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Discussion Starter · #1 ·
Came across some interesting info regarding final drive ratio and some discrepancies as reported by AlfaOBD. Both my 2014 ADII Limited V6 and my 2016 Trailhawk V6 have the tow package. According to Jeep Tow Package Cherokees get 3.51 final drive ration gears and the non tow package cherokees get 3.25. AlfaOBD says otherwise. It would appear both my vehicles have 3.25 gearing despite having the tow package. I just need access to a non tow package cherokee to confirm. I'm inclined to believe this is correct as all the other info being reported regarding the drivetrain is correct. More food for thought

2016 TH



2014 ADII V6

 

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Discussion Starter · #4 ·
Hmmm, so is that a miss by Jeep or a miss with the AlfaOBD software? How do you confirm independently?
I'll do some logging and use some online calculators to figure out if its accurate or not. Everything else in AlfaOBD has been accurate. Its got the proper low range gearing (2.92) now I just need a non tow package vehicle to play around with.
 

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Old school easy way
Computers only know what they are told "PUT GARBAGE IN GET GARBAGE OUT"
Same applies to front wheels


Step 1

Slide a floor jack under the rear of the vehicle and raise it until the rear wheels are off the ground. Place jack stands under the rear axle on both sides of the vehicle. Lower the jack so the weight of the rear of the vehicle is supported by the jack stands.

Step 2

Crawl under the vehicle and locate the drive shaft where it attaches to the front of the rear axle. Place a 6 to 8 inch length of masking tape longitudinally on the drive shaft where it will be easily visible from the side of the vehicle near one of the rear wheels.

Step 3

Place a piece of masking tape on the edge of one of the rear tires.

Step 4

Rotate the tire by hand one complete revolution using the piece of masking tape on the tire as a guide.

Step 5

Count the number of revolutions the drive shaft turns (using the masking tape attached to it as a guide) to one tire revolution. For instance, if the drive shaft rotates three and a quarter turns to one tire rotation, then your rear axle gear ratio is 3.25:1.

Remove the masking tape from the drive shaft and tire. Raise the rear of the vehicle with the floor jack and remove the jack stands. Lower the floor jack until the rear wheels are on the ground.

Tip

You can pinpoint the exact rear axle gear ratio of your vehicle by finding out what axle ratios were available for your particular vehicle from the owner's manual or a repair manual. Using the example in the above procedure, if ratios 2.80:1., 3.28:1 and 4:11.1 were available for your vehicle, you can determine that the 3.28:1 ratio is the one closest to your calculations.
Items you will need

Floor jack
Jack stands (2)
Masking tape
 

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Old school easy way
Computers only know what they are told "PUT GARBAGE IN GET GARBAGE OUT"
Same applies to front wheels


Step 1

Slide a floor jack under the rear of the vehicle and raise it until the rear wheels are off the ground. Place jack stands under the rear axle on both sides of the vehicle. Lower the jack so the weight of the rear of the vehicle is supported by the jack stands.
That only works if you have a limited slip differential. If you have an open differential (like in the KL) you need to keep one wheel from moving, otherwise the opposite wheel will just turn the other direction and the drive shaft stays put. So, as above, but with only one side jacked up.
 

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Discussion Starter · #9 ·
Old school easy way
Computers only know what they are told "PUT GARBAGE IN GET GARBAGE OUT"
Same applies to front wheels


Step 1

Slide a floor jack under the rear of the vehicle and raise it until the rear wheels are off the ground. Place jack stands under the rear axle on both sides of the vehicle. Lower the jack so the weight of the rear of the vehicle is supported by the jack stands.

Step 2

Crawl under the vehicle and locate the drive shaft where it attaches to the front of the rear axle. Place a 6 to 8 inch length of masking tape longitudinally on the drive shaft where it will be easily visible from the side of the vehicle near one of the rear wheels.

Step 3

Place a piece of masking tape on the edge of one of the rear tires.

Step 4

Rotate the tire by hand one complete revolution using the piece of masking tape on the tire as a guide.

Step 5

Count the number of revolutions the drive shaft turns (using the masking tape attached to it as a guide) to one tire revolution. For instance, if the drive shaft rotates three and a quarter turns to one tire rotation, then your rear axle gear ratio is 3.25:1.

Remove the masking tape from the drive shaft and tire. Raise the rear of the vehicle with the floor jack and remove the jack stands. Lower the floor jack until the rear wheels are on the ground.

