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05/26/2008
As fuel prices continue to climb I thought I would
write about what I have done to optimize the mileage on
my TJ and perhaps other people could chime in and
express their views. As you probably know I have a ’03
TJ with 33’s and the 4 speed auto and 3.73 gears. The
reviews about this transmission have been mixed, some
comments based on fact and some based on Chrysler’s
lack of testing this transmission in the TJ.
The 42RLE is a direct descendant of the A604 or the
“ultradrive” tranny that Chrysler put out in 1989. Its
claim to fame was that it had electrically controlled
shifting with adaptive memory. It was remamed the 41TE
(4 forward ratios, load range 1, transverse mount,
electronic) and later was modified becoming the 42RLE.
The transmission was modified for rear drive vehicles;
“R” designation, the transverse placement was modified
to be longitudinal (parallel with the drive shaft, thus
the “L” designation) and the “2 “ designation was given
to it because the internals where redesigned to take a
larger load on it.
Despite these changes the 42RLE also inherited some
problems from the 41TE, specially the dreaded
“shuddering” that seems to come up commonly on these
forums. Apparently, (if I understand correctly) this
due to inputs going to the Transmission Control Unit
(TCM), the TCM gets confused when to lock up. Between
38 and 42 miles per hour the torque converter locks up
but the then slips because of signals getting to the
TCM are pulse modulated erratically during these
speeds, while I am not entirely sure what that means,
apparently this causes the shudder. There is a write
up on how to hack the electronics to fix this at
allpar.com. Personally, this is inexcusable on the
part of Jeep not to offer a fix for this problem.
On the positive side, this transmission has potential
in this world of $4 gas. It has a deep overdrive
ratio of .69. So theoretically one could go higher
(numerically) with gears around town and gain that
feeling of power and still be able to settle down to a
lower RPM on the highway to save fuel. Additionally,
it has the option of turning off 4th gear entirely and
you get lockup in 3rd. Unfortunately, lockup in third
is not possible with OD on. I live in rural part of
Michigan where speeds are usually about 45 mph, so
turning off the OD gives me lockup and thus better mpg
around town. I just started this and have noticed
about .5, well in the margin of error but over the next
couple of weeks I’ll be able to tell if it helps.
I’m going to re-gear to 4.88’s soon and again will
remark on any changes in MPG’s I observe.
UPDATE(5/30): Using the OD disengage around the city
and only turing it off when I reach 55MPH has gotten me
11.6% increase in MPGs. This is about 1-1.5 mpg
increase, not bad, considering all I am doing is
pressing a button....
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