TJ 3-speed to 4-speed AT swap

[kis86]

What would be needed to swap the original auto transmission from a 98 TJ to a 4-speed...say from a newer TJ or a cherokee?

Other than that, any way to improve gas mileage on the highway...gear switch?

 

[SteelHeadz]

Did you install bigger tires?, if so, re-gearing the axles is the way to go and cheaper rather than swapping Transmissions. All transmissions that are electronically actuated and hydraulically controlled are dependent to a computer known as the TCM(Transmission control Module). If your Jeep has the SBEC(single board electronic controler), you will need to buy the whole PCM(Powertrain Control Module) which controls both engine and transmission. they are very expensive. next, you might end up looking for a harness that will link up to the trans. from the existing harness you have on the jeep. thats a headache. i'm sure i've barely touched the tip of the iceberg here but all in all, i would recommend re-gearing the axles.

 

[kis86]

No bigger tires, just looking at a 98 with the auto and wanted to get a bit better mileage on the highway, so I figured putting in another auto with OD would be nice. Which ones will bolt up and are good transmissions...would I just need to shifter, transmission and TCM then?

 

[SteelHeadz]

2003 TJ is the first to get the 4 speed. 42RLE. Here are SOME of the specs.42RLE AUTOMATIC TRANSMISSION
GENERAL SPECIFICATIONS Transmission Type Four-Speed Automatic, Electronically Controlled, Fully Adaptive, Electronically Modulated Torque Converter
Lubrication Method Pump (internal - external gear-type)
Cooling Method Water Heat Exchanger and/or Air-to-Oil Heat Exchanger

GEAR RATIOS 1st Gear 2.84:1
2nd Gear 1.57:1
3rd Gear (Direct) 1.00:1
4th Gear (Overdrive) 0.69:1
Reverse Gear 2.21:1
I'm not sure how to go about on the swap when it comes to the TCM. somehow or another, the PCM needs to see the 4 speed 42RLE instead of the 30RH or 32RH trans. Since your TCM is also a part of your PCM, It might be able to be re-programmed by a Dealer.

 

[kis86]

What does TCM and PCM stand for I'm used to TCU and ECU...is the PCM the engine computer? ...probably safe to assume TCM is the transmission computer.

 

[SteelHeadz]

You are correct.

 

[Bounty__Hunter]

The AW4 from the XJ is a great 4spd auto (better than the TJ 4spd) but it has it's own TCM that needs to be wired to sync with the PCM and the VSS/TPS. Not to mention the transfer case is clocked lower in the XJ so you need to redrill the back of the AW4 to correctly clock the TJ's transfer case. You would need a different shorter input shaft in your transfer case, or have the stock input shaft turned down by a machine shop.

Here's a good writeup. www.stu-offroad.com/engine/aw4/index.htm