88 MJ to 96 XJ Swap

[jeepy2013]

Hello all, this is my 1st MJ and 1st post. I've had a bunch of XJ's over the years, but never managed to get my hands on a decent MJ. Well I just purchased a 100% rust free '88 MJ. It was originally a 2.5 , auto, 2wd.  The previous owner started to swap in the complete '96 XJ drive train (4.0 , AW4 , NP231, D30) into this truck. They swapped over the entire engine harness, dash harness, cluster and steering column, brake booster / mc  from the '96 XJ. Alot of stuff was loose, unhooked, etc, so I pulled  the drivetrain out and engine harness so I can clean it all up and start fresh. I know I have to adapt the 96 xj interior harness to the MJ rear body harness. My biggest hurdle seems to be the fuel system.  Whats the best way to go about this? It has a brand new MJ tank in it, supposedly a new pump too, but I think its for the 2.5. Can I simply swap in a '92 pump / sending unit (rock auto lists one for '92 MJ 4.0) into the tank and change out my '96 fuel rail with one from a '92 with the regulator?  will the '96 motor run on the lower fuel pressure? Or am I going to better off trying to swap in a different tank? modify a 96 xj sending unit? I've been searching and looking for a week, alot of conflicting info out there....and my head hurts!  I don't want to run a fuel cell, I'd like to keep a stock style tank of some kind underneath.  All of the original renix stuff is gone , none of it came with the MJ so I don't have any of that to work with. This is kind of a mess with this '96 swap, but the body is in super good shape so I think it will be a great foundation for a nice build.  Any help would be greatly appreciated.

 

[DirtyComanche]

The pump and sending unit that Rockauto lists is likely for an XJ and won't fit.

 

No other advice.

[Smokeyyank]

A 96 swap is going to be tough. It was a crossover year so everything is going to be a mess. What might be a 96 for one thing is a 97 for another maybe 95 another time. 

[barimpact]

I've never done the swap, so take this more as suggestions rather than hard facts. Hopefully someone will be able to confirm my suspicions.

 

With that said, my first thought is that if you're using a 96 engine, ecu, wiring, etc, just keep using the 96 fuel rail.

 

Second, among other things, the harness for the sending unit for the 92 MJ is different than the harness on the 96 XJ (the XJ harness has an extra 2 wires I believe). The connection point for the wires on the fuel pump itself is also different between the two (the 92 MJ just has 2 studs and nuts where the wires connect, the 96 XJ has a female harness connector directly on the pump). However, this pump is also a 2 wire design, and I've seen some online that come with the male harness and a few inches of wire so you can splice it if needed. You may be able to get this pump and splice the wires to the MJ sending unit.

 

There's a little more discussion in this thread I'm pasting below, where someone mentions having adapted the XJ sending unit to fit the MJ, but mentioned that you can't let it get to 1/4 tank or it will be sucking air. They also have a picture of the difference between sending units. Hope this helps.

 

[jeepy2013]

1 hour ago, DirtyComanche said:

The pump and sending unit that Rockauto lists is likely for an XJ and won't fit.

 

No other advice.

 

Upon further research I think you are right. RA shows the same TYC part # for the '92 MJ and for a '92 XJ

[jeepy2013]

53 minutes ago, Smokeyyank said:

A 96 swap is going to be tough. It was a crossover year so everything is going to be a mess. What might be a 96 for one thing is a 97 for another maybe 95 another time. 

 

I knew the '96 swap was a can-o-worms, probably why the previous owner bailed on the project. I bought it cheap so it was well worth it still for a solid rust free MJ, plus I have a ton of stuff I can sell off if I go another direction. I guess I have some options, try to complete the '96 swap , try a '98 swap ( I have a complete  98 XJ ) , find a renix donor XJ or.....who knows.......lol

[jeepy2013]

41 minutes ago, barimpact said:

I've never done the swap, so take this more as suggestions rather than hard facts. Hopefully someone will be able to confirm my suspicions.

 

With that said, my first thought is that if you're using a 96 engine, ecu, wiring, etc, just keep using the 96 fuel rail.

 

Second, among other things, the harness for the sending unit for the 92 MJ is different than the harness on the 96 XJ (the XJ harness has an extra 2 wires I believe). The connection point for the wires on the fuel pump itself is also different between the two (the 92 MJ just has 2 studs and nuts where the wires connect, the 96 XJ has a female harness connector directly on the pump). However, this pump is also a 2 wire design, and I've seen some online that come with the male harness and a few inches of wire so you can splice it if needed. You may be able to get this pump and splice the wires to the MJ sending unit.

 

There's a little more discussion in this thread I'm pasting below, where someone mentions having adapted the XJ sending unit to fit the MJ, but mentioned that you can't let it get to 1/4 tank or it will be sucking air. They also have a picture of the difference between sending units. Hope this helps.

