94 drivetrain swap from a 91

[Cole94]

I have a 94 explorer auto with 3.27 diffs that just rolled 275k. Body and interior is in nice condition for these trucks. But the motor, shes getting a little tired. Burning oil and being generally weak are the main concerns, and collapsing lifters are on the horizon. The (stock) A4LD is still doing alright, but leaves a little to be desired.
Now i just went and bought a 91 5 speed w/3.55s and 87k (likely 187k), and shes a little rough on the outside, but i bought her for a parts car for a reason.
Holy crap, I never realized how much fun the 5 speeds are. I just spent an hour tearing thru the field and up and down the road, and I've decided I want to do a complete powertrain swap. Its too much fun to not do it.

So my question to you, can I rebuild the 91 engine and put the 91 engine, tranny, t-case, driveshafts and axles along with the associated brains (pcm+ecm+harnesses) into the 94 with no issue? They're both 4 door btw. How many electrical gremlins would I be looking at? The 91 doesent have ABS, would I be forfeiting that by swapping all? Ahhhh so many questions!

Blue is 91 xl, green is 94xlt

 

[Cole94]

Bump

I'm just worried about stuff like the speedometer or other instrument panel stuff not working, I notice there's a few things different on the instrument panel.

 

[MrQ]

Why not just rebuild the 94 motor if you are going to do the 91? Makes more sense and you can use the 94 PCM. (94's had the camshaft position sensor, 2 O2 sensors, and EGR, none of which the 91 has.)

If you do use the 91 engine you will need to swap the computer, y-pipe, and the injectors (91-92 same, 93-94 different)

Everything else is drop in. Unless the 91 has a manual t-case you can use the one in your 94. Don't need to change driveshafts, but you will need to swap the front pumpkin and the rear diff over to match the gears if you want 3.55's. Just strip all the 5-speed stuff out of the 91, (suggest replacing the shift rail plugs and the shifter bushings while it's out). There are a lot of great threads here on the M5OD conversion.

Speedometer is gear driven and did not change 91-94. Nothing of any consequence changed in how the instrument panel functions between 91-94.

 

[Cole94]

Why not just rebuild the 94 motor if you are going to do the 91? Makes more sense and you can use the 94 PCM. (94's had the camshaft position sensor, 2 O2 sensors, and EGR, none of which the 91 has.)

If you do use the 91 engine you will need to swap the computer, y-pipe, and the injectors (91-92 same, 93-94 different)

Everything else is drop in. Unless the 91 has a manual t-case you can use the one in your 94. Don't need to change driveshafts, but you will need to swap the front pumpkin and the rear diff over to match the gears if you want 3.55's. Just strip all the 5-speed stuff out of the 91, (suggest replacing the shift rail plugs and the shifter bushings while it's out). There are a lot of great threads here on the M5OD conversion.

Speedometer is gear driven and did not change 91-94. Nothing of any consequence changed in how the instrument panel functions between 91-94.

Thank you for your response. That would be ideal, yes, but the 94 is already tagged and insured. The 91 is only titled. I would like to rebuild the 94, but there's no way to cheaply get around the insurance and tag part of the 91. I'd pay just as much for that as I have the rest of the truck. The only out i see is to rebuild the 91 and transfer the ecm and pcm over with the motor and tranny.
How difficult is it to switch over the ecm? Is it one big plug, or many plugs, or the entire harness?

 

[MrQ]

What I meant was, why not just rebuild the 94 engine instead of the 91, not the whole truck.

ECM is the PCM and it's easy to remove; it's inside the passenger side kickpanel. You'll need a 7mm for the retaining bracket and a 10mm for the connector. R&R.

 

[Cole94]

What I meant was, why not just rebuild the 94 engine instead of the 91, not the whole truck.

ECM is the PCM and it's easy to remove; it's inside the passenger side kickpanel. You'll need a 7mm for the retaining bracket and a 10mm for the connector. R&R.

Awesome, I'm glad you're helping me and being patient. Will the 91 ecm plug right up and work, or am I going to have to work on the wiring harness?

Or what if I just drop the 91 motor in there long enough to rebuild the 94, will the 94 ecm run the 91 motor?

I know i sound senseless, but I cannot drive the 91, and I really want the 94 motor and the 5 speed. I just need to get around for a couple weeks while the 94 motor gets rebuilt

 

[MrQ]

Here is what you can do. Swap in the 91 pcm (you are gonna need to swap some pins around, see below) and drop the 91 in as it sits. (94 wire harness will work with the 91 engine.) Swap in the 91 y-pipe. Move (do not remove) the extra O2-connector, Cam position sensor, and EGR connectors out of the way. Depending on how old the 91 is you will need to install the barometric sensors (the rectangle looking thing in the firewall behind the coil) should be a block off plate in the 94 in the same area.

On the O2 sensors there will be a dead pin for the second sensor on the 91 ECM. Find a pinout of a 91 ecm and use a multimeter on the CONNECTOR (not the ECM) to find which is the correct connector for the correct O2 sensor based on the ECM pin out. Otherwise ECM will throw error codes.

 

[MrQ]

Electrical Diagrams

1991:

1994: