Front Diff Grenaded

[Brian_B]

I just grocked what you said here - is this right, the FAD system on the Bronco only disengages (i.e. the wheel/axle from the FDU) on one side? This would mean that one side (whichever side the FAD is on) is getting a touch less resistance then the other, which effects steering and alignment, at least theoretically...maybe the difference is not enough to matter practically, but then that would beg the quesiton if the FAD itself has any material effect at all - maybe it is all for show for the EPA??

Here's the breakdown of a M190 - it only disengages passenger side. FAD is #28

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[crenca]

lateral loads quickly increase when CV angles increase (lift). OEM SAS CV angles are near zero at oem ride height, hence very little lateral loading. No one wants to hear this but it is simple geometry. Better lateral support is required as CV angles increase.

What do you think of the idea that with no practical hub disconnect solution (FAD is not it - the other side is always spinning the FDU internals) there is some sort of stress/wear the FDU is not designed for and thus users have to limit 4A/4H/4L miles to some as of yet unstated number?

I'm thinking that Ford would not intentionally go with such a design/implementation - it is not like they don't have extensive 4x4 and AWD design/implementation experience. This leads us back to manufacturing defects being the most likely cause.

 

[RIDE]

Here's the breakdown of a M190 - it only disengages passenger side. FAD is #28

Okay. That diagram is a huge help!! So, two questions:

1) am I right that by disengaging one sideaxle of the diff only would have the engaged axle side and hub free spin inside the diff, but would not spin the diff itself or the driveshaft going back to the xfer? (Say, in 2H)

2)does activating 4A (4H&4L too) actuate that solenoid (#28 in diagram) to then actuate both hubs and differential and the front drive shaft back to the xfer?

edited for clarity again. Am failing horribly on initial posts.

 

[87-Z28]

What do you think of the idea that with no practical hub disconnect solution (FAD is not it - the other side is always spinning the FDU internals) there is some sort of stress/wear the FDU is not designed for and thus users have to limit 4A/4H/4L miles to some as of yet unstated number?

if the CV axles remain perfectly horizontal then they don’t need much lateral support. The FDU is probably designed well enough to handle this scenario, with or without a hub disconnect.

we like to add large tires and induce significant CV angles. Loads into FDU just go up. Not bashing ford just stating that better lateral support is required when outside the design envelope.

unlikely, bigger tires are usually the culprit

The issue is the the cv axle stubs shafts are poorly supported. This causes the seals to wear out and put stress on the internal spider gears. Couple this with no front hub disconnects you get substantial wear as seen in the metal found during oil changes.

This can’t be overstated.

 

[Brian_B]

Okay. That diagram is a huge help!! So, two questions:

1) am I right that by disengaging one axle of the diff only would have the engaged axle side and hub free spin inside the diff, but would not spin the diff itself or the driveshaft going back to the xfer? (Say, in 2H)

2)does activating 4A (4H&4L too) actuate that solenoid (#28 in diagram) to then actuate both hubs and differential and the front drive shaft back to the xfer?

1) yup

2) I don’t think any packages have both 4A and FAD, but for 4H/L, yes, tcase engages and FAD engages to lock in the front drive

 

[Ducati1098]

I will have to test this out. I recall that with almost any throttle input over about 40mph no torque is sent forward (as indicated by the gauge), but perhaps I was not mashing it down enough

Yeah definitely takes more acceleration then lower gears, but still doesn’t take too much for it to send it to the front.

 

[RIDE]

1) yup

2) I don’t think any packages have both 4A and FAD, but for 4H/L, yes, tcase engages and FAD engages to lock in the front drive

Right. But theoretically 4A would have to engage that solenoid also since without it, it couldn’t just engage at speed once sensors picked up on wheel slippage, yes?

You guys might see where I’m gong with this but my suspicion 4A may play a part in early wear is b/c it keeps the front diff engaged where (2H doesn’t). If the front diff isn’t turning, there’s no wear, thus no reason for it to wear out.

No way do I think it’s ok that it’s not designed to hold up to that use pattern but I’m also not gonna be surprised at all if Ford finally comes out with a recommendation on how, where, and when to use 4A.

 

[Brian_B]

Yeah definitely takes more acceleration then lower gears, but still doesn’t take too much for it to send it to the front.

Back to the future!

 

[Brian_B]

Right. But theoretically 4A would have to engage that solenoid also since without it, it couldn’t just engage at speed once sensors picked up on wheel slippage, yes?

It would - then you would be more or less in the same boat as the non-FAD or m210 options though, and they run that way all the time, even in 2H

 

[BigMeatsBronco]

Right. But theoretically 4A would have to engage that solenoid also since without it, it couldn’t just engage at speed once sensors picked up on wheel slippage, yes?