 

 

Thanks for the link. I'll be using the MJ rear body harness that goes to the tank. The tank in the MJ is brand new and I was told the fuel pump was as well, but I don;t know what the PO put in for a fuel pump. It may be the replacement for the 2.5 or he put a 4.0 pump in there, I couldnt really get accurate info.  I'll have to drop the tank and see whats going on. My concern was the difference in fuel pressure, the pre '96 models run lower pressure and the 96 model runs 49 psi I believe. Maybe I could run a aftermarket high pressure pump with external regulator and draw fuel from the stock tank pickup.  Really getting the fuel system sorted out seems to be my biggest hurdle. 

[barimpact]

Yes the earlier models run 31 psi at idle I believe, but it's also the fuel pressure regulator that makes this pressure. If you take the vaccuum hose off the regulator, the pressure jumps up to about 39 psi. I think theoretically, you could get a comanche 4.0 pump and use the 96 XJ fuel rail and regulator and I think it should work. 

[jeepy2013]

10 hours ago, barimpact said:

Yes the earlier models run 31 psi at idle I believe, but it's also the fuel pressure regulator that makes this pressure. If you take the vaccuum hose off the regulator, the pressure jumps up to about 39 psi. I think theoretically, you could get a comanche 4.0 pump and use the 96 XJ fuel rail and regulator and I think it should work. 

 

This is what I was thinking at first as well. Then I was looking at some other options like retro fitting a higher pressure pump to the stock sending unit assembly, I saw mention of someone using a Mustang pump or something. Or.....a external high pressure pump, with a external pressure regulator. Would just have to figure out plumbing.  Then there is the issue of the fuel gauge reading backwards. Is the gauge issue the difference between a MJ and XJ sending unit / gauge ? or just the years. 

 

I'm stuck with what I have to make this work..... I guess, other than buying another parts Jeep to use for swap parts.

[barimpact]

1 minute ago, jeepy2013 said:

Is the gauge issue the difference between a MJ and XJ sending unit / gauge ? or just the years. 

 

I'd guess just the years. It makes no sense to make 2 totally different ways to read the fuel level from the manufacturing perspective. You'd have to make too many specialized parts where they weren't needed.

 

As far as reading backwards....You may be able to reverse it. Before explaining my thought process, are you 100% sure it reads backwards, or does the gauge just always read full? Just want to make sure.

[jeepy2013]

1 hour ago, barimpact said:

 

I'd guess just the years. It makes no sense to make 2 totally different ways to read the fuel level from the manufacturing perspective. You'd have to make too many specialized parts where they weren't needed.

 

As far as reading backwards....You may be able to reverse it. Before explaining my thought process, are you 100% sure it reads backwards, or does the gauge just always read full? Just want to make sure.

 From everything I've read online, the gauge reads backwards with the newer XJ cluster in a MJ,  ie; Full is empty , Empty is full.  I haven't got that far with my wiring yet. I pulled out the engine, trans, transfer case and engine bay harness to pressure wash and repaint the compartment. Good thing I did, the torque converter was bolted to the flex plate with 4 different bolts, the holes in the flexplate were all egg shaped, the drivers side motor mount was was completely broken, the trans mount was wasted.  I don't know why someone would swap all this stuff in and not look at or change anything that needed to be replaced before doing it. 

 

 

[DirtyComanche]

1 hour ago, barimpact said:

 

I'd guess just the years. It makes no sense to make 2 totally different ways to read the fuel level from the manufacturing perspective. You'd have to make too many specialized parts where they weren't needed.

 

As far as reading backwards....You may be able to reverse it. Before explaining my thought process, are you 100% sure it reads backwards, or does the gauge just always read full? Just want to make sure.

 

Early (Renix) XJ/MJ gauges use 0-88 ohms for the sender.  0 is empty, 88 is full.  In practice I find they're normally a little higher than that, similar to the GM 0-99 ohm setup that is super easy to find.

 

Early HO (91-96) uses 88-0 ohms.  88 is empty, 0 is full.  I have no idea why Chrysler did this, as I don't believe it even harmonized with any of their other vehicles.  So yes, it reads backwards if a Renix cluster or sending unit is swapped.  He needs the HO sending unit for it to work.  The gauges also mount differnent so it's not as simple as swapping them between clusters.

 

97+ used the even more illogical 70-270 ohm sending units.  Or something like that.  That is harmonized with modern Dodge/Chrysler products, and also is an absolute PITA because it does not use 0 as a data point.

 

Solutions really are to find a HO pump/sending unit, modify an XJ HO unit to work, or install a GM ranged universal unit into the tank separate from the current pump/sending unit assembly.  The current sending unit should accept a FI pump from the 4.0L and it will make enough pressure.

[barimpact]

3 minutes ago, DirtyComanche said:

 

Early (Renix) XJ/MJ gauges use 0-88 ohms for the sender.  0 is empty, 88 is full. 

 

Early HO (91-96) uses 88-0 ohms.  88 is empty, 0 is full.  

 

 

If this is the case, it may be as simple as switching some wires around on the sending unit. 

