In the summer of 2006 Larry acquired a 1993 Wrangler (there is a description of how we four-linked his YJ in another part of the Tech-Tips section of the web site). After the previous modifications the YJ is both a great highway and off-road vehicle. It will take moderate to difficult obstacles with finesse. However, about a year later the manual five speed transmission started acting up with what seemed to be a synchro problem. About 50% of the time it just would not properly shift into first or second gear.

We had been urging Larry, an old die-hard manual transmission man, to put an automatic transmission in the YJ. And it just so happened that when his tranny started acting up we had a 1997 Cherokee AW4 four speed automatic transmission just sitting in the lot. Although Larry is all for manual transmissions, he is also cheap. So when it came to the cost of rebuilding or replacing the manual versus installing a virtually free auto… Well, free won out. We’ll let Larry tell the rest of the story.

These guys have me all wrong. It’s not that I think that manual transmissions are the only way to go. It’s more that I’m of the opinion that if it ain’t broke, don’t fix it. This jeep had a perfectly functional transmission that allowed me to do anything I felt capable of doing on the trail. That is until recently when the tranny finally gave up the ghost. So, as was mentioned previously, there was this automatic transmission available. The two main advantages of using this transmission were that it was a direct bolt on to my engine and transfer case and, unlike the regular Wrangler automatic transmission, it had overdrive.

It did present one problem, though. The transmission was electronically controlled via a computer and my YJ did not have the capability of controlling it. So I decided to do a little research to find out if this swap was even feasible. I discovered that indeed it was. As it turned out, even though the tranny is computer controlled it does not use the vehicle computer, more properly known as the power control unit (PCU), but is instead controlled by a separate computer called the transmission control unit (TCU). How’s that for an original name?

A little more research on the internet revealed that other people had successfully integrated the Cherokee TCU and transmission into TJ Wranglers. To do this, however, in addition to the transmission I needed a TCU and the appropriate mating connectors for the TCU and transmission. As luck would have it, there was a full Cherokee wiring harness and a TCU lying right beside the transmission. The TJ swap articles also provided wiring information to integrate the TCU and transmission into the Wrangler electrical system. I was good to go, or so I thought.

It turned out that the TJ swap diagrams didn’t match anything I had. It appeared that my transmission was an earlier model with different connectors and of course the wiring for the YJ was completely different from that of the TJ. However, being an old electronics man, I was completely undaunted. I went to AllData and downloaded the appropriate wiring diagrams for the 1993 Wrangler and the 1997 Cherokee, and cross referencing them, I developed my own interconnection diagram. As it turned out, all of the signals needed for the XJ transmission and TCU were available right at the YJ PCU connector. So it was simply a matter of splicing the transmission and TCU wires into the appropriate wires of the YJ PCU harness. There was only one signal that had to go to a place other than the PCU and that was the 12 volts to illuminate the backup lights. I tied that into an existing YJ connector just to the right of the transmission.

The first task was to extract the transmission and TCU portions from the XJ wire harness. Others have just cut the connectors off the harness leaving short pig tails. I wanted to keep as much of the harness together as possible. Using the appropriate connecter pin-out diagrams I identified each wire. The wires that ran between the two transmission connecters and the TCU connector I left intact. Any wires that went from these connectors to other parts of the XJ harness, I cut and labeled, as they would be spliced into the YJ PCU harness.

This is the end result. The two connectors on the upper right attach to the transmission harness, a 6-way and an 8-way. The other 21 pin rectangular connecter at the top plugs into the TCU. The wires that are unattached at one end are to be spliced into the vehicle harness. At this time I left the firewall grommet on the harness as I wasn't sure whether or not I would use it. As it turned out, I later removed it and routed the harness through the firewall through an existing grommet.

Now comes the fun part, splicing the transmission/TCU harness into the YJ PCU harness. While the PCU is mounted in the engine compartment, the TCU isn’t weather proof and needs to be mounted inside the vehicle. Since the PCU is located on the driver’s side of the firewall, I decided to route the TCU harness through an existing grommet in the same vicinity. I pulled the grommet out and cut a cross shaped slit through which the TCU connector would fit.

I then attached the TCU up under the instrument panel near the gas pedal. I routed the TCU connector through the firewall and replaced the grommet. All is neat and tidy.

I used solder connections insulated with shrink tubing to splice the wires into the PCU harness. The photo below shows my high tech soldering station.

Below is the PCU harness with the loom removed and all of the splices in place. While making the connections I selected the proper wire by color. To verify that I had the correct wire I inserted a sharp probe through the wire insulation and checked the resistance at the the appropriate pin on the PCU connector. This proved helpful in two instances. One was the start/run wire color at the PCU was incorrect on the wiring diagram. The other was that there are two wires in the PCU harness that are the same color. As I made each connection I marked that wire on the diagram with a highlighter to keep track of where I was in the process.

Once I had made all of the connections, I replaced the PCU loom and added loom material to the cables that run down to the transmission. I then tied it all in place with plastic tie wraps. The harness going to the right goes to the PCU. The one going to the left goes to the transmission.

Everything is now ready. It's time to remove the manual transmission and install the automatic transmission.

I’m not going to go into the nitty-gritty of removing the manual tranny and installing the automatic tranny. Since the AW4 bolts right up to the 4.0 liter engine it’s pretty much a standard operation. Of course we replaced the fly wheel and clutch with a flex plate to which we bolted a new torque converter. The old torque converter had been left out in the weather for quite awhile so we thought it best to replace it.

The YJ’s NP231 transfer case also bolts directly to the AW4, however, there’s about a ten degree difference in clocking angle between the Cherokee T-case and the Wrangler T-case. Now this angle would probably work fine on the Wrangler, but the T-case would hang lower, reducing ground clearance, and would require a whole new skid plate. Therefore, we drilled new mounting holes for the T-case on the tail housing of the transmission.

Here Marc is drilling the new T-case mounting holes using a hand drill and a jig.

Here you can see the new T-case mounting holes. There was just enouqh space on the mounting tabs to perform this operation.

The transmission and transfer case bolted into position.

To finish up the mounting process we fabricated a new cross member and transmission mount.

A note on drive lines. The new transmission is 1.25 inches longer than the manual transmission. This of course, will move the T-case back by the same amount. In my case, I had to replace the rear drive line with one slightly shorter. The front drive line still had enough spline left so I didn't have to replace it.

Unlike the XJ and the TJ, the YJ doesn’t have a center console to house the transmission and transfer case shifters. In any event, the one thing I didn’t have on hand was the XJ shifter. It really didn’t make any difference because the guys here insisted that I go with an Art Carr gated shifter. Now, Art Carr doesn’t make a shifter specifically for the AW4 tranny. So I purchased a generic Art Carr six gate shifter. I probably could have found an XJ shifter at a junk yard for about one tenth the price, but I went with the Art Carr anyway. So much for me being cheap.

Below is the new mounting bracket for the transfer case shifter.

This is the new linkage for the transfer case.

Our engineer, Andy, had to design a new shifting arm for the transmission. He put all of the angles and dangles into AutoCad and came up with a shifting arm that integrated perfectly with the Art Carr six gate shifter.