You guys might see where I’m gong with this but my suspicion 4A may play a part in early wear is b/c it keeps the front diff engaged where (2H doesn’t). If the front diff isn’t turning, there’s no wear, thus no reason for it to wear out.

No way do I think it’s ok that it’s not designed to hold up to that use pattern but I’m also not gonna be surprised at all if Ford finally comes out with a recommendation on how, where, and when to use 4A.

Naw, turning of the parts doesn't put enough load on them to cause any significant wear...for example my Carrier berrings only show wear on the race that is on the thrust side of the case. I run in 2WD almost exclusively, So it wearing out from too much side thrust and possibly case stretch causing ring gear deflection. A heavy foot and big tires pushes the parts to their limits, or beyond.

 

[crenca]

You guys might see where I’m gong with this but my suspicion 4A may play a part in early wear is b/c it keeps the front diff engaged where (2H doesn’t). If the front diff isn’t turning, there’s no wear, thus no reason for it to wear out.

In trims with 4A, the front diff is always turning - in 2H,4A, 4H, 4L.

In models without 4A, The front diff is always turning because FAD only disconnects one side (in 2H), so the other axle is always spinning the internals of the diff.

So no matter what mode is selected, no matter what the transfer case is selected to, no matter the trim, and all 6g Broncos the internals of the front diff are always spinning if the car is moving

Unless someone corrects me but I think this is right.

 

[mpeugeot]

"Excessive" 4A use doesn't cause the front differential to fail. If that was true, my front diff would have been toast by now!

Or maybe I am just lucky at 105k miles of 4A sport mode daily driving. There has been 1 day that I haven't run sport mode 4A, that was 19 November 2021 when I drove it home from the dealership 2.4 miles.

I have used other modes off-roading, mostly Baja 4H or 4L, but to get there and back, 4A Sport Mode.

 

[xtreme_exploder]

In trims with 4A, the front diff is always turning - in 2H,4A, 4H, 4L.

In models without 4A, The front diff is always turning because FAD only disconnects one side (in 2H), so the other axle is always spinning the internals of the diff.

So no matter what mode is selected, no matter what the transfer case is selected to, no matter the trim, and all 6g Broncos the internals of the front diff are always spinning if the car is moving

Unless someone corrects me but I think this is right.

You're kind of right. The detail difference is that the FAD models only spin the spider gears 100% of the time (when the FAD is in disconnect mode) but the rest of the differential is not moving (ring and pinion, driveshaft). In 4A models, the spider gears aren't spinning but everything else is. In all models, the CV axles are rotating so they will all put roughly equal wear on output seals and axle support bearings. So all Bronco trims wear on the CV axles the same. FAD differentials will wear the spider gears more, and 4A differentials wear everything else from the ring/pinion gears and bearings and seal to the driveshaft going into the transfer case more (everything except the spider gears basically). The design of the FAD axle is exactly the same as basically all other 4wd vehicles that don't use locking hubs instead.

There is no reason for any of these differentials to fail without a maintenance, manufacturing defect, or design flaw. Unless you drive around in 4H or 4L on dry pavement all the time, that's the only time you may have a problem.

 

[NC_Oak]

Oh, when that guy drained the oil onto the engine plate, I had JUST TOLD HIM to remove the plate and how easy it was... Like literally standing there, when he was underneath. Nod, nod, "Ok, sure", go back inside and .

That was actually beat by that one time the same place used a ladder to take the spare off while it was jacked up off the ground. I seriously thought someone was going to get hurt that time.

No doubt you no longer use that service center...

 

[harpo]

I wish I had paid more attention to the FDU at my 5k oil change. I know I looked at it, but I honestly can't remember exactly what I saw, just the thought "it's fine" . I am planning on a remote wheeling trip tomorrow, now thinking I better get the skid off and do a thorough inspection today...

Maybe drain/inspection/fill on these diffs every 10k is not overkill after all.



As I understand it your right, no hub disconnect on trims with 4A so FDU is 'free wheeling' the whole time, but that is a separate issue from whether 4A is sending torque or not to front axle - unless someone wants to explain different



I will have to test this out. I recall that with almost any throttle input over about 40mph no torque is sent forward (as indicated by the gauge), but perhaps I was not mashing it down enough

I suspect that if your diff was leaking you would have noticed when you changed the oil. I'm not a car guy but I do change my oil because it's easy, I'm cheap and retired.
Mine started leaking somewhere between about 16000 and 24000. It wasn't much of a leak and I still noticed. It has gotten worse and about 2 weeks back I started noticing a burning smell, probably the gear oil burning on the exhaust.

I'm planning on taking it in but need it for a trip this week.

Checked the diff oil yesterday and it was low but didn't seem to take much to fill it up.

The drip quantity and staining was way bigger then when I changed the oil about 3000 miles back.

fyi I hardly ever use 4a or 4h, and only a few off-road excursions. stock 7mt badlands.

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