To top off the swap, we put in a LiteGrip billet aluminum shift knob. As of this writing, it is hard to find a shift knob with the pattern for this transmission short of paying the exorbitant dealer's prices. Even then, we weren't sure the stock shift knob would go on whatever we ended up with for a stick. So, the LiteGrip knob was a perfect choice, and we didn't get gouged at the dealership.
In the first part of this six-speed swap story, we showed you how we bolted an '06 NSG370 six-speed transmission and NV241OR transfer case into our '89 Comanche. The actual bolting in of the transmission and getting the truck running again went fairly easily.
The idea for this installment of the swap was going to involve finishing it off, using junkyard-sourced Jeep parts for not only a clean, factory look, but for a swap that wouldn't break the bank. Well, let's just say we got the parts installed.
It turns out that getting a mechanical speedometer to read off an electric sending unit without going to the aftermarket is harder than we'd ever imagined it would be. And, after spending about 300 bucks on various and sundry junkyard parts trying to make it work, not to mention enough time to swap the stock transmission back into the Jeep if we wanted to, we had to go to press with this story.
The issues we had with this swap all stem from the fact that the Rubicon transfer case doesn't have any provisions for a mechanical speedometer, and the '87-'90 Comanches and Cherokees run on a mechanical speedometer. If you are doing this swap on a '91-and-up Jeep, which we figure most of you will be, you won't have half the problems we encountered.
Again, trying to save ourselves a buck, we used a stick from an NV3550 that a friend had given us. Cutting off and rewelding the shifter to get it in the right spot is tricky thanks to a rubber isolator inside the "cup" portion, which starts to melt with just a good tack weld. Take it slow, and have a dip cup handy to drop the shifter in when the rubber starts to melt. Even though we had to clearance the hole that the shifter passes through from the outside of the truck, we were able to reuse the stock seal (shown inside console around shifter).
At the beginning of this project, when it was still just an idea, we were going to swap a GPS speedometer into the factory cluster after ripping the stock speedometer out. Then we decided that for someone who paid 500 bucks for an '87-'90 truck, a $500 speedometer might not be a good idea.
Even from the start we didn't want to trash all the factory gauges by going to a complete aftermarket array. It just added too much money to this swap. So, we moved on to the idea of using an electric motor to drive the stock mechanical speedometer. Availability simply wasn't there when we started this swap, and we were confident we could save a buck by going at it our way, so we didn't look too closely into it.
Now, with about $300 of burnt or non-functional electrical gauges and sending units in the trash, we wish we'd gone with one of our first two options. If you have an '87-'90 Jeep, be it a Wrangler (which also uses the mechanical speedometer), a Cherokee, or a Comanche, don't do what we did. Just keep your stock gauge cluster.
As of this writing, there isn't a GPS speedometer that would exactly replace the stock unit. GPS speedometers are big in boats and often don't feature odometers. They are just coming into the automotive realm with odometers, trip odometers, and other things that we need. Autometer makes a few units, but most of them have a white face with white rings; Nordskog has a couple of black-face offerings, and while none matches exactly, all would be better than what we ended up doing to all of those poor gauges and sending units.
Terf now makes an electronic to mechanical speedometer adapter. When we started this project, the unit was unavailable. Jaguars That Run also has such a unit to drive a mechanical speedometer from an electronic signal. Either one would have been a better option than the route we took.
The backup light switch (arrow) is on the driver side of the transmission, up near the shifter. We ended up splicing into it by soldering wires to it. Do yourself a favor and scour junkyards for the male side of the plug off another Chrysler or later-model Mercedes vehicle. Another thing you might want to do, if you do solder wires to the switch, is do it before the transmission is in the vehicle. | 
This is the electronic speedometer hookup, the root of all our problems. If we had Factory Service Manuals for all the years involved, maybe we'd have figured it out. As it was, we melted and burned a lot of electrical components with naught to show for it but melted electrical stuff. |
It seemed pretty easy. Get an electric speedometer instrument cluster from a later model Jeep and swap it in. We chased the wires back to the gauge in an effort to figure out what wire in the truck went to what wire on the new cluster. To the right is our '89 cluster; to the left, the first '91 donor. | 
We caught a break right out of the gate. The wires going into the Driver Information Center had the same plug on the early and late cluster, and they went to the same bulbs. So, a simple swap of the indicator panel from the '89 (right) to replace the '91 panel (left), and we had all our lights working again. |
After lopping off the old instrument cluster plug, we soldered and heat-shrank all of the wires together behind the dash to attach the '91 plug to our '89 wiring. | 
We knew going in that the '87-'90 fuel tank sending unit ran on a different ohm range than the later '91-'95 one. The early reads 1 ohm at empty and 88 ohms at full, whereas the later one reads 88 ohms at empty and 1 ohm at full. Since the sending unit can't be taken off the fuel pump assembly reliably (it took destroying two before we admitted it) and the sending unit isn't available on its own (you need to buy the whole fuel pump/sending unit assembly), we just ended up swapping in the entire HO fuel pump assembly. We were concerned with the higher pressure causing issues, but the truck ran fine and nothing leaked when we were done. |
After talking to some other Jeep engineers, we knew that there was a 5-volt signal going to the speedometer pickup in the transfer case. What we didn't know was the amperage. So, we got a bunch of potentiometers to dial in 5 volts at different amperages. We fried two potentiometers, and on our third one, we think we fried the speedometer sensor. It got hot, smelled burnt, and we decided that there were other things to look at. | 
This was taken right before going to press. The truck is running. We never did figure out if the two junkyard clusters had fried tachs or if we'd just wired it up wrong, and we never did get the tach to work. We accidentally fried the fuel gauge by crossing two wires in one of our install/uninstall cycles on the cluster. And the speedometer? Well, we'll just keep with the flow of traffic for the time being. |
Since you can't really mix and match gauges from the early cluster to the late cluster thanks to the printed circuit board on the back, we swapped the entire cluster in. We were told that the '87-'90 and '91-'95 water temp and oil pressure sending units were different. We found out that the '87 and '88 are different from the '89-and-up ones and confirmed it with our inside source at Jeep by comparing the part numbers in the Engine Bill of Materials book. In our case, the '89 sensors played fine with the '91 instrument cluster. | |