FAD Missing

[heretic]

my 24 Rubicon X has no FAD actuator, was this the first year they started doing this?
It has the rock-trac 4W Hi Auto.
I have not pulled the plate off to see if the factory uses a straight shaft, I may next oil change...LOL
Is 4W Hi Auto done with clutches or gears in the T-case?

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[Janster]

Is 4W Hi Auto done with clutches or gears in the T-case?

Clutches…. In the tcase

 

[heretic]

Janster...thank you
so when in 4 Auto if you have a FAD it locks the axels together?
And when in 2 High the collar is "unlocked"
I ask B/C I didn't see any sync grears like in a reg transmission.

 

[JTdiRtyD]

Janster...thank you
so when in 4 Auto if you have a FAD it locks the axels together?
And when in 2 High the collar is "unlocked"
I ask B/C I didn't see any sync grears like in a reg transmission.

FAD was removed in 2024, which is also the first year X models were available with 4-Auto. So there is no FAD and 4-Auto combination.

But yes, for those of us with a FAD when you engage 4HI a collar is moved that locks both axle halves together.

 

[wuykats]

Didn't Jeep, many years ago, have a similar FAD? It was vacuum operated and did not work out well if I recall correctly. So the purpose of the FAD is to theoretically increase gas mileage? But it forces the spider gears to spin much faster? And would not locking hubs accomplish this free-wheeling in a better manner? I gues I don't understand the concept of FAD.

 

[JTdiRtyD]

Didn't Jeep, many years ago, have a similar FAD? It was vacuum operated and did not work out well if I recall correctly. So the purpose of the FAD is to theoretically increase gas mileage? But it forces the spider gears to spin much faster? And would not locking hubs accomplish this free-wheeling in a better manner? I gues I don't understand the concept of FAD.

On-the-fly 4x4 engagement. Locking hubs require you to stop and get out of the vehicle, a FAD does not.

How would a FAD force spider gears to spin faster?

 

[Janster]

FAD was removed in 2024, which is also the first year X models were available with 4-Auto. So there is no FAD and 4-Auto combination.

But yes, for those of us with a FAD when you engage 4HI a collar is moved that locks both axle halves together.

I have a 2024 Mojave X…..
Just crawled underneath to take a photo… If that’s not a FAD, then what’s in the box?

 

[heretic]

the actuator is missing, its the motor that moves a fork back and forth to lock the passenger axle together. I still don't know if jeep uses a solid axel or just made the fork so it can't move out of its locked position. Additional you don't see any wires going to the "box".

 

[AustyPosty]

I have a 2024 Mojave X…..
Just crawled underneath to take a photo… If that’s not a FAD, then what’s in the box?

Looks like a block off plate to me. Nothing in there, just the axle shafts passing through the tube.

 

[AustyPosty]

Didn't Jeep, many years ago, have a similar FAD? It was vacuum operated and did not work out well if I recall correctly. So the purpose of the FAD is to theoretically increase gas mileage? But it forces the spider gears to spin much faster? And would not locking hubs accomplish this free-wheeling in a better manner? I gues I don't understand the concept of FAD.

It doesn't just help increase gas mileage(which is obviously debatable if it even does), it also is incredibly beneficial for lifted Jeeps. Jeeps with the FAD can run much higher caster angle enabling better steering while lifted, this is because the FAD allows the front driveshaft to NOT spin in 2WD. So when you hit freeway speeds you won't get driveshaft vibrations. Even when you shift into 4WD and the front driveshaft is spinning, it would be pretty rare you'd be going at a fast enough speed to see any vibrations.

The inverse is usually true with a lifted Jeep without FAD, it may be difficult to run higher caster angles since the front shaft is always spinning, it may vibrate at highway speeds. This means you may have to sacrifice lower caster angles for a front driveshaft that doesn't vibrate. Aftermarket driveshafts may help with this, but there's usually always some degree of sacrifice between running the caster angle you want for driveability and keeping the driveshaft angle good with a Non-FAD jeep.

 

[heretic]

I'm confused...how does FAD control or help with caster? that's done with the control arms right?
I understand the vibration thing, but still caster is rotation of the axle for steering influence. Right?

