I've heard some say that shift kits (such as Transgo's) will improve a tranny's longevity while others say they're detrimental. Which is it? Anyone have the knowledge/expertise to clear this up?

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2006 Jeep Liberty Limited CRD w/275000 miles

It depends .... lol

Here's the explanation of how & why ....

An automatic transmission provides a smooth shift by doing two things - overlapping the shifts slightly, and allowing a certain amount of slippage of the clutch packs/bands during a shift. While this provides a smooth shift, the slippage of the friction materials generates heat; during some conditions (such as towing) this slippage will generate EXCESS heat. EXCESSIVE HEAT KILLS AUTOMATICS. A shift kit reduces or eliminates this slippage that causes heat, which is beneficial. It works mainly by one (or both) of the following - it increases the line pressure (which is the hydraulic pressure used to engage the clutches / bands in the transmission), and it alters the timing/overlap of the ship. Since the slippage of the clutches and bands is reduced, or eliminated, heat goes down, wear from slippage goes down, and the performance and longetivity go up.

Here is where a shift kit can cause issues:

Slight slippage & shift overlap can provide a cushioning effect for the "hard parts" of the transmission - i.e. planetary gearsets, input shaft, output shaft, et cetera. In extreme cases (if the shift kit shifts ROCK HARD, and the transmission is behind a torquey engine such as our CRD) you can find the weaks parts of the transmission - a good example is the input shaft on GM's 4L60E that they ran in the late model Camaro and Firebird - a moderately modified engine, a rock-hard shift kit, and good traction would cause breakage. Also, increasing line pressure can cause extra strain on the transmission pump, which provides not only the hydraulic pressure but the lubrication for the transmission.

In our case, a shift kit should be helpful to a point - the only weak link of our transmission is the front pump and TC ....

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2006 CRD Sport 4X4, bought 6 July 2006, born 5/20/2006 1900 hrs.
Mods N Things:
235/70r16 Wrangler SilentArmor, ProVent, Gasser airbox, CAT filter, Kennedy lift pump, fuel cooler, Evic from wrecked 05 KJ, factory skidplates, towhooks, OME 2" CRD lift, Magnaflow 12256 muffler, Alpine stereo & XM "stealth antenna" install

It stikes me as odd that DC doesn't provide a tow/haul mode for the Liberty like the WK. This mode does exactly what MightyCRD was describing as necessary for towing.....and it works very well. I would think any vehicle rated to tow 5,000 lbs should have this feature for all the reasons outlined above.

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2006 Libery Sport CRD, Lt Kakhi, nicely equipped
*****GDE Hot Tune at 38,879 miles
*****Stock TC.....for the time being!!

Excellent explanation! Thanks!

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2006 Jeep Liberty Limited CRD w/275000 miles

Anytime ... glad to help!

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2006 CRD Sport 4X4, bought 6 July 2006, born 5/20/2006 1900 hrs.
Mods N Things:
235/70r16 Wrangler SilentArmor, ProVent, Gasser airbox, CAT filter, Kennedy lift pump, fuel cooler, Evic from wrecked 05 KJ, factory skidplates, towhooks, OME 2" CRD lift, Magnaflow 12256 muffler, Alpine stereo & XM "stealth antenna" install