1991 YJ Engine swap - PCM / ECM question - JeepForum.com
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post #1 of 12 Old 05-24-2021, 07:12 AM Thread Starter
mudtug
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1991 YJ Engine swap - PCM / ECM question

Hi All,

I have a 1991 YJ that came with a 2.5L, AX-5, NP231. The Jeep runs great, but it is grossly underpowered with 4:10 gears and 33's. I have a 4.0L and an AX-15 (plan to use the existing NP231) from a 1997 TJ. I have the powertrain harness from the TJ (Cam position sensor, crank position sensor, O2 Sensors, etc.) from the front all the way back to Fuel pump/gauge and in between.

My question pertains to the PCM / ECM. Will the PCM from the TJ work with the YJ? I am thinking no, primarily because the dashes are so different, I know the TJ uses a digital 2-wire CCD communication to the gauges on the TJ instrument panel. Moreover, will the 2 YJ body harnesses work with the newer (97) PCM?

So, do I need to use the PCM from, say an 1987 YJ that came with a 6-cylinder? Can I use the PCM from the 1997 TJ in the 1991 YJ? Could use some guidance from those who have done this before (I will say the 1987 YJ's 4L had a carburetor and not a throttle body, so not sure it would work?)

Would appreciate feedback from those who have done this before, so I can track down whatever I need before I have another unfinished project in the shop!

I know I have to cut the 2.5L engine mounts and weld in new mounts for the 4L, no issues there. I believe I can reuse the skid plate I have that came in the 91 YJ (AX-5) and mount the AX-15 to it. I know I will need a new exhaust, no mechanical issues I can foresee. Just the PCM / ECM question.

Appreciate any knowledge the mechanical gurus can pass along!

Thank you,
Mudtug

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post #2 of 12 Old 05-24-2021, 09:16 PM
Luuca
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Quote:
Originally Posted by mudtug View Post
Hi All,

I have a 1991 YJ that came with a 2.5L, AX-5, NP231. The Jeep runs great, but it is grossly underpowered with 4:10 gears and 33's. I have a 4.0L and an AX-15 (plan to use the existing NP231) from a 1997 TJ. I have the powertrain harness from the TJ (Cam position sensor, crank position sensor, O2 Sensors, etc.) from the front all the way back to Fuel pump/gauge and in between.

My question pertains to the PCM / ECM. Will the PCM from the TJ work with the YJ? I am thinking no, primarily because the dashes are so different, I know the TJ uses a digital 2-wire CCD communication to the gauges on the TJ instrument panel. Moreover, will the 2 YJ body harnesses work with the newer (97) PCM?

So, do I need to use the PCM from, say an 1987 YJ that came with a 6-cylinder? Can I use the PCM from the 1997 TJ in the 1991 YJ? Could use some guidance from those who have done this before (I will say the 1987 YJ's 4L had a carburetor and not a throttle body, so not sure it would work?)

Would appreciate feedback from those who have done this before, so I can track down whatever I need before I have another unfinished project in the shop!

I know I have to cut the 2.5L engine mounts and weld in new mounts for the 4L, no issues there. I believe I can reuse the skid plate I have that came in the 91 YJ (AX-5) and mount the AX-15 to it. I know I will need a new exhaust, no mechanical issues I can foresee. Just the PCM / ECM question.

Appreciate any knowledge the mechanical gurus can pass along!

Thank you,
Mudtug
Your 1991 has a mechanical speedo. Ours did.

The ECU you have will run the 6 cylinder. It is provisioned for the extra 2 cylinders. You can use your 91 engine bay harness and simply add the 2 necessary injectors. You will want to use the YJ sensors not the TJ sensors.

You can get two injector connectors from the Junk yard cheap, and wire them in. Below is a link to a PDF for the 1991 YJ FSM. look at the 6 cylinder wiring diagrams. You will need to extend the coil wires and the alternator wiring on the passenger side of the engine.

I was lucky to find an actual 6cylinder 1991 YJ engine bay harness when we did our swap, but I know all this can be achieved.

https://www.dropbox.com/s/xa98fp8pjb...%20SM.pdf?dl=0

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post #3 of 12 Old 05-25-2021, 04:38 AM
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Quote:
Originally Posted by Luuca View Post
Your 1991 has a mechanical speedo. Ours did.

The ECU you have will run the 6 cylinder. It is provisioned for the extra 2 cylinders. You can use your 91 engine bay harness and simply add the 2 necessary injectors. You will want to use the YJ sensors not the TJ sensors.

You can get two injector connectors from the Junk yard cheap, and wire them in. Below is a link to a PDF for the 1991 YJ FSM. look at the 6 cylinder wiring diagrams. You will need to extend the coil wires and the alternator wiring on the passenger side of the engine.

I was lucky to find an actual 6cylinder 1991 YJ engine bay harness when we did our swap, but I know all this can be achieved.
@Luuca The 1991 2.5 ecu will run the 4.0?!! I didn’t know that!

OP: I used 1991 2.5 harness in my ‘89 and modified as luuca indicated to run a 4.0. Wasn’t hard to do.

1991 had/has a cable-drive speedometer that ALSO has an electronic speed sensor for the ecu. It will throw a code and usually trigger the CEL if it does not ‘see’ the speed (vss) sensor.

The YJ also has two engine temp sensors whereas tj has only one. One is for the gauge (in head) snd other is on the t-stat housing. There’s a couple of ways around this.

It’s a fun project imho

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Cranking IS turning over
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post #4 of 12 Old 05-25-2021, 11:48 AM Thread Starter
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That's great news! Thank you for the info.

