Eagle

NOOOOOOOOOOOOOO!!!!!!!!!!!! That truck is MUCH too nice to be cut into.

What are "running lights"? Headlights? Parking lights? Taillights? All of the above? It sounds to me like a bad headlight switch.

Not that I want to do an IFS swap, but keep in mind that the MJ is an AMC design, and AMC was building unibody cars that were around the same size as the XJ/MJ for decades before the XJ came out. I would look at adapting the front suspension out of a Javelin, Hornet, or Concord. Avoid Rambler Americans -- they had a very strange coil spring/trunnion setup that had some problems, and would likely be impossible to find replacement bushings for today.

Oops.

The problems are that (a) the glass is curved, not flat, and (b) the rear glass is tempered, not laminated. You can't use epoxy injection to repair tempered glass like you can with laminated windshield glass. Your options are to find another factory MJ window, install an aftermarket slider, or use plexiglas to make a replacement panel. If there are any old-school glass shops in your area, you might find one that will cut a flat replacement for you using laminated safety glass, and that would be better than plexiglas, bit it'll still be flat and not match the original.

What upper piece? I thought 1990 was the year they changed the grille to incorporating the upper trim strip into the grille itself rather than having it as a separate strip like on my '88s.

Like most of the other votes, I vote to keep the nice one nice, and use the rust bucket as the off-road project. It's going to get beat up anyway, so why bash up a nice, solid, straight MJ when you have a beater candidate already?

Anticipating the next question: No, they are not the same as 2-door XJ rear flares.

I'm still thoroughly confused because Dzimm refers to the tires by what he thinks the diameter is rather than what the actual size is. He did mention a 225/70 (15?), but (a) I don't know which truck those are on or if those are the tires he used to get his speed and RPM readings for, and (b) that wasn't a size that was offered by the factory, so the speedoemter can't be accurate.

I think I got the updated version uploaded to Google. If the top says copyright through 2018 ... it's the new version. It's as correct as I can make it from the limited real-world revolutions per mile I could find. When I first made up the spreadsheet, almost all the tire manufacturers gave that data. Now almost none do. For tire sizes not listed, I did some comparisons. I spot checked a number of entries in another spreadsheet I have, of tire specs. For ten entries for which I had actual revolutions per mile, I compared those to the revs/mile calculated based on nominal tire size. The difference ranged from 2.64% to3.53%. So, to get in the ballpark, if all you have is the calculated revolutions per mile ... just add 3 percent and you should be close enough for most purposes. I'm not sure if you lads can just view the spreadsheet, or if you can edit it. If you find that you can edit it -- PLEASE DON'T! If you want to play, download your own copy, don't change the one I uploaded. And, please respect my copyright. A fair amount of work went into that spreadsheet. It says it's copyrighted because I am not releasing it into the public domain. Please don't pass it on to anyone else.

Cooper Tire still publishes revolutions per mile in their specs. For anyone wishing to accurately figure out RPMs relative to speed, that's the number to use. https://tiresize.com/tires/Cooper/Discoverer-AT3.htm For some sizes, they list three different tires, I guess based on load rating (or speed rating), and the numbers are different. I'm going to have to update my spreadsheets. I'm not going to list three of the same size, because if you look at another manufacturer's tires of the same size, their numbers will be different. A difference of 5 or 10 or even 25 RPM doesn't matter, so where they list multiple entries for the same nominal tire size ... I averaged them. Now I have to get a good night's sleep and come back to triple check my work. And then I have to try to remember how to update the document on Google Docs

Here ya go -- a 195/60-15 has a theoretical diameter of 24.2 inches. I have a spec for that tire -- 833.1 revolutions per mile. You can run the math on it and see what kind of RPMs you get.

2600 RPM would be about right (2636) for 1:1 transmission ratio -- which is 4th gear, not fifth. Then you factor in the overdrive ratio. 2636 x 0.75 = 1977 RPM at 70 MPH. Are you not using fifth gear?

I have no idea whatsoever. Find me the revolutions per mile for the tire you have in mind and I'll have an answer for you.

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.