Eagle

My math is based off tire manufacturers' specs. It's tough to find today, but they used to include in their specs things like "static radius" (which is the height the axle actually sits above the pavement when the tire is loaded to a normal load), and revolutions per mile. That second one is what I used. Take, for example, the 235/75-15. Some manufacturer's spec sheet told me that tire in that size turns 698.5 revolutions per mile. 60 MPH is one mile per minute, so at 60 MPH the tire is turning 698.5 revolutions per minute. Now multiple that by the drive ratio ... 3.73 times 698.5 = 2605.4 RPM. But that's for direct drive, 1:1 -- which is fourth gear. Now multiply that by the overdrive ratio (which in the version of the spreadsheet on Google Docs is 0.75:1, but some years of the XJ/MJ had 0.72:1). 2605.4 x 0.75 = 1954.05 RPM at 60 MPH. To get any other speed, do a straight proportion. For 70, multiply by 7 and divide by 6 ==> 2279.73 So I completely ignore the mathematics of figuring out the tire diameter from the size, because it's meaningless. How meaningless? Back when I was doing the spreadsheet, one of the tire companies (Cooper, IIRC) offered the same tire and size in both a P version (Passenger) and an LT version (Light Truck). Same tire model, same size -- but the LT version had a higher static radius and turned fewer revolutions per mile. Why the difference? More plies ==> stiffer sidewall ==> less tire squat under load. Theoretical tire size calculations can't take that into account.

Full synthetic, Castrol SynTec. I ran 5W50 when it was available, but nobody carries that grade any more so I typically run 5W30 in winter and 10W40 in summer. Connecticut. Oil change interval 5,000 miles.

By the way -- you won't be using the same tires for autocrossing and street driving. Autocrossing eats tires. You'll be far better off using normal MJ size tires for daily driving, and keeping another set of wheels with your autocross tires on them just for event days. In my case, my brother worked for Firestone back when we were racing, and he got the store manager to give us tires for free. The poor manager didn't have any idea what he was getting himself into.

I don't believe your numbers. I think your speedometer is off. What size are the "29-inch" tires you're running? I made up my spreadsheet because the on-line RPM calculators are so far off. They use round numbers like 29" for tire size, but almost no tires are actually an even number. And a 29" tire doesn't sit or roll with the centerline of the axle 14.5" off the ground. The spreadsheet I made up used actual revolutions per mile from tire manufacturers wherever I could find such data. And your numbers are way out of whack. 235/75-15 tires, which are almost exactly 29" in diameter, only turn 1940 RPM in 5th gear at 70 MPH with 3.07 gears. The same 235s will only turn 2356 RPM at 70 with 3.73 gears. My '88 MJ is rolling 3.73 gears, and I wish all my XJs and MJs had that ratio. It makes 5th gear useful for something beyond the interstate. My spreadsheet doesn't go any smaller than 215/75-15, because it was made up for street and off-road and I didn't think anyone would ever put smaller tires than that on an XJ (or MJ). 215/75-15 is roughly a 28" tire. With 215s and 3.73 gears, the engine would be turning 2460 RPM at 70 MPH. That's right where it wants to be, IMHO. You could easily run 27" tires with 3.73 gears and not be straining the engine at all. Remember, the Jeep 4.0L engine is basically an AMC engine that was developed back in 1964, decades before overdrive transmissions were anything but an expensive and very rare option. My family all had AMCs. The usual setup generated 2500 RPM for 60 MPH, and 3000 RPM was 72 MPH. And they got decent (or better) gas mileage -- without overdrive.

To begin, something is very wrong with your numbers. A 235/75-15 is roughly a 29-inch tire (28.9"). With 3.07 gears in overdrive, 235s should run roughly 1940 RPM at 70 MPH, not 2500. See if this link will open for you: https://docs.google.com/spreadsheets/d/1pHFuhGgj6dQDfzyfFJH5z7NCDLW2KX3ABQgAJr3lBvM/edit#gid=0 Before I would venture to offer suggestions, I would like to know what size tire you actually plan to use (not just "smaller"), and how much of what kinds of driving you plan to do. And are you going to be autocrossing the tires you drive on the street every day? When I was autocrossing (back when Henry first invented the Model T), I had one set of tires for racing on Sunday, and another set for driving to work during the week. Mine happened to be the same size, but they don't have to be.

