New here to this forum, but not new to other forums. I just recently bought a 1987 ranger, reg cab, 4x4, 5 speed with a bad 2.9. Local salvage yard has a couple early 90's 4.0 motors for $300 with wiring harness. I have been reading on how to do this swap and am still worried about all the wiring. I know it will bolt up since its basically the same mounts and bell housing. How difficult is it going to be to splice several wires from the 4.0 to my truck? Thanks in advance for any insight.

 

There won't be many splices.
The engine and computer with wiring are fairly autonomous.

The 4.0l used MAF sensor and was distributorless when introduced in 1990
So you will need the air filter box and MAF sensor
Also the EDIS-6 ignition module, usually on rad support
And the Power box(fuse box)

Also the engine wiring harness with computer, hopefully computer is for a manual trans, if it is from an automatic 4.0l you will get false CEL(check engine light) all the time because transmission controls are not plugged in.
Alternator wiring should be close to the same if not exactly the same, just the reconnect in the Power Box instead of the starter relay

Your '87 power distribution is basically a few Fusible links attached to the starter motor Relay on the fender, these will be moved to the Power Box for the most part.

The oil gauge and temp gauge senders just use the same 1 wire setup, you may need to lengthen the wires but not a big deal.

Current fuel pumps will work fine with 4.0l
The fuel pump relay should be in the Power Box, you may need to splice the wire from this relay to the inertia switch in the passenger compartment.

 

I wonder if I should rebuild my fm145 too or just see how long she lasts?

 

The 4.0l engine came with M50D-R1(5-speed manual), better trans than FM145, see how much to include that.
If not available then just see how long the FM145 lasts, it wasn't a bad trans, just not as strong as the M5OD

 

Could I put a new clutch for a 4.0 in my Trans to beef it up any?

 

Yes, but there are differences.
4.0l clutch disc and pressure plate can be used on 2.9l engine(or 4.0l), it is a better clutch, so an upgrade from the 2.9l clutch.

Differences are:
FM145 trans uses a different throw bearing/slave than will come with 4.0l clutch Kit.

FM146 and M5OD use the same throwout bearing/slave, that would come in a 4.0l clutch Kit.

So if you keep the 2.9l and use the FM145 trans get throwout bearing/slave for FM145 and 4.0l clutch disc and pressure plate, so not a Kit.

If you get the 4.0l but keep the FM145, then its the same thing, separate parts, throwout bearing/slave must match the trans you are using.