 

[AustyPosty]

I'm confused...how does FAD control or help with caster? that's done with the control arms right?
I understand the vibration thing, but still caster is rotation of the axle for steering influence. Right?

Caster angle is directly related to the pinion angle of your front driveshaft, the more caster you run, the more severe the pinion angle will be, a severe pinion angle can cause vibrations. With the FAD you don't need to worry about the pinion angle as much because the front driveshaft isn't spinning in 2wd, thus you can generally run the caster angle you want (such as 6+ degrees) for driveability without worrying about vibrations from the driveshaft caused by high caster angles (Within reason). Obviously the pinion angle still matters even with the FAD, however, it is more forgiving compared to a non-FAD Jeep when it comes to the balance between caster angle and pinion angle. Aftermarket front driveshafts (such as a double cardan shaft) help with lifted non-FAD Jeeps to reduce vibrations when running higher caster, but there still needs to be a balance with the caster angle and pinion angle to ensure you don't get vibrations.

 

[DylanM]

On-the-fly 4x4 engagement. Locking hubs require you to stop and get out of the vehicle, a FAD does not.

How would a FAD force spider gears to spin faster?

With locking hubs disengaged neither the front axle shafts, differential, spider gears, nor front driveshaft are spinning when moving in two wheel drive. With a FAD delete, the front driveshaft, axle shafts, and differential all spin at wheel speed when driving and the spider gears only do so when there is a difference in axle shaft speed between the two sides, such as when turning.

With the FAD setup in our Jeeps, while driving in two wheel drive you have the left axle shaft and right outer axle shaft spinning at wheel speed going forward, the driveshaft and differential not spinning, and the right inner axle shaft spinning at wheel speed backwards via the action of the spider gears. This means that at all times when driving with the FAD disconnected the spider gears are spinning in the front differential, instead of only when going around a corner as is their intended design function. When you shift into 4HI on the fly while driving, the front driveshaft and differential have to suddenly spin up to match wheel speed, and the right inner axle shaft has to completely reverse direction and get up to wheel speed going forwards so the FAD can lock the two halves of the right axle shaft together.

 

[JT1]

I'm confused...how does FAD control or help with caster? that's done with the control arms right?
I understand the vibration thing, but still caster is rotation of the axle for steering influence. Right?

With no fad the front driveshaft spins all the time, like a JK

Because the front shaft is spinning at highway speeds, you have to balance enough caster for good road manners with low driveshaft angles to minimize vibrations. The higher you lift, the worse the problem gets.

 

[wuykats]

On-the-fly 4x4 engagement. Locking hubs require you to stop and get out of the vehicle, a FAD does not.

How would a FAD force spider gears to spin faster?

On-the-fly 4x4 engagement. Locking hubs require you to stop and get out of the vehicle, a FAD does not.

How would a FAD force spider gears to spin faster?

"With the FAD setup in our Jeeps, while driving in two wheel drive you have the left axle shaft and right outer axle shaft spinning at wheel speed going forward, the driveshaft and differential not spinning, and the right inner axle shaft spinning at wheel speed backwards via the action of the spider gears. This means that at all times when driving with the FAD disconnected the spider gears are spinning in the front differential, instead of only when going around a corner as is their intended design function. When you shift into 4HI on the fly while driving, the front driveshaft and differential have to suddenly spin up to match wheel speed, and the right inner axle shaft has to completely reverse direction and get up to wheel speed going forwards so the FAD can lock the two halves of the right axle shaft together. "

So it seems like the spider gears which are designed to not spin very quickly, just enough to allow the front 2 wheels to turn at slighlty different speeds when turning, are spinning at much higher speeds when the FAD is disconnected. I would think that oil drag in the diff would also cause to driveshaft to turn, albeit at a much slower speed than if everything were connected. I would prefer locking hubs over this design, we have to stop and air down when going wheeling, so locking the hubs would just be a few seconds more time when preparing for the trail. I think FAD is a similar design to the vacuum actuated disconnect Jeep used years ago, and from what I have read, that design was not very successful.

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