When you say use the YJ sensors, I assume you mean swap over the Cam Position sensor, Crank position sensor, Throttle Position sensor? Anything else I need to move over to the 4L?

I fully understand grabbing the injector connectors/wires from the junk yard, will likely just yank those out of my donor motor harness.

As for the 97 4.0L only have the temp sensor in the thermostat housing, I am interested in any suggestions for cheating/working around the missing cylinder head temp sensor?

fishadventure, yes, should be a fun project, that YJ is pretty much unstoppable off road in low range, but getting there with those 33's I get a lot of waves from folks in such a hurry that they only wave with 1 finger! I think the 4L and AX-15 will coupled with those 4:10's should actually allow me to see the speedo actually rise in 5th gear!

Thanks again for the great info, very very helpful!

Mudtug
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post #5 of 12 Old 05-25-2021, 12:46 PM
Luuca
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Quote:
Originally Posted by mudtug View Post
That's great news! Thank you for the info.

When you say use the YJ sensors, I assume you mean swap over the Cam Position sensor, Crank position sensor, Throttle Position sensor? Anything else I need to move over to the 4L?

I fully understand grabbing the injector connectors/wires from the junk yard, will likely just yank those out of my donor motor harness.

As for the 97 4.0L only have the temp sensor in the thermostat housing, I am interested in any suggestions for cheating/working around the missing cylinder head temp sensor?

fishadventure, yes, should be a fun project, that YJ is pretty much unstoppable off road in low range, but getting there with those 33's I get a lot of waves from folks in such a hurry that they only wave with 1 finger! I think the 4L and AX-15 will coupled with those 4:10's should actually allow me to see the speedo actually rise in 5th gear!

Thanks again for the great info, very very helpful!

Mudtug
I don't have any pictures on hand, but while we had the engine pulled, we drilled out and tapped the boss on the TJ engine to fit the YJ temp gauge sensor.

We removed the rear upper freeze plug from the head and ran a shop vac into that freeze plug while we drilled along with coating the drill bit and tap with red-tacky grease. We were able to simply thread in the factory YJ sensor when we were done.

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post #6 of 12 Old 05-31-2021, 05:36 AM
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I removed a 1/2” pipe plug in the block under the manifolds and threaded it into an adapter there. Thanks jsawduste

[size=3]Shackles & D-rings are different things.
Cranking IS turning over
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post #7 of 12 Old 08-01-2021, 09:03 PM
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fishadventure, can you confirm that the 2.5 ECM will run the 4.0?
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post #8 of 12 Old 08-01-2021, 11:30 PM
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So you are saying an ECM from a 2.5 will work with a 4.0? Are you sure? I know the wiring harness is almost the same and the harness will work from the 2.5 with some modification, but I don't believe the 2.5 ECM will work with the 4.0..?
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post #9 of 12 Old 08-02-2021, 05:04 AM
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Quote:
Originally Posted by Mackie View Post
fishadventure, can you confirm that the 2.5 ECM will run the 4.0?
No, I cannot confirm that. @Luuca may have been referring to your reference of the tj ecm. I have not used tj components so I don’t know
Quote:
Originally Posted by Mackie View Post
So you are saying an ECM from a 2.5 will work with a 4.0? Are you sure? I know the wiring harness is almost the same and the harness will work from the 2.5 with some modification, but I don't believe the 2.5 ECM will work with the 4.0..?
A 1987 anything YJ will not work for you. MPFI came out in 1991- those ecm/ecu units will both plug into your 1991 harness.

However, to run a 4.0 from a post-1996 4.0 with a 1991 2.5 YJ harness straight up you need these things done:
1) add injector wires
2) move one or two wire locations in the 60-pin
3) move / make new motor mounts
4) add a second coolant sensor
5) if XJ 4.0 change out water pump, fan, some cast aluminum brackets, power steering pump etc from the XJ config to the 1991-1995 YJ config.

This involves of course adding two more injector wires to the 60-pin connector, plus one or two wires need to moved to a different position in the connector. Three wires needed to be extended iirc.

I don’t know if the TJ 4.0 has YJ water pump etc.; I think it does, but I don’t know.
@Luuca will have to confirm the tj ecm use as I haven’t stolen any tj 4.0s

(Since you’ll be making new motor mounts ‘my’ suggestion is to lift the motor and trans at the same time to get rid of the factory low-slung skid. However, this predicates an SYE, driveshaft, and floor pan mods as well as a new crossmember so maybe you don’t want to spend that much or bite off that much work.)

[size=3]Shackles & D-rings are different things.
Cranking IS turning over
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post #10 of 12 Old 08-02-2021, 07:40 AM
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What about the coil packs that the TJ motor uses?
Can you remove those and run plug wires and a distributor?
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93 YJ SOA 2" springs front, XJ springs rear w/main leaf added, High pinon 9 inch rear detroit locker front Dana 44 ARB 4.56 Gears, 36 inch Irok tires too much to list.
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post #11 of 12 Old 08-02-2021, 10:46 AM
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Quote:
Originally Posted by TSEJEEPERS View Post
What about the coil packs that the TJ motor uses?
Can you remove those and run plug wires and a distributor?
The first few years of the TJ had a distributor.


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post #12 of 12 Old 08-02-2021, 02:52 PM
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Quote:
Originally Posted by TSEJEEPERS View Post
What about the coil packs that the TJ motor uses?
Can you remove those and run plug wires and a distributor?
I can Confirm this. You can pull the coil packs off a TJ4.0 and run a standard distributor in the hole where the TJ has the OPDA. We have a 98 XJ that we did this on.



You can see the coil pack Bosses in the above pic.

Below is the engine with distributor and plug wires. I drove this Jeep yesterday.

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