Originally Posted by Rolfsted
How difficult to do the lock up switch?
Two ways to go about it.
1] Buy a TS Performance torque converter lockup gizmo. Then follow instructions:
2] Find DIY instructions somewhere on the internet. They were easy to find several years ago, but the newest 4R100 tranny is now over 11 years old, so most of those old DIY instructions have been deleted. But even with good instructions, you must be an automotive electronics guru to be able to figure our exactly which wires to locate and tap into, and which wires must be protected by a diode or fuse. And you must wire in a dash light so you know when the torque converter is locked and unlocked.
I just checked the old threads on TheDieselStop, dated 1999 through about 2006, and none of the links in those threads still work. So apparently the DIY instructions have been deleted.
Here's a post I made to a thread back in 2006:
Originally Posted by TheDieselStop
Re: Torque Converter Lockup
What do they do exactly and why do people install them?
The torque converter in front of your 4R100 automagic tranny has a computer-controlled clutch to lock or unlock the torque converter to the tranny. When the torque converter is locked, then the engine crankshaft is tied almost directly to the driveshaft that drives the rear wheels. But with the torque converter unlocked, the only thing that ties the engine to the rear end is a bucketfull of ATF inside the torque converter, so the pickup can almost free-wheel down the mountain as that ATF sloshes around inside the torque converter.
The PCM in most '99-up PSDs will lock the torque converter at about 37 MPH when accelerating, and unlock it at about 37 MPH when slowing down. But it will also unlock the torque converter any time you apply the brakes. Some PCM codes will relock the torque convedrter after you get off the brakes if you are still going more than 37 MPH. Others won't relock the torque converter unless you mash on the go pedal while going at least 37 MPH.
For towing, you want that torque converter locked any time you are coming down the mountain with a heavy trailer pushing on you. Even if you apply the brakes to slow down a bit, you want that torque converter to stay locked. The locked torque converter combined with the coast clutch in the tranny will give you at least some engine braking. And if you have an exhaust brake, then the locked torque converter can add a lot of engine braking.
So the main purpose of a torque converter lockup switch is to assist with braking by increasing engine braking when being pushed down the mountain by a heavy trailer. The only caveat is you must remember to unlock it every time before your speed drops below about 20 MPH. Else it would be like coming to a stop in a stick shifter without mashing the clutch. Not a good.
With my PCM code and a really-good coast clutch in my tranny, I don't need an exhaust brake or a torque converter lockup switch to tow my 8,000-pound 5er all over the USA - including the so-called mountains back east and the real mountains out west. But with a heavier trailer I would. Or with a different PCM code, or with a less-than-perfect 4R100 tranny, I might.