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2.5 to 4.0 swap


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Alright, so to start off with, I'm new to ComancheClub. I'm no amazing mechanic or anything...let's just say my dad didn't teach very well. So, I know alot more than the average joe, but a heck of alot less than you respectable persons. So, my position here is thus: I have an '88 2.5l, 4 speed, 2wd that I blew the engine on. Now I bought a '90 MJ for $300, no title, no glass, 4.0, 5 speed, 4x4. I bought it for its guts and drivetrain, to swap into the 2.5. The '90 does run and has good compression. The 2.5 I have the title to, and it has a decent body, which is why I'm using it for the body/chassis. My situation is such that I don't have a running, legal vehicle, and I need one badly, don't have more than a couple hundred to spare every month, and this is my best bet for a cheap vehicle. That being said, I've read a ton, done research, compared diagrams...etc, but the simple truth is that I don't know what I'm even looking at sometimes. I've read Mjeff87's write-up, and it helps, but doesn't spell out for ME what to do with my set up. Sorry to repeat what's already been gone over so many times but I need someone to rectify my ignorance. So my question is:

 

What all do I have to swap over? I've heard such things as having to swap the entire vehicle wiring system over, to the underhood and underdash harnesses, to only the underhood harness, and nobody says where to start, for example: should I pull both engines/transmissions, and swap the drivetrain, then do the guts, or should I do engine/tranny first...all "writeups" I've found are more like documentation of what they did, not actually a write up of what they did, in what order, and how they did it. So, just not nearly detailed enough for me, and very detailed is what I need.

 

I live in Alaska, and perma-snow will be coming down as soon as October, and that is when I need to be done by. I also have some other questions, mods I might make since at some point almost everything will have been removed atleast once anyway. (I hope only once!!!)

 

The radiator from the 4.0 is in bad shape, so I was interested in knowing if anyone knows if a second gen. Chevy Lumina rad and electric fans would work, and if so, how you wire such a thing up (all fan writeups simply say 'install', but nothing about what's involved in the install). Also, I have a moderate interest in knowing if anyone has put a double tranny setup in their MJ, and if it worked. Emissions have been done away with here, so I'd like to know if anyone knows if there would be a benefit to entirely removing the cat out of the exhaust line.

 

I'd also like to know what any of you would suggest I do mod or replacement-part-wise while I do the swap, outside of a rebuild, which I won't be doing, atleast not now anyway. Long-winded post and subject that's been hashed and rehashed, so I apologize, but any help would be huge for me. Thanks in advance.

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per my writeup, I swapped an 89 4.0 underhood harness/ECU into my 87.....you need to find the pinouts of the C100 connector for both model years (the C100 is the big connector on the backside of the fuse panel, on the firewall under/behind the brake booster. My research on 87/89 showed that all the C100 pins were the same (2.5 to 4.0) so all I did was swap the underhood harness and left the original 2.5 in-cab harness in place. Everything worked, but I had to switch the HVAC blower fan because it had a different style weatherpack connector.

 

My advice (and what I did) was to pull the old 2.5 drivetrain out completely, then swap the engine harness (and frame horns) while the engine bay was empty. Once the new harness was installed, I dropped the 4.0 in and hooked everything up. By using the 89 harness I also eliminated the dreaded C101 connector too. Same should apply for your '90 harness.

 

Jeff

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My advice is if you are going to do all of this and not just what Pete suggests is that you get steppin'.

You ain't got much time. These large projects like this always seem to take longer than you think.

 

Good Luck with whatever you do. :thumbsup:

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.....you need to find the pinouts of the C100 connector for both model years (the C100 is the big connector on the backside of the fuse panel, on the firewall under/behind the brake booster.

The problem with this is that the '88 will have a C101 connector, but the '90 won't. It went away away in the middle of the 1989 production year.

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true, but both styles still terminate at the C100 connector (and connect to the interior harness the same way). The 89 harness I used doesn't have the C101 and it plugged up 100%.

Bah ... reading comprehension is important. You wrote "C100" and I read "C101," probably because the C101 connector is such a common topic of discussion.

 

My apologies for the inadvertent hijack.

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Thanks everybody for the help. If someone does have those pics of the connector's that would be great. I'll be starting my long haul on this project either mid-week or this coming weekend. Wish me luck, and insights/advice from past experience always welcome :thumbsup:

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If you kick @$$ on this, you should be able to get it done quite quickly.

Drink lots of 5 hour energy drinks and don't get frustrated.

Sounds real easy just by the info jeff gave. Only issue you may have to handle is the driveline. Hopefully it just goes right in too. You should have all the parts you need now.

 

My 2 cents... don't mess with that lumina radiator setup.

 

You could do away with the expansion tank setup while doing this conversion also.

The radiator for 91 comanche is only 80 bucks on rockauto.com

Then a few hoses from your local parts house and youll have a cleaner engine bay and nice normal cooling system.