Tip

You can pinpoint the exact rear axle gear ratio of your vehicle by finding out what axle ratios were available for your particular vehicle from the owner's manual or a repair manual. Using the example in the above procedure, if ratios 2.80:1., 3.28:1 and 4:11.1 were available for your vehicle, you can determine that the 3.28:1 ratio is the one closest to your calculations.
Items you will need

Floor jack
Jack stands (2)
Masking tape



The other issue you have is.............

You will have to ensure the PTU is engaged so that the driveshaft is actually connected to the RDM and that the RDM is engaged to allow the driveshaft to spin the rear tires. Its not like a direct connection like most vehicles.
 

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The other issue you have is.............

You will have to ensure the PTU is engaged so that the driveshaft is actually connected to the RDM and that the RDM is engaged to allow the driveshaft to spin the rear tires. Its not like a direct connection like most vehicles.[/QUOTE]


Ok, block the front tires, start the Cherokee, put in Sport or snow mode, raise one rear tire put in nuetral , spin rear tire watch driveshaft rotations
 

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Discussion Starter · #11 ·
When you put the vehicle in sport mode or any other mode it will not engage the PTU/RDM until you are in drive........so nearly impossible to use this method.
 

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The other issue you have is.............

You will have to ensure the PTU is engaged so that the driveshaft is actually connected to the RDM and that the RDM is engaged to allow the driveshaft to spin the rear tires. Its not like a direct connection like most vehicles.

Ok, block the front tires, start the Cherokee, put in Sport or snow mode, raise one rear tire put in nuetral , spin rear tire watch driveshaft rotations[/QUOTE]


None of that has a chance of telling you the final drive ratio anyway. The final drive gears (the ones that give the 3.2 or 3.5 ratio) are in the transmission case and take effect before the PTU/RDU. Yes the RDU does have its own gear reduction ratio, but it is only there to compensate for the overdrive gear ratio in the PTU. Meaning the PTU has gears inside to spin the rear drive shaft at a higher RPM than the wheels on the road (enables the use of a lighter cheaper driveshaft) so the RDU has to gear that RPM back down to road speed. For example the PTU might have a 1:2 overdrive (doubles the driveshaft RPM vs wheel RPM) so the RDU would have to have a 2:1 final underdrive ratio to match.


Anyway... 5th gear in the transmission is a 1:1 ratio. So get in 5th gear on the highway... at 3000 RPM in 5th you should be going about 73 with a 3.5 final drive, and just about 80 with a 3.2 final drive (based on the 705 revolutions per mile of a stock Trailhawk tire). 73 is spot on in my TH with the tow package, so it's got a 3.5 final drive. Adjust your math as needed for a different tire or if you're trying to verify a 4 cyl final drive like 4.whatever:1.
 

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Discussion Starter · #13 ·
Ok, block the front tires, start the Cherokee, put in Sport or snow mode, raise one rear tire put in nuetral , spin rear tire watch driveshaft rotations

None of that has a chance of telling you the final drive ratio anyway. The final drive gears (the ones that give the 3.2 or 3.5 ratio) are in the transmission case and take effect before the PTU/RDU. Yes the RDU does have its own gear reduction ratio, but it is only there to compensate for the overdrive gear ratio in the PTU. Meaning the PTU has gears inside to spin the rear drive shaft at a higher RPM than the wheels on the road (enables the use of a lighter cheaper driveshaft) so the RDU has to gear that RPM back down to road speed. For example the PTU might have a 1:2 overdrive (doubles the driveshaft RPM vs wheel RPM) so the RDU would have to have a 2:1 final underdrive ratio to match.


Anyway... 5th gear in the transmission is a 1:1 ratio. So get in 5th gear on the highway... at 3000 RPM in 5th you should be going about 73 with a 3.5 final drive, and just about 80 with a 3.2 final drive (based on the 705 revolutions per mile of a stock Trailhawk tire). 73 is spot on in my TH with the tow package, so it's got a 3.5 final drive. Adjust your math as needed for a different tire or if you're trying to verify a 4 cyl final drive like 4.whatever:1.[/QUOTE]


The RDM has gears to match the final drive in the transmission, that is why you can't just toss an RDM in any vehicle, its needs to match what was in there before.

I can't put an RDM from a 2.4L trailhawk in my cherokee because it has 4.08 gears vs the 3.51 gears in my cherokee.

The RDM in each cherokee will have the same final drive as the ones in the transmission
 

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....


The RDM has gears to match the final drive in the transmission, that is why you can't just toss an RDM in any vehicle, its needs to match what was in there before.

