At light loads the shift points wont change much. If the shift happens at about 45 mph with 3.73 gears it'll happen at 41 with 4.11s. Under light loads the transmission is going to shift at 1800-2000 and as you push it those numbers will go higher regardless of the final drive. So yes, road testing is really the best way to see what RPM the shift is going to happen with the way you drive. You can use Grimm Jeeper to figure out at what speed it's going to shift with the new gears, it'll be pretty close to the simple change in gear ratio.
Changing the 42RLE shift points?
- Thread starter 2005RubiconTJ
- Start date
One more key input for when the transmission shifts is the gear ratio in the axles. If the Jeep is undergeared, as in geared with too high of a ratio like 3.73 for 33" tires, etc., it will shift prematurely and the engine rpms will be too low for it to perform well. I bought my current Jeep used with 35" tires and its previous owner only regeared it to 4.88 which caused the engine to lug and shift prematurely. Regearing to 5.38 to bring the rpms up helped immensely.
If you regear from lets say 3.73 to 5.13 and accelerate at the same "feel" the load on the engine will be less because of the increased mechanical advantage meaning the shit will be at a lower RPM because the 42RLE will see a reduction in load. It's not easy stuff for most people to understand.
Apparently not.
So... if the shift point under a light load is lower with deeper gears, then how does going deeper elevate the cruising RPM for over-the-road driving? I can see how the re-gear would help with crawling in that case, but the purpose of my re-gearing is to cause the engine RPMs to run higher. My engine (a) lugs uphill b/c it won't downshift automatically where I'd want it to and also (b) upshifts sooner then I want it to when accelerating and (c) settles into a higher gear then I want it to when at a steady state. I was thinking that objective of re-gearing is at least to bump the RPM brackets up for (a) and (c) so that the engine's running at higher RPMs.
Top gear the RPM will be higher when you are cruising, the transmission can't up shift anymore so it revs as higher. In all other situations after a regear the RPM will be slightly lower due to the automatic up-shifting under the lighter load earlier. Overall like I said at first it doesn't matter much. The shift points will be "about" the same RPM but the fact is they do tend to be earlier after you re gear slightly because of the lighter load.
You don't pick gears because of any of this in my opinion. You pick the RPM you can live with on the freeway and any higher ratio will give you better performance on the street and trail. With an automatic it's generally better just to ignore the shift points, let the transmission do the work and enjoy it.
Ok, that simplifies this, makes sense and is basically what I've been doing with Grimjeeper - charting the 3rd and OD RPMs at highway speed at differing gearing and tire sizes.
Thanks for these replies - really helped to understand more about how that transmission operates.
See if I've got this right?? The 42RLE is actually a 3 speed transmission with an overdrive. Turning off O/D locks out overdrive/4th gear and shifts lock-up from O/D to 3rd gear. It also may change the shift points for the transmission. Proper gearing (4.88-5.13-5.38) makes the O/D livable except in mountains with 70mph in the 2500rpm +/— range. In mountain/steep hill conditions where O/D is still too steep a gear and down shifting to 2nd leaves the engine running well above 3000rpm if you want to/can maintain 70+mph.
Well stated summary and I'm looking forward to reading replies from other folks with know-how. This is my curiosity in a nutshell.
I assume the tune of the transmission on the 2.4 liter 4 cylinder motor (not the old iron 2.5, the DOHC 2.4) is actually tuned for a low torque 4 banger. Wonder if we could swap the 2.4 liter TCM into our 4.0 TJs and experience “4 banger like” shift points …
PS: the 2.4 liter motor used the 42RLE as well.
PS: the 2.4 liter motor used the 42RLE as well.