What exactly does the torque converter lockup switch do?

[Retiredwarhorses]

What real world situation is pressing the brake and the throttle at highway speeds simulating though? Why would I need to disengage the TCC while pressing the throttle while also simultaneously pressing the brake? Sorry, I'm not trying to be difficult, just trying to understand what possible purpose this test serves. Right now it does not do this but there seems to be no adverse effects that I can discern.

What is it about the word “test” your not getting…it’s a test, not a way of driving the vehicle , the same way i test every truck that comes through my shop, or a transmission shop checks after a rebuild.

 

[diesel_dave]

What is it about the word “test” your not getting…it’s a test, not a way of driving the vehicle , the same way i test every truck that comes through my shop, or a transmission shop checks after a rebuild.

Right, a test is performed to make sure a device performs a certain way so you know it will perform properly in the course of using said device. In this case, I don't understand what the "use" is. Why do I need the TCC to unlock while pressing the throttle and brake at the same time while driving at highway speeds. That's what I am not understanding since I would never actually do that. Sorry, I must not be explaining clearly.

 

[Mogman]

The switch is a safety device that you want to be working same as with all safety devices, the "test" is just that, a test, it is a way to prove that safety device is working that is all.

 

[diesel_dave]

The switch is a safety device that you want to be working same as with all safety devices, the "test" is just that, a test, it is a way to prove that safety device is working that is all.

Okay that makes sense. In the unlikely event the engine is stuck at full throttle, I'll just put it in neutral which is what you'd do anyway. Thanks for clearing that up.

 

[REF]

On my M1123 at 60 MPH when I lift my foot on the throttle I get dynamic braking from the engine until I touch the Brake the converter unlocks and RPM drops to an idle, and I lose my engine braking. If your losing engine braking when you back off the throttle you may have more going on here than just the brake switch IE the throttle position sensor and the brake switch. As Steve said this trans makes decisions based on several inputs perhaps all should be tested because your truck is acting different than mine and yours doesn't sound right.
Good luck

 

[Mogman]

As RWH said pull your codes and see what the TCM is up to

 

[diesel_dave]

On my M1123 at 60 MPH when I lift my foot on the throttle I get dynamic braking from the engine until I touch the Brake the converter unlocks and RPM drops to an idle, and I lose my engine braking. If your losing engine braking when you back off the throttle you may have more going on here than just the brake switch IE the throttle position sensor and the brake switch. As Steve said this trans makes decisions based on several inputs perhaps all should be tested because your truck is acting different than mine and yours doesn't sound right.
Good luck

See this is what THOUGHT should be happening. I was pretty confused how I'd ever get any kind of engine braking with how it acts now. I'll dig into it when I get a chance. Kind of have to triage things so this is lower in the list. Gotta figure out the brakes at the moment.

 

[blutow]

See this is what THOUGHT should be happening. I was pretty confused how I'd ever get any kind of engine braking with how it acts now. I'll dig into it when I get a chance. Kind of have to triage things so this is lower in the list. Gotta figure out the brakes at the moment.

I don't believe my 1123 has engine braking except in 1st and 2nd gear and I think that is as designed. The manual linked below does a nice job of describing the intended behavior of the transmission, including the lockout testing procedure by pressing the brake pedal while at speed.

https://gear-report.com/wp-content/uploads/2017/03/Supplement_7_4L80E-Transmission_Troubleshooting.pdf