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Flexplate Engine or Transmission Year?


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Hey guys, new member here! If this has already been answered somewhere then I apologize, I have not been able to find it! So I’m trying to figure out if the flexplate follows the engine year or the transmission year. I’ve got a ’91 MJ, 4.0, auto, 2wd, column shift, short bed, etc. I recently grenaded a wheel bearing and ruined a spindle, so decided to go ahead and do a 4wheel swap. I bought an almost exact 1990 Comanche but 4x4 and worse body to part out. In doing some research I noticed flexplates are listed by ’87-’90 then ’91-’00. Both seem to have the same specs (164 teeth, 13.83” OD, 12-14 pitch) but have two different part numbers, different weights, and two very different prices. So if I’m using the 1990 transmission/transfer case but keep the 1991 engine, which flexplate do I need to use?? Thank you in advance!

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Use the one that goes with the engine.

 

What's going on is that the crankshaft position sensor (CPS) reads off of windows on the flexplate (or flywheel in a manual trans application). In 1991 Jeep implemented Chrysler's OBD1 engine control system on the 4.0 in the XJ and MJ, rebranding it as "high output" (HO) as it picked up a handful of horsepower. Among the many changes from the old AMC-implemented Renix system is a different style of CPS, which reads differently from the Renix CPS, and so the arrangement of windows around the flexplate is different between the two and you'll need to use the flexplate (or flywheel) that matches your CPS and the rest of the engine control system. If you have the two flexplates side by side you'll see that the two are physically compatible (nearly identical) except for the window arrangement — the Renix has a bajillion with one slightly larger than the next, IIRC the HO has like, three.

In your case the engine control is the HO/OBD1, so you'll want the flexplate that goes with the engine control, i.e. the '91+. The used one will probably be okay, but they are a wear item and eventually will crack, so if it's got a ton of miles it might be better to replace it while you've got everything apart.

 

Also note that the CPS bolts to the bellhousing, so you'll have to pull that off the older transmission and stick it on the new trans. Unplug it before dropping the trans, but your life will be one hell of a lot easier if you don't unbolt it until you've got the trans out, and remember to transfer it to the 4x4 trans before it goes in. It takes some serious contortion to get at in with the trans in the vehicle.

There may also be some differences in the TCU control circuitry between the '90 and '91 trans. You might want to consult this thread for more info on that.

 

One more thing, unless that MJ is completely pooched, we CC members tend not to like seeing running and driving MJ's parted out. All the parts you need (except the rear driveshaft, you should be able to get the old one cut down) can be found in an XJ. If you can find one with an NP242 transfer case as well it's a pretty solid upgrade over the 231, not necessarily in terms of strength, but in terms of function in normal driving in low-traction conditions the full-time 4x4 is a hell of an improvement over the 231's lack of centre diff.

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Use the one that goes with the engine.

 

What's going on is that the crankshaft position sensor (CPS) reads off of windows on the flexplate (or flywheel in a manual trans application). In 1991 Jeep implemented Chrysler's OBD1 engine control system on the 4.0 in the XJ and MJ, rebranding it as "high output" (HO) as it picked up a handful of horsepower. Among the many changes from the old AMC-implemented Renix system is a different style of CPS, which reads differently from the Renix CPS, and so the arrangement of windows around the flexplate is different between the two and you'll need to use the flexplate (or flywheel) that matches your CPS and the rest of the engine control system. If you have the two flexplates side by side you'll see that the two are physically compatible (nearly identical) except for the window arrangement — the Renix has a bajillion with one slightly larger than the next, IIRC the HO has like, three.

In your case the engine control is the HO/OBD1, so you'll want the flexplate that goes with the engine control, i.e. the '91+. The used one will probably be okay, but they are a wear item and eventually will crack, so if it's got a ton of miles it might be better to replace it while you've got everything apart.

 

Also note that the CPS bolts to the bellhousing, so you'll have to pull that off the older transmission and stick it on the new trans. Unplug it before dropping the trans, but your life will be one hell of a lot easier if you don't unbolt it until you've got the trans out, and remember to transfer it to the 4x4 trans before it goes in. It takes some serious contortion to get at in with the trans in the vehicle.

There may also be some differences in the TCU control circuitry between the '90 and '91 trans. You might want to consult this thread for more info on that.

 

One more thing, unless that MJ is completely pooched, we CC members tend not to like seeing running and driving MJ's parted out. All the parts you need (except the rear driveshaft, you should be able to get the old one cut down) can be found in an XJ. If you can find one with an NP242 transfer case as well it's a pretty solid upgrade over the 231, not necessarily in terms of strength, but in terms of function in normal driving in low-traction conditions the full-time 4x4 is a hell of an improvement over the 231's lack of centre diff.

Very well said. And that was my first thought reading this, why part out an MJ for 4x4? XJs all day long.

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Wow, awesome! Thank you! Makes complete sense. And you answered my follow-up question if I’d run into any wiring/communication issues.

And yeah, I hear ya about parting out Comanches! About this ’90 MJ though: the reason I picked this one up was out of complete convenience and it only cost $360 (if that tells you anything about the condition). This truck was far from running and driving! Interior a mess, no floorboards, tons of components missing from the engine, wiring harness cut out, etc. There wasn’t a front driveshaft when I bought it but I obviously didn’t care about that, but what I didn’t notice was someone had previously stole the axel shafts out of it. So the front end was pretty useless to me at this point! The rotors were too thin to turn, the calipers were so rusted that the bleeder screw sheared off when I tried opening it… One fail after another! Long story short – the only parts I’m able to salvage off this truck is the tranny, t-case, and rear driveshaft. And it’s a complete gamble on the tranny/t-case since I have no idea the true condition (odometer reads 225K). I did end up throwing a junkyard Dana 30 from a ’97 XJ in my ’91 and that went pretty well, I’ve been driving it around like that for about a month now with no issues. But I’m looking into doing the rest of the job this upcoming week, hence the question. And just to go full circle on this, when I’m done with the ’90 MJ, I’m trailering it up to a guy in upstate NY so he can take what he needs. The box has some rot, but it’s better than anything you can find in NY!

But anyway, thanks again for the response!

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