Minuit

3/16" is the line size, not the fitting thread size. I do not know what the fitting sizes are other than the brake bleeders, which are also 3/8-24.

Time to make this the most viewed topic in CC history.

For a primarily new system using R-134a and an SD709 compressor, PAG 100. No additives or stop leak crap. The only thing in your system should be refrigerant and oil. Ester oil is generally used in retrofits, but you're building a new system. Drain the existing oil from the compressor and add roughly 6 to 7 oz of oil to the system. You can put it all in the compressor, half in the compressor and half in the condenser, or whatever. Just make sure there is some oil in the compressor when you start the system. If you add the entire charge to the compressor, rotate the compressor 10 full revolutions right before beginning the charge. Mineral oil will not be carried through the system by R-134a. It is for R-12 or R-12 compatible refrigerants only. As a side note - for a custom system, it'll be damn impossible to tell you HOW MUCH refrigerant/oil you will need. That is calculated based on the internal system volume and was done when the system was designed. You are deviating from the factory setup so I cannot tell you an exact amount of refrigerant or oil.

What I call a "parallel flow" is what @ghetdjc320 calls "plate and fin". The refrigerant flows evenly from one side to the other in separate channels, rather than back and forth. Roughly a 20% to 30% efficiency improvement. APDI part number 7014173 is a direct fit and uses the threaded line connections. Almost all aftermarket reproduction condensers are "serpentine" tube and fin condensers. The APDI is the only one I'm aware of that is different from the original design. I do not know if you can install the aftermarket parallel flow evaporators in a 1990 heater box. If you can, that would be a worthwhile upgrade - I cannot confirm personally but @ghetdjc320 had good results and managed to retain the evaporator probe, which is essential to a properly functioning system. If your compressor spins freely without resistance, noise, or binding, that's about the best you can hope for. I'd rather have a used compressor that spins smoothly over a new reman. I am no fan of the Sanden A/C compressors that Jeep used, but if it works it works.

My 750s are 3.5" by 6".

Nope, the fixed ones don't have the same problem with leaks. You might get a little wind whistle if the weatherstripping along the leading edge is worn out or not installed properly, but it's extremely rare for them to leak water. You might want to use some silicone lube on the new ones when you install them. The weatherstripping can be very tight, which is a good thing but can make it hard to install.

Easier to just switch the wing assemblies out for ones that don't pivot. About a 15 minute job including taking the door card off. Unless you're dead set on having full windows.

No. Just gotta look the part numbers up. A lot cheaper than ordering "kits" though - that's all they did to assemble the kit I'm sure.

About all I can say is to do a very thorough water test with someone inside the truck and someone spraying from every possible angle with a hose. If any water can get in anywhere, it will... and ruin all of your hard work.

Mouser and the Delphi catalog (you can just plug the part numbers from the catalog into Mouser) is my usual source, but I buy these in bulk (often tens or hundreds of contacts at a time), your mileage may vary

So what else have you been hiding from us this entire time?

12V T-1 bulbs, 6 in total. Three in twist-lock bases (easy), and two soldered to the board behind the LCD assembly (not so easy), and one behind the tape door. Take care to not destroy the LCD assembly or front panel ribbon cable if you choose to replace the LCD bulbs.

You'll want the expansion valve matched to whatever refrigerant you're using. R-134a XV for R-134a use. All pre-97 XJs use systems with the same basic operating principle, although the parts do change a bit. In 1994 they switched from R-12 to R-134a. The pressure switch changed, receiver-drier changed, the expansion valve changed, the compressor technically changed but remained cross compatible, and there may have been some changes to the lines at the same time. The condenser did not change, and that really kneecapped the stock R134a system's performance since R-134a isn't as good a refrigerant as R-12. I strongly suggest using a parallel-flow condenser. I suggest using R-134a for environmental, cost, and parts availability reasons despite this. All of our Jeeps use a receiver-drier and expansion valve instead of an accumulator and orifice tube. That's better anyway. Buy a new receiver drier and make it the last thing you install. Pull the plugs on it right before you button up the system. It must not be exposed to open air for any longer than strictly necessary. Renix 4.0 used an SD508 compressor in an ear mount configuration. HO used an SD709 compressor in a direct mount configuration. Brackets are therefore different. The SD709 compressor has roughly 10% larger displacement. The job MUST BE DONE CLEANLY. Keep your hands clean when working on the A/C components and do not allow any dirt to get inside the system in any way. There's no special magic to A/C lines other than the specific type of hose used. It's called "barrier hose". Just let the guy know it's an A/C hose and he should know what to do. If not, find a guy who specializes in A/C hoses. All of the factory hoses use crimped-on hard fittings at connections. Tools needed for a successful job will be a set of manifold gauges, vacuum pump, HNBR o-ring assortment and an R-134a can tap (or R-12 can tap if you decide to go that way) at the bare minimum. If you have access to a dry nitrogen cylinder it will make for a more accurate leak check before you add refrigerant. This isn't all you need to know but should get you started on choosing parts. I don't think any certain way is better than any other for most uses, and you can have a good working system with pretty much whatever configuration of parts you pick. There are things about the stock Jeep system that do not impress me, so I'll be doing a semi-custom build on my 91 soon, but the stock parts will do most people just fine.

