Regear rear axle only?

[Chocolate Thunder]

Yes, I know that when used in 4WD the Jeep MUST have the same gearing on both axles or tragedy will occur in short order. However…

If not driven in 4WD at all, is there any problem with changing the gear ratio only on the rear axle? The reason I ask is because I don’t wheel every weekend, or even every month like many if you do. If I can regear the rear axle to 4.88 now, then follow suit with the front axle later, will it make any difference so long as I don’t shift into 4WD until the front has the new gears?

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[JimLee]

Yes, I know that when used in 4WD the Jeep MUST have the same gearing on both axles or tragedy will occur in short order. However…

If not driven in 4WD at all, is there any problem with changing the gear ratio only on the rear axle? The reason I ask is because I don’t wheel every weekend, or even every month like many if you do. If I can regear the rear axle to 4.88 now, then follow suit with the front axle later, will it make any difference so long as I don’t shift into 4WD until the front has the new gears?

Shouldn't affect a thing. I would personally pull the front drive shaft just to make sure.

 

[bjm00se]

As long as you have the Front Axle Disconnect, and 4WD isn't engaged, the front diff won't spin.

If it don't spin, there's no issue.

But, catastrophe will ensue if you ever move that 4wd lever. There's a snowstorm and you forget, or you lend the Jeep to someone who can't resist a nearby field, or just wants to "try it out"

It just seems like it's asking for trouble to me, to leave a user selectable lever in the cockpit that, if ever engaged for any reason, would cause catastrophic driveline damage.

I think if you were to go through with this, the move would be to remove the front driveshaft completely, until the front diff is re-geared.

 

[grimmjeeper]

Why not just save up and do both at the same time?

 

[Chocolate Thunder]

Thanks all. As I thought.

 

[fourseasons1621]

Generally you’ll save some labor having both done at the same time too. Many 4WD shops have “Gear packages”

 

[Vinman]

I would pull the front driveshaft out.

A couple of thoughts come to mind, first is having 2 different ratios effect how the wheel speed sensors communicate with the ABS or Traction Control modules and second, if you don’t remove the front driveshaft would there be any negative effect on the transfer case having the front and rear outputs turning at different speeds?

 

[Chocolate Thunder]

I would pull the front driveshaft out.

A couple of thoughts come to mind, first is having 2 different ratios effect how the wheel speed sensors communicate with the ABS or Traction Control modules and second, if you don’t remove the front driveshaft would there be any negative effect on the transfer case having the front and rear outputs turning at different speeds?

The potential transfer case issue makes sense, thanks for that input. I hadn’t thought about that. Regarding ABS and TC modules and sensors, are you suggesting that different gear ratios front and rear might throw off their algorithm and that normal forward motion would appear to the system as spin on one axle because they’d calculate that the wheels on that axle are turning faster or slower than the other because they factor gearing to be the same in both axles?

 

[grimmjeeper]

I would pull the front driveshaft out.

A couple of thoughts come to mind, first is having 2 different ratios effect how the wheel speed sensors communicate with the ABS or Traction Control modules and second, if you don’t remove the front driveshaft would there be any negative effect on the transfer case having the front and rear outputs turning at different speeds?

Gears have no effect on wheel speed sensors. The wheels don't turn faster or slower when you change gears. The driveshafts change speed but there are no driveshaft speed sensors in the Jeep.

The computer uses the wheel speed, tires, axle gears, and low range to figure out when to shift the transmission. But if the Jeep doesn't shift into 4wd (hi or low) the front axle ratio makes no difference there, especially with FAD where the driveshaft isn't even spinning.

 

[D60]

I can't think of any way the vehicle has to measure actual driveshaft speed.

If there were a VSS in the t-case one could argue that allows the vehicle to see REAR d/s speed, but I've never seen a t-case with TWO VSS's, ie one for each output (rear and front). Engagement of even AWD is typically determined by WSS's at each corner....

Plus, I'm guessing the JL gets its vehicle speed info from a tone ring in the diff? This I should know but I don't......

Either way it hardly matters because as long as you're in 2wd and have a functioning FAD the front d/s won't necessarily spin at all, or it might spin intermittently but even then I doubt it spins "full speed"

 

[mgroeger]

Following.
I think it's a great idea. Please leave front driveshaft in place as I'm curious to see the pics when you you forget and throw it into 4WD.

 

[grimmjeeper]

I can't think of any way the vehicle has to measure actual driveshaft speed.

If there were a VSS in the t-case one could argue that allows the vehicle to see REAR d/s speed, but I've never seen a t-case with TWO VSS's, ie one for each output (rear and front). Engagement of even AWD is typically determined by WSS's at each corner....

Plus, I'm guessing the JL gets its vehicle speed info from a tone ring in the diff? This I should know but I don't......

Either way it hardly matters because as long as you're in 2wd and have a functioning FAD the front d/s won't necessarily spin at all, or it might spin intermittently but even then I doubt it spins "full speed"

The wheel speed tone rings and sensors are built in to the unit bearing assemblies in the front, next to the wheel mounting flange.

On the rear, the rings are on the axle shaft outside the bearing and seal, right next to the wheel mounting flange.

From there, it's just arithmetic to calculate what the relative speeds are.

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