I can't put an RDM from a 2.4L trailhawk in my cherokee because it has 4.08 gears vs the 3.51 gears in my cherokee.

The RDM in each cherokee will have the same final drive as the ones in the transmission

I see the same OE part numbers listed for RDUs across 4cyl and V6. The differences appear to only be for AD1 / ADII / ADL which makes sense.



Here's a silly test: Put a big zip tie on your driveshaft, put the car in some mode that isn't auto to ensure the driveshaft is engaged, put it in drive, and creep forward exactly one revolution of the tires. I'd be shocked if that zip tie flips around 3.5 times. Perhaps I'll do this on mine.



To have a 3.5 gearset in the RDU for the rear axle and 3.5 in the transmission for the front axle, the transmission would have to have 2 outputs... one going through the 3.5 gears for the front, and an unreduced output to feed the PTU/driveshaft. Technically doable but that would be stupid complicated. Having the final drive be in one place (transmission) means they only have to make/stock 3 RDUs (AD1 / AD2 / ADL) and 2 PTUs (AD1 single speed and AD2/ADL 2 speed). Otherwise they'd have to stock who knows how many RDUs to account for the 4 final drive ratios AND AD1/AD2/ADL, on top of already having to stock 4 different transmissions to cover tow/non-tow ratios for 4 and 6 cyl.
 

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Discussion Starter · #15 ·
I see the same OE part numbers listed for RDUs across 4cyl and V6. The differences appear to only be for AD1 / ADII / ADL which makes sense.



Here's a silly test: Put a big zip tie on your driveshaft, put the car in some mode that isn't auto to ensure the driveshaft is engaged, put it in drive, and creep forward exactly one revolution of the tires. I'd be shocked if that zip tie flips around 3.5 times. Perhaps I'll do this on mine.



To have a 3.5 gearset in the RDU for the rear axle and 3.5 in the transmission for the front axle, the transmission would have to have 2 outputs... one going through the 3.5 gears for the front, and an unreduced output to feed the PTU/driveshaft. Technically doable but that would be stupid complicated. Having the final drive be in one place (transmission) means they only have to make/stock 3 RDUs (AD1 / AD2 / ADL) and 2 PTUs (AD1 single speed and AD2/ADL 2 speed).
Well i have something sort of like that....

But it was testing something other than we were talking about



Drive shaft appears to spin the same regardless of selec terrain mode.
 

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Discussion Starter · #17 ·
To have a 3.5 gearset in the RDU for the rear axle and 3.5 in the transmission for the front axle, the transmission would have to have 2 outputs... one going through the 3.5 gears for the front, and an unreduced output to feed the PTU/driveshaft. Technically doable but that would be stupid complicated. Having the final drive be in one place (transmission) means they only have to make/stock 3 RDUs (AD1 / AD2 / ADL) and 2 PTUs (AD1 single speed and AD2/ADL 2 speed).
I just went to a bunch of different online parts places and punched in PTU and RDMs for 2016 Cherokee with both the 2.4 and 3.2 engines, You are correct the same part number comes up. You sir just cost me some more money! If its just a trans swap for 4.08 gears that is something i'm probably going to do!
 

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I just went to a bunch of different online parts places and punched in PTU and RDMs for 2016 Cherokee with both the 2.4 and 3.2 engines, You are correct the same part number comes up. You sir just cost me some more money! If its just a trans swap for 4.08 gears that is something i'm probably going to do!

That sounds ambitious. I just did the ziptie thing... it's roughly 2.3 turns of the driveshaft for 1 turn of the wheels. I didn't have an observer, just me inching forward then getting out to check over and over so I didn't get an exact number.
 

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If you ever get your hands on a non Tow Group KL, just remember that 2017+ (IIRC) THs and ADIIs all have 3.5 final gearing, Tow or no Tow.

An easy way to tell (??) : get your RPM at a specific highway speed, in a specific gear, with a 3.2, and compare with others here. Non Tow should preferrably be non TH, non ADII.
 

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Discussion Starter · #20 ·
That sounds ambitious. I just did the ziptie thing... it's roughly 2.3 turns of the driveshaft for 1 turn of the wheels. I didn't have an observer, just me inching forward then getting out to check over and over so I didn't get an exact number.

A while back I did some testing to see if sport mode send more power to the rear (it doesn't as subsequent testing shows it doesn't but I made this video to see if for some reason the backs were spinning faster than the fronts)


Red wheel paint makes it easy to count the wheel revs,
 
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