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  #1  
Old February 14th, 2019, 14:50
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Exclamation differences in 91-95 xj

I am currently having an issue with my pcm in 1991 jeep Cherokee 4x4 auto trans. I was wondering if a pcm from a 1995 that is virtually identical will work in my Cherokee. my dad has a 95 and mine is a 91.
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Old February 14th, 2019, 16:56
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differences in 91-95 xj

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Originally Posted by Club31XJ View Post
I am currently having an issue with my pcm in 1991 jeep Cherokee 4x4 auto trans. I was wondering if a pcm from a 1995 that is virtually identical will work in my Cherokee. my dad has a 95 and mine is a 91.


They are close but Iíve understood you needed to match the computer numbers. At least that was the parts business rule

I do not know if there is any real difference in the plugs or pins, mechanically the transmissions were interchangeable 91-96.


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Old February 14th, 2019, 16:57
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differences in 91-95 xj

The pins are the same. I know one difference that the 95 had was that it had an airbag in the steering wheel. Otherwise pretty much the same I believe


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Old February 14th, 2019, 18:53
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differences in 91-95 xj

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The pins are the same. I know one difference that the 95 had was that it had an airbag in the steering wheel. Otherwise pretty much the same I believe


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Far as I know that air bag wiring shouldnít really come in to play, the way I remember the TCU set up was almost a separate system to the main ECU, hell it looks just like the Toyota tcu and wiring of the period.

My guess would be if the tcu pins are the same then 91-5, maybe 96 should work. Renix will be close be different enough to cause headaches.

Isnít there a fuse in the TCU itself? I want to say there is but havenít look at one in years.


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Old February 14th, 2019, 18:55
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differences in 91-95 xj

Tcu? Sorry not familiar with the abbreviation


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Old February 14th, 2019, 18:55
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differences in 91-95 xj

Pcm Iím pretty sure is 91-92 then 93-95, 96 is OBD II


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Old February 14th, 2019, 19:02
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differences in 91-95 xj

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Tcu? Sorry not familiar with the abbreviation


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Sorry my mind is tired and wandering, I was think of the transmission control computer, it just about a separate entity and I do think if the pins are the same itíll work 91-96, Engine PCM will be damn close but Iím almost certain itís 91-92 then 93-95 in compatibility. But 93 may be on its own, I canít remember. When I looked for a replacement for a 92 pcm the 91 was compatible, I donít think the 93 was, 94-95 I k ow will work together because I have a 95 harness running a 94 computer.
I may be over exhausted after 12 hr shift so please feel free to correct me. Dunno why I jumped to TCU ..


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Old February 14th, 2019, 19:02
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differences in 91-95 xj

Yes thatís correct


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Old February 14th, 2019, 19:05
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differences in 91-95 xj

No that all sounds right. I took the pcm out of my dads jeep and put it in mine and my problems were solved minus a weak spark. I have the proper year pcm coming tomorrow for my Jeep I just needed to do a check with his from his 95 to make sure it was the computer causing no spark. Thanks for all the help man!


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Old February 14th, 2019, 20:05
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Re: differences in 91-95 xj

Late to the party, but for future reference for anyone else who may come across this thread: The airbag is a non-issue in this picture. The '95-'96 airbags are "mechanical". They do not interact with nor depend on the computer. They are self contained units.
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Old February 15th, 2019, 04:41
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Re: differences in 91-95 xj

Computer talks to all the electronic bits--sensors, ignition parts, injectors, etc. If you swap in a later computer you may need to replace some of the other different bits too.
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Old February 15th, 2019, 12:56
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Re: differences in 91-95 xj

91-95 are pretty close. The only things I can think of off the top of my head:
- 91-92/93ish have a 2 wire VSS that's just a reed switch to ground. 93/94ish-up have a 3 wire Hall effect VSS. Same number of pulses per revolution and the ECU *may* have a pullup resistor built into it, I'm not sure, so it might not matter, or you might have no VSS signal.
- I think the brake light switch changed at a certain point, not sure when. If it did, the ECU may not read it right but it probably will read it fine.
- 91-93ish had a fuel pump ballast resistor and a relay to bypass it during starting and wide open throttle, 94ish and up don't. If this gives you trouble you can just bypass the ballast resistor. I believe the O2 sensor heater might be involved in this circuit as well, might want to compare the wiring diagrams and see if it'll affect the O2 sensor.
- there are a few TCU changes in that year range but I don't believe they'll matter for the ECU.
- cruise control is different 91-94 vs 95-96. Old ones used the GM steering column and cruise switch, 95 used the resistor multiplexed setup similar to 97-98 and 99-01, but each of those year ranges has different multiplexing resistors.

I'm 99% sure everything else will work. I haven't checked the resistance curve on any of the sensors but I'd be pretty surprised if they actually changed them instead of just changing the connectors occasionally. For example 91 through 01 use the same CPS reluctor slot pattern on the flywheel and the same (or compatible) CPS electronics, but they changed the connector like 3 or 4 times over the years. I know people who have spliced old CPS connectors onto new sensors and vice versa as an emergency repair and had it work just fine.
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Old February 15th, 2019, 13:00
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differences in 91-95 xj

Thank you for the info. So basically if the pcm from a 95 is good the 91 jeep should run maybe just with minor issues?


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Old February 15th, 2019, 16:17
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Re: differences in 91-95 xj

Not likely.

'95 was a crossover year for the drivetrain/ electronics.
'96 was all new style.
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Old February 17th, 2019, 20:02
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Re: differences in 91-95 xj

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Originally Posted by WB9YZU View Post
Not likely.

'95 was a crossover year for the drivetrain/ electronics.
'96 was all new style.
You're off by one year. Go +1year for both those.



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