What about the guy at Jeep who was in charge of waterproofing? I think that's a hate that everyone can understand.

Yep, love those too. If memory serves, the 0.062" version is the connector used by the S10 blazer dome lights that a lot of people use to replace the originals. I have a sneaking suspicion that the Renix diagnostic port is a variation on that connector too.

They made the dashboards too. Wouldn't it be nice to be able to reproduce some interior parts? A pipe dream, I know. But I'd really love to have a new production dash pad or some B-pillar trims that don't have to be treated like they're made of sugar glass.

Agreed. If I were in your place, I'd go with some unsealed Metripack 150 connectors. That connector line was heavily used in Jeeps anyway so it would look proper, generic crimp tools work just fine for the contacts, and they can be had by the boatload cheaply as components on Mouser. Just make sure to pick the connector so that you can't accidentally plug something in incorrectly. AMP has a few lines of connectors that are good for this purpose as well, and I especially like them for audio installs.

It was for this exact reason that I eventually decided the "I'm looking for leather seats" piece that was in my signature for a while was a complete waste of time. Glad to know that the admins are on top of this.

I will now proceed to defend what I said. I intend no disrespect and I trust your judgment on matters of structural rigidity probably more than any other member on this forum, but I have my reasons for saying what I said. For my specific situation: 1. I have an education in mechanical engineering too, and I concur that we're talking about maybe a few percent difference at most. But, I currently have no realistic way to prove that the upper frame brace doesn't have a significant effect on the stiffness of the truck sitting in front of me, so I choose to err on the side of caution for my build. If I still had access to the FEA programs I used in school, it would be a fun project to build a model of the floor system with and without the brace and see just how much that piece matters. What I can say is that there's no way it doesn't reduce the truck's stiffness in at least some way, and that's good enough for me. 2. I'm partially doing it the stubborn, difficult way as an exercise in fabrication. I haven't done much work with sheet metal before and I want to learn. I just got told I can't drive on public roads for 6 months for medical reasons, so I've got nothing but time to do things the slow way. 3. Plenty of people do exactly what you mention and end up just fine, including some people who are doing what I'd consider to be pretty high grade restorations. This is just another case of me being picky. 4. My upper frame brace has catastrophic damage past where the seat mounting studs attach to it, and I don't really want to redesign the way the seat attaches to the floor pan, so I'm just going to copy the factory piece when I rebuild it. 5. Duplicating the factory design will make the carpet fit like it should. Yeah, that's something I'm willing to go to the effort for. 6. I take a lot of pleasure in the concept of "doing a lot of work to make it look like I did nothing" and that extends to duplicating the factory design of the floor pan and not adding any extra bracing even if it would make the truck stiffer. I want my truck's floor to look completely original when I'm done, even if there couldn't be anything further from the truth. We'll see if I achieve that goal in the end, but it'd be nice if I did. 7. Approximately NOBODY puts that brace back in when they do a floor pan in these trucks, and I just like doing things a little different. I'm the Jeep restoration world's version of a hipster. 8. The MJ's unibody was designed to handle much heavier payloads than the XJ's unibody and those loads are also concentrated in different places, which I'm sure is the actual explanation for why the MJ's floor pan has so much extra bracing that isn't present on the XJ, and I'm sure that layering in some more stampings was probably the cheapest way to achieve that for AMC. It was also designed after the XJ, so we might be seeing some things that the engineers wished they would've done on the XJ in the design of the MJ. I wasn't there, so I don't know. But for those reasons I don't consider "the XJ did it like this, so it's fine on an MJ" a valid argument if we're really going to be picky. For our OP's situation: 1. The OP is clearly someone who thinks something through and is interested in doing things the right way. Very few people on this forum go to the effort to give us annotated pictures of their truck's rust situation to start off a help thread. When I see something like that, I think "hmm, that person seems like a detail-oriented guy. I'm a detail-oriented guy. Maybe I should open up a little more on my point of view than normal." I don't think TheJeepNut would be dangerously affecting his vehicle's structural strength if he decided to just throw that upper frame brace in the garbage, but the way he phrased his post made me think that I'm not wasting my time when I write up a long post about why he might want to think about doing it the way the factory did it. 2. In the grand scheme of things, his truck isn't rusted too bad. But he really needs to look under that brace to make sure there isn't more rust being hidden by the brace, so it's coming up anyway. If anyone stands a chance of being able to drill the spot welds on the brace, clean it up with a wire wheel and make a simple patch for the section with pinholes, and then weld the brace back in place without doing any re-engineering, it's him. My floor pan is a total loss, and I'm choosing to duplicate the factory design because I want to. But for him, that's legitimately the easiest way to go about this and still do a complete job. We can still be friends, right?