That right there sounds like a story that really needs to be told. Have you visited the Pub lately?

As others have commented -- check all fluids (don't forget the transfer case -- and don't ask me how I know this). Unless you've replaced it recently, replace the serpentine belt, or at least buy a spare to carry with you. I would pack a couple of quarts of oil and a gallon or two of pre-mixed anti-freeze (you don't have to buy the pre-mix, just buy one gallon, pour half into an empty container, and add half a gallon (sorry, 2 liters) to each. Check your u-joints. Enjoy the trip.

As already noted, you have something confused. The Comanche used a V6 engine only in 1986, so either you have an in-line 6-cylinder, or you don't have a 1988 MJ. And the MJ never had a V4 engine. Neither did the XJ. Let's assume that you have an '88 4'0L in-line 6-cylinder. The '92 is a 4-cylinder. Plus, when the change was made from the 1990 model year to the 1991 model year, Chrysler dumped the 1987-1990 AMC/Renault/Bendix injection/ignition system and switched to a more modern, Chrysler system. So, basically, you can't use anything out of the '92 4-banger. You could use the transfer case, but the transmission and bell housing from the '92 won't fit your MJ, and you won't be getting the parts you need to modify your transmission to hand the transfer case off it. You could use the front axle, but the ratio won't match what's in your truck. To swap the XJ axle into your truck you would have to move the spring perches, and (again as already commented) it's probably a Dana 35, so not an upgrade to what you have. In fact, the '88 isn't a C-clip Dana 35, but the 92 is a C-clip, so many people would consider it to be a downgrade. Bottom line, look for a 4.0L XJ with better equipment as your donor.

There's really not much information there to go by. It's the equivalent of only about 80,000 miles, which is extremely young for a 4.0L. The AW4 tranny is very reliable and durable. Body looks solid, but you should ask for photos of the underside of the cab to see if the floors have rusted out. If so, they can be repaired but it's a negotiating point. Not sure what model it is. It has idiot lights, which suggests base model, but it has the optional "wagon" wheels and dual, XJ-style mirrors (the base model came standard with a single "flag" style mirror screwed into the sheet metal of the driver's door). It certainly looks to be worth considering, but it's a real risk when it's so far away that you can't check it out before buying. Can you get more information, or maintenance records? Does Carfax cover Canada?

Yepp. Part of the emission control system.

Another Olde Pharte checking in to confirm. In January of 2017 I was in a hotel room in Las Vegas (I was there for the annual SHOT Show, not to gamble). Got up to walk across the room for a glass of water to take my evening heart meds and both heard and felt a tremendous "POP" from my hip. By the time I made it home after the show, I could hardly walk. I've been through 16 weeks of physical therapy, two months of walking with a cane, and three cortisone injections. A year and a half later, I'm finally more or less able to move normally with excessive pain. How did it start? It started in 2014, doing cardio rehab after my heart surgery. I overdid it on a recumbent bicycle machine and over-stressed the right hip. For awhile I almost couldn't drive at all. Couldn't move my right foot from the gas peddle to the brake, so I was limited to driving my wife's XJ, which is automatic. I had to learn left foot braking. They tell me getting old beats the alternative, but there are days when I question that. Hang in there, Keyav8r. You'll drive the MJ again.

There's a reason most gear makers don't offer 5.13s for the D30 -- the pinion gear gets so small that it's very weak and prone to breakage. Since the reason for going to 5.13s is BIG tires, it makes for a very bad combination. If you really need 5.13s, IMHO you're far better off going to a Wrangler Rubicon "D44" even if it is low pinion.