 

And since it all basically plugs right in... should be running in no time.

 

SWEEEET. :)

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  • 2 weeks later...

Alright, to continue my post, I'll let you guys know where I'm at. To keep it simple, I'll refer to the 4.0l (donor) as the 'blue' one, and the 2.5l as the 'black' one. So, I have the engine/tranny and gas tank out of the black one, front clips taken off of both for ease of access, the blue one has engine unbolted and gas tank almost ready to come out (I'm really peeved at the metal armor tub on the blue truck's gas tank, it's a real :headpop: , the black's was easy), both truck's driveshafts are disconnected (the blue truck didn't have a front DS, I'll have to get one), and so now I have a couple more questions for anyone that can answer them. Mjeff87, you mentioned splicing gas lines. What exactly did you mean :help: ? Did you mean you actually combined them, or just took the 4.0's lines that go from the tank to the engine bay and put them in the 2.5, or what... I'm just :???: on that point. Also, there is an undershroud type thing that was on the 4.0 (under the front clip) that wasn't on the 2.5. Far as I can tell it's just metal plate that doesn't do much and could be done away with ( :no: ??) unless someone knows and will tell me the point of it? Oh, and the last and most important question: when you say I have to swap the underhood harness, that means all the wiring connected to the firewall and wheel wells, etc have to be taken out and put in the 2.5's bay, correct? As in, when I look at blue's bay, it should be virtually empty, correct? Oh, and probably the most gratifying, yet small, update is that my dad managed to open my passenger door :banana: :banana: :banana: :banana: :banana: which has been perma-shut since 2 winters ago, that stupid :wall:. Though that could easily be attributed to my impatience... ahem, like slamming the door because it wouldn't close and it was like -40. I deserve a :thwak: . Also, found that the Lumina's radiator is far too deep in any case, which leads to another question: are 4.0 XJ rads the same as 4.0 comanche.gif rads? If so I'll see what I can find at a junk yard (which in AK are :cry: few and far between, and don't have much in them anyhow. Just a :dunce: asking questions, please :help:

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The fuel lines on the 2.5 have quick (dis)connects down along the framerail where the flexible lines meet the hardlines back to the tank, but the 4.0 ones are press-fit. I popped off the 2.5 lines and cut the 4.0 lines out of the donor MJ I used, and joined the 4.0 lines to the metal hardlines with barb fittings and double clamps. Make sure you attach the lines correctly, one is supply and one is return.

 

I can snap a pic of what I did later on tonight when I get home from work if you want to see.

 

Jeff

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I do have a question for Mountainman though. What do you mean by saying that I should switch to a "normal" cooling system? Is this the distinction between "open" and "closed", and if so what's the difference? Or, if you prefer, refer me to a thread describing what you mean? I'd appreciate that, thx :thumbsup:

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  • 2 weeks later...

Ok, another update, and some more questions. I'm closing in on my goal. After I pulled the 4.0's engine/tranny/tcase all together, I noticed that there was a crack in the top of the tcase, it's about an inch from the seam that meets the tranny. The crack itself is about 3 inches long, and doesn't spider outwards. My question is, can I weld it and use it? It is at the top of the case, I don't see any reason to discard the case if it works, all the crack does is make me worry about leaking fluid. Also, which numbers do you use from the front diff to tell what ratio you have? The numbers I got are as follows: on the 'top' line 53005574 and on the 'bottom' line 3 07 610317-5.

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Also, which numbers do you use from the front diff to tell what ratio you have? The numbers I got are as follows: on the 'top' line 53005574 and on the 'bottom' line 3 07 610317-5.

 

 

It has 3.07:1 gears.....not good.

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Oh, gotcha. Seems I mixed up the ratio formula from the diff gears and the diff tag :oops: . My bad. Ok, so what's so bad about 3.07? Wouldn't that be a good thing for mpg? And thx :cheers: PS, I realize I'm coming across as a :dunno: :help: , but can't help it, sorry :doh:

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3.07 are on the other side of the sweet spot for mpgs. I got better mpgs when I bumped up to 3.55s. :thumbsup: but, as with everything else in the Jeep world, individual results may vary.

 

and just to double check, are you sure that's a crack in the t-case rather than a casting flaw? I had what I thought was a crack in a trans a while back but it turned out that it just looked like a crack. :yes:

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or you can count the spins. brace one axle shaft still and spin the other around exactly twice while counting the times the pinion rotates. if it spins around 3 times its a 3.07. If it spins around 3 and a half times, it's a 3.55. If the axle has a posi, then just spin one side (the other will spin with you) around once.

 

or if it's easier with the axle on the ground (admittedly I usually seem to be giving this tip when the axle is installed under a Jeep) you can hold one shaft still and spin the pinon around until the opposing axle shaft spins twice. either way should work. :thumbsup:

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