bad_idea

1990 4.0l, Automatic Trans, 231 transfer case, 4.10 gears, 33" tires.

Runs about 2700 rpms at 70. I assumed this was normal but another thread on the forum leads me to believe it should be running much lower RPMs. My transmission fluid is filthy, no idea the age (bought the Jeep 6 months ago). The trans shifts smooth and does not hunt for gear or shift up and down at steady speed. I'll change the fluid this week and see if that helps. What components do I need to check? I'm not familiar with the auto trans in the Jeeps, and I know the Renix Jeeps are a whole different animal.

cruiser54

First off, the Renix Jeeps are not a different animal.

They just have a different fuel injection system.

Count the shifts.

1,2,3, OD, and then look at the tach and feel for another "shift". It's the torque converter locking up.

pvsprme

My 89 with Auto, 231, 225/70R15 tires, all stock runs 2200 rpm at 65 mph. Has the factory towing pkg so I think it's a 3:55/1 rear end. If you're running 4:10 and 33's that sounds about right.

Fred/N0AZZ

You may not be getting into overdrive/lockup with the 4.10 gear all the time, mine with 33" tires and 4.56's @ 70mph runs about 2750 rpm's with lockup max.


Time for a full flush and clean the filter in the trans, and TC both. I drained then refilled the trans 3 times to get all the old fluid out then changed the filter and refilled the final time. It takes this many times to clean it out completely.

bad_idea

So what years of XJ have the same transmission controlled in the same fashion? Are they all AW4? Aren't later years controlled differently by the computer?

cruiser54

XJs used the AW4 from 87 til end of production. All controlled by TCM as far as I know.

bad_idea

As far as gear ratios:

(Axle Ratio x Vehicle Speed x Transmission Ratio x 336.13) / Tire Diameter = Engine RPM

pvsprme, your setup would be as follows:
3.55 x 65 x .75 x 336.13 / 27.4 = 2123.04 RPMs at 65 MPH

Fred, your setup would be as follows:
4.56 x 65 x .75 x 336.13 / 33 = 2264.29 RPMs at 65 MPH

My setup should be as follows:
4.10 x 65 x .75 x 336.13 / 33 = 2035.88 RPMs at 65 MPH

This assumes all three vehicles are in overdrive. Assuming mine is in third, not OD:
4.10 x 65 x 1 x 336.13 / 33 = 2714.50 RPMs at 65 MPH

As you can see, the 4.10 gear I have brings the engine 88 RPMs below the stock setup. The 4.56 gears Fred has brings the engine 141 RPMs above stock. Theoretically my 4.10 should be better for fuel efficiency while Fred's should be better for acceleration.

Fred, at 70 MPH your Jeep should be turning 2438.47 RPMs. I wonder if your converter is locking up? Or perhaps your tach is inaccurate.
4.56 x 70 x .75 x 336.13 / 33 = 2438.47

With all of that said, what can I do to trouble shoot why the Jeep isn't going into overdrive?

cruiser54

Re-read Post 2?