lockup holds, the less heat goes into the fluid. The transmission ECU is driving most of the AT activities w/ solenoid selections to specific ATF valve circuits. Heat also comes from K0 slip during starts & from lockup slip in controlled-slip mode, which fits with this hybrid driveline running warm even while using a thermostat & coolant-based heat exchanger to control the ATF temperature.

I’m @ 11,700 miles. Changing TC, diffs & AT fluids around 15K. If AT fluid smells burned I’ll add the PPEPower Tacoma bypass.

 

[jefrose]

I see your prior transmission posting. Were you experiencing similar transmission shift patterns when the dealer discovred the selanoid issue?

I was having hard shifting, but thought it was normal.
Waiting in drive thru line. Felt a jolt, thought someone rear ended me.
The 3 warnings came on - "System maltunction", "Hybrid system malfunction"
", and "Drive/Start Control Malfunction".
Cruiser was stuck in 2nd gear. Drove to parking lot, turned off truck. Restarted and seemed to shift ok.
Drove to dealership diagnostics said solenoid failure in transmission. took 3 weeks to get transmission. Gave me a new Tacoma as a loaner. Some owners get new transmission, hybrid module and torque converter replaced. Dealership said only needed new transmission.

 

[Esquire]

I was having hard shifting, but thought it was normal.
Waiting in drive thru line. Felt a jolt, thought someone rear ended me.
The 3 warnings came on - "System maltunction", "Hybrid system malfunction"
", and "Drive/Start Control Malfunction".
Cruiser was stuck in 2nd gear. Drove to parking lot, turned off truck. Restarted and seemed to shift ok.
Drove to dealership diagnostics said solenoid failure in transmission. took 3 weeks to get transmission. Gave me a new Tacoma as a loaner. Some owners get new transmission, hybrid module and torque converter replaced. Dealership said only needed new transmission.

That’s an issue to have to go through on a new, expensive vehicle.

The over complexity of these cars makes one want to keep them under warranty.

 

[amw]

I've been doing some testing over the last few days and believe I've figured out a "fix".

When you are slowing down from say 40mph to a stop you will tap the brakes as normal to begin braking. When you feel the brakes "grab" (which might be the regenerative braking activating I honestly wouldn't know), the instinct is to let off the brake pedal because the car is braking harder than expected. However, what you need to do is depress the brake pedal further (you should see you don't actually slow down more if you slightly depress it further, you just match the current braking force applied by the regen brakes).

In other words the car is braking harder than your input and if you do not depress the brake pedal to "match" the current brake level, when the regen brakes switch off as you come closer to a stop the car will lurch forward because the friction brake level from the brake pedal input is much lower. When this lurch happens also the weird gear shifting tends to happen as well.

If when you feel the car being grabbed by the brakes you depress the pedal further, when the regen brakes switch off the car doesn't lurch and the weird shift doesn't happen.

This has worked for me. It takes some getting used to and is not ideal uphill. I was able to do a drive with this method and could get the lurch to happen when i didn't do the method and the lurch would not happen when i did use this method. Go ahead and give it a try hopefully it can work for you as well.

I view this as a temporary fix though, i think a brake calibration is certainly needed.

The transmission is still a bit clunky especially with upshifts in the morning but same for other toyota and lexus cars i've owned in the past so I'm not concerned about a sporadic hard up/down shift.

 

[MSINAL]

There is a lot going on in the LC hybrid driveline, & to me it is much more complicated than the normal RWD or P/T 4WD systems. Been digging into the why these make heat & what I cn do to better control it. For less cold markets, the PPEPower Thermal Bypass (Thank You @Bad1975) just might be my ticket for towing w/ the LC AT, along with/ holding gears in Manual mode.


Below is my summary from playing in AI collecting info on how the heat is being made.

The LC hybrid driveline layout order is:

• T24A Engine
• K0 Clutch
• Hybrid Electric Motor
• Torque Converter
• 8spd AT
• Transfer Case
• F/R Driveshafts
• F/R Differentials

K0 clutch: This is a wet multi-plate clutch that couples or decouples the engine from the e-motor & the rest of the driveline.

• Closed: engine & e-motor both drive the transmission.
• Open: the engine is disconnected, so the vehicle runs on the e-motor alone (EV mode) & engine doesn't drag during regen braking.
• Engine start: the hybrid e-motor spins the engine by slipping K0 closed, so K0 sees heat during starts.

Hybrid Electric Motor: The e-motor's rotor is always mechanically connected to the converter impeller, so it spins whenever the driveline is moving, but it isn't always powered. It has three states:

• Driving: current flows and it adds torque (EV launch, engine assist).
• Generating: it's loaded to charge the battery, such as during regen braking or when the engine is producing extra power.
• Zero torque: the rotor spins along with everything else but the inverter commands no torque, so it's essentially a passenger. The only cost is small losses like bearing drag and some magnetic drag.

When the vehicle is stopped with the engine off, the rotor isn't turning at all. And with K0 closed and the engine running, the engine turns the rotor, so the e-motor is spinning even if it isn't contributing.

AT Torque Converter: The e-motor's rotor drives the torque converter impeller, so the converter sits after the e-motor & smooths launch & multiplies torque from a stop. It has a 3-disc friction system & lockup clutch that ties input to output mechanically once speeds match. The 3-disc converter has more friction discs to give much higher clutch capacity in a smaller compact package, which matters with the combined engine + e-motor torque. It can lock earlier & hold, or can run a controlled slip to damp vibration, supposedly without overheating the clutch.

Electronics matter. Converter slip is one of the biggest heat sources in a AT. The more time the lockup holds, the less heat goes into the fluid. The transmission ECU is driving most of the AT activities w/ solenoid selections to specific ATF valve circuits. Heat also comes from K0 slip during starts & from lockup slip in controlled-slip mode, which fits with this hybrid driveline running warm even while using a thermostat & coolant-based heat exchanger to control the ATF temperature.

I’m @ 11,700 miles. Changing TC, diffs & AT fluids around 15K. If AT fluid smells burned I’ll add the PPEPower Tacoma bypass.

Another heat generating component is the electric motor. Trans fluid is cooling it as well.

 

[Esquire]

The transmission is still a bit clunky especially with upshifts in the morning but same for other toyota and lexus cars i've owned in the past so I'm not concerned about a sporadic hard up/down shift.

This is true. Toyota / Lexus transmissions have never been smooth shifters. They are generally very reliable and hold up well, however, there are smoother transmissions out there.