I have a ZJ box in my 91. The 12.7 steering ratio and lower assist valving are big winners and are a 100% improvement for street use. No more 1-finger steering. I hate the overboosted AMC power steering so much, so that was a major upgrade. Maneuverability is also helped in a noticeable way thanks to the faster gear ratio. I'm not 100% happy with how it turned out - I bought a junkyard box with unknown miles on it and adjusted it to the top end of the factory spec. It's still got just enough looseness over center to bother me. The box probably has well over 200k miles on it like almost every junkyard Jeep around here, and the sector shaft seal leaks. I see absolutely no point in buying a rebuilt one or having it rebuilt - no rebuilder I know of replaces the actual important part, the gearset. So since I hate money I've been thinking harder and harder about buying one of those new production Borgeson boxes. They're the same box as every GM muscle car uses (and Borgeson essentially makes them to the Jeep spec, with metric line fittings and 3/4" input shaft) so there's a big customer base, and I've heard almost nothing but good things about them. They're the one source of Saginaw boxes that I actually believe when they claim to provide a "modern rack-and-pinion" style of steering feel.

I mean, I'm sure I can probably guess what owning it is gonna be like, but I've got to say that's a pretty good looking car in an '80s wedge spaceship kind of way.

Also, it's very important that the rusty flat arm that your finger is pointing to is horizontal with the valve hooked up. Otherwise there will be almost no rear braking.

What people neglect to mention (and where the HO system is a huge upgrade) is everything else where it pertains to having a functional vehicle. No more fusible links. No more 1970s-grade wiring harness jank. A proper engine bay fusebox. Proper grounding out of the box. Much more organized wiring runs. More sealed electrical connectors. Better availability of used parts. No more bouncing speedometer. A cooling system that isn't a pain in the @$$ to work with. Larger A/C compressor. Less 1980s emissions "technology." Less vacuum lines. Much higher "it just @#$%ing works" factor. I don't really care about having 8 more horsepower. I own my trucks to use them, and the HO era makes that easier. The Renix era has some cool stuff about it, and my own Renix truck isn't getting an HO swap. The interiors were a lot more interesting. I like the 1987 grille a lot more than the 1991 grille. The radios are cool. But a 1991 is a better vehicle than a 1990 in practice.

For what it's worth, there are oodles of boutique steering box rebuilders out there that will promise the world and may or may not deliver on those promises. You hear about all of the parts these guys will replace, but as far as I know nobody replaces the thing that really wears out with age - the gearset. If I was going to spend $500 on a steering box, I'd go to Borgeson - they own the original Saginaw tooling and as far as I know are the only ones that can produce new Saginaw steering boxes. They come with the ZJ's 12.7:1 steering ratio already and presumably the valving to give a nice tight feel. I've heard good things, and that might be one of my projects in the future, honestly.