Since this place was created, it has grown and matured a lot, the forum software has been updated and changed, and the place has come to look and act a lot like ... well, a lot like a real Internet forum. And we have attracted a lot of members since the place was created. I recently had a discussion with one of our member by private message, and I discovered that apparently some members (one, at least) aren't aware that we owe this whole thing to one person: Pete M. I had to check, because I couldn't remember just when Pete started this place. It was actually 13 years ago -- 2005. How time flies when you're having fun (or just busy living life). I can hardly believe that it's been so long, but it has. I wanted to mention this because, without Pete's initiative, this site wouldn't exist. We should all be grateful to Pete for starting this place, and now we also owe thanks to the volunteers who help him out with the technical issues of keeping the doors open and the lights on. I guess because I knew it from early on I more or less assumed that everyone knew the Comanche Club was Pete's creation. One should never assume. So, for those who didn't know it ... now you do. Pete ... thanks.

I don't know of any way to remove that plate/baffle/shelf (whatever it's called) from an old tank and install it in a new tank. The new tank I bought for my '87 SWB had the baffle, but I don't remember where I got it so that's not much help.

Then go for it. But not for $1,000. Back in 2000 (or so) I bought a 1988 4.0L 5-speed MJ that was running. I drove it home from where I found it. I paid $500. Back then, Connecticut still had safety inspections to register vehicles more than ten years old. I spent another $2,500 (so total $3,000) just to get it registered -- that didn't even begin to address the fix-up stuff that I felt it needed. The next summer I stumbled into a routine check point for seat belts. I always wear a seat belt, so no problem there -- except that one of the cops spotted something and handed me a warning for defective equipment. That cost me another $500. So I was into it for $3,500 just to make it a driver, and only after that could I start fixing it up. I would love to see you get it and restore it, because if you don't take it, it will probably rot behind that barn. But go into it with your eyes open.

So use the cross member and tranny/transfer case mount out of the '93.

Why are you looking at Cherokee hoses for a Comanche? They aren't the same.

There shouldn't be any differences. The factory didn't make any changes to the XJ unibody from '84 thru '96 -- they stiffened it when they updated to the '97 body, and even then I don't think there were any dimensional changes, they just used a different forming method (hydroforming), and possibly some thicker sheet metal.

No, it isn't. The cross members are the same length and can be swapped from an XJ to an MJ. What is different is that different transmissions and transfer cases use different rear mounts for the tranny or transfer case, and there are two sets of mounting holes in the frame rails, to position the cross member either farther forward or farther back. Be sure you're using the cross member and mount that correspond to your "new" engine and drive train, and check to see if you need to use the other set of mounting holes.

Congratulations.

The switch is the LAST thing I would look at. When the dash tirn indicator glows or lights up at inappropriate times, it's a sign that you have a bad ground path at the lights, and something, somewhere, is seeking a ground by backfeeding the turn indicator circuit. That's not a switch issue, it's a socket or ground issue.

Since the Renix speedometer is mechanical, there are no printed circuit connections to it. So it should be possible to mount the HO police speedometer without any butchery of the cluster. So then the issue is making it read. On the transfer case end, the electronic sender mounts the same way the mechanical quill mounts for the cable speedometers, so that's not an issue. I don't know where my '94 FSM is at the moment, so I can't look up a wiring diagram to see if it feeds through the ECU. If that's the case -- wouldn't it be possible to mount an HO ECU somewhere, give it a power and ground connection, and then just use the pins that are associated with the speedo?

Not to pick nits, but Windows Calculator Plus says 180 kilometers is 111.8468146027201145311381531854 miles. So now you can properly calibrate your speedometers.

At the time, the word from the folks at the dealership was that Chrysler killed off the MJ because it was cutting into Dakota sales ... they didn't want Jeep trucks competing with Dodge trucks in the same price range and market segment. Let's face it -- if they had offered an extended cab with the MJ, they would have tripled or quadruples their sales, easily. The lack of an extended cab was why my first "modern" (at the time) Jeep was a Cherokee rather than a Comanche.

When the dash turn indicators light up as soon as you turn on the headlights or parking lights, that's a sure sign that you have bad grounds in the front end wiring, and possibly bad parking light sockets, as well. Start by cleaning up the ground connections, then clean up the sockets -- or just replace the sockets. You can find replacement sockets that fit in the HELP! line at the parts chains. I don't have the number, but I believe the ones that fit are for a Ford. I don't know about replacing a broken tab in the fuse panel. My guess is that it would be easier overall just to replace the fuse panel.