Join Date: Apr 1999
Location: Midland County,TX, USA
Feedback Score: 0 reviews
Re: Torque Converter Lockup
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What do they do exactly and why do people install them?
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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 PCM 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. [img]/ubbthreads/images/graemlins/crazy.gif[/img]
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.
My Sierra Blanca in the sig pic was a great pickup for 11.5 years. I sold it a coupla years ago. I drove a hand-me-down 2003 F-150 SuperCrew 4.6L 2V for a while, but it was unacceptable for towing more than a rowboat. Replacement is a 2012 F-150 EcoBoost SuperCrew Lariat that tows my 5,000-pound TT like a dream.