 

The piece of the sending unit that reads the fuel level is basically a slide potentiometer, which is a variable resistor that acts as a voltage divider. Here's a fairly simple diagram of one from a quick Google search:

As the float goes up and down in the tank, it moves the slide (top piece in the picture) up and down the length of the potentiometer, which changes the number labeled 7.5V DC by changing the resistance of this path. The closer the slider is to the zero resistance path, the closer it is to giving the gauge the full 12V reading (per this picture - actual voltage numbers might be different). So, without moving the slider, if you switch the battery positive and negative leads, the power must move through the opposite path, which in this picture, would theoretically switch the 7.5V DC to reading 4.5V DC, provided this is a linear resistor.

 

Long story short, if you switch the positive and negative leads on the ends of the float resistor from the power source, it should make it read correctly. 

[jeepy2013]

OK, I thought a HO sender would work, and I got all happy that I found a listing for one on Rockauto for '92 MJ, but apparently its not quite the right one as they list the same one for '92 XJ.

[DirtyComanche]

17 minutes ago, jeepy2013 said:

OK, I thought a HO sender would work, and I got all happy that I found a listing for one on Rockauto for '92 MJ, but apparently its not quite the right one as they list the same one for '92 XJ.

 

You can mod them to fit.  Somebody on here was selling them, IIRC.

[Jeep Driver]

10 hours ago, barimpact said:

 

If this is the case, it may be as simple as switching some wires around on the sending unit. 

 

The piece of the sending unit that reads the fuel level is basically a slide potentiometer, which is a variable resistor that acts as a voltage divider. Here's a fairly simple diagram of one from a quick Google search:

As the float goes up and down in the tank, it moves the slide (top piece in the picture) up and down the length of the potentiometer, which changes the number labeled 7.5V DC by changing the resistance of this path. The closer the slider is to the zero resistance path, the closer it is to giving the gauge the full 12V reading (per this picture - actual voltage numbers might be different). So, without moving the slider, if you switch the battery positive and negative leads, the power must move through the opposite path, which in this picture, would theoretically switch the 7.5V DC to reading 4.5V DC, provided this is a linear resistor.

 

Long story short, if you switch the positive and negative leads on the ends of the float resistor from the power source, it should make it read correctly. 

Single wire, resistance to ground. 

[jeepy2013]

Well, I haven't given up on this project. I was able to get the engine bay all cleaned up and painted. I just ended up going satin black, was a little easier being the engine bay had 3 different paints on it, so I did some cleaning and sanding here and there and called it good. Not sure if I'm going to stick with the original red body color anyways. 

 

 

I have given up on the '96 Swap though.......as I dug into it further, nothing was done right. Things were loose, missing, broken or just done wrong. So....I took the dash back out, removed the '96 dash harness, '96 steering column, seat and rubber mat to clean up the floor pan and get it painted.  

 

So, now my options are to find a older parts XJ because I can find them dirt cheap here, probably like a '92 so I can swap everything into this MJ, or I already have a good running '98 XJ that I can do the full swap (wiring, engine, interior, front clip, doors, etc).  I know the full '98 swap is a huge undertaking, so I'm on the fence.

[MJXJjeepguy]

On 7/30/2019 at 3:46 PM, Smokeyyank said:

A 96 swap is going to be tough. It was a crossover year so everything is going to be a mess. What might be a 96 for one thing is a 97 for another maybe 95 another time. 

Your right, myself i have 2 96 one is current drive other is parts out, the current drive build date june 96 most sensors, piggy plug, wires, and other thing that are same with 97+ while parts out jeep build in march 96 all are pre 96. So that why people called 96 is a bastard year.

[jeepy2013]

I probably woulda have stuck with the '96 conversion, but everything was just done hodge podge. Thats not my way.  I got to cleaning up the inside of the cab and I have rust (yes even here in FL) Guess thats what 30 years of water leaking does.....lol  It's not bad though and it's the only rust anywhere on this MJ. 

 

Overall not too bad....

 

 I

 

Driver's side is the worst, I'll clean it up and weld in some small patches....

 

 

Passenger side just a couple of very small holes....

 

[Eric sicca]

On 7/30/2019 at 4:57 PM, jeepy2013 said:

 

Thanks for the link. I'll be using the MJ rear body harness that goes to the tank. The tank in the MJ is brand new and I was told the fuel pump was as well, but I don;t know what the PO put in for a fuel pump. It may be the replacement for the 2.5 or he put a 4.0 pump in there, I couldnt really get accurate info.  I'll have to drop the tank and see whats going on. My concern was the difference in fuel pressure, the pre '96 models run lower pressure and the 96 model runs 49 psi I believe. Maybe I could run a aftermarket high pressure pump with external regulator and draw fuel from the stock tank pickup.  Really getting the fuel system sorted out seems to be my biggest hurdle. 

Run a 95 rail with return line and a dodge caravan fuel regulator should but you in ball park of 49 psi

[jeepy2013]

I am actually picking up a complete '94 XJ for parts tomorrow, I think I'm going to swap the column, wiring and everything over the MJ just to make it easier and not deal with the '96 gremlins.