Clunk when engaging 4wd

[DT400]

'21 F-150 XL 2.7EB
When engaging 4wd there is a clunk as soon as the truck moves.
Stopped foot on brake-> engage 4wd hi-> allow time for truck to do it's shift process and tell you it's in 4wd-> let foot off brake and gently apply throttle there is a noticeable clunk in the driveline. It only does it during initial engagement...and doesn't matter if you are going forward or back.

My '22 Lariat with the 4WD auto T-case does not do this.

Has anyone else experienced this?

Darrell

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[hotrodmex]

The the ESOF in the 21 has to move a shift fork and engage dog-teeth to lock in 4H. There are synchros, and it's meant to be done at speed, so I wonder if it would be smoother at 3-5 mph (though it should also go at much higher speeds)

The TOD in the 22 has nothing to engage, the 4H is always turning in the T-case, so it just energizes the clutch to engage. Nothing to clunk.

The 21 also as IWEs that need to engage teeth, and the 22 Lariat does not.

 

[Sneezy]

'21 F-150 XL 2.7EB
When engaging 4wd there is a clunk as soon as the truck moves.
Stopped foot on brake-> engage 4wd hi-> allow time for truck to do it's shift process and tell you it's in 4wd-> let foot off brake and gently apply throttle there is a noticeable clunk in the driveline. It only does it during initial engagement...and doesn't matter if you are going forward or back.

My '22 Lariat with the 4WD auto T-case does not do this.

Has anyone else experienced this?

Darrell

Yes mine does this as well, and it’s a really harsh clunk if I’m actually rolling down the road. My truck is only 2 months old and I just figured it’s not a smooth transition from 2H to 4H, but maybe it’s supposed to actually be smooth

Edit: My truck does not have 4A either, just the standard transfer case like in your ‘21

 

[Snakebitten]

2Hi to 4A is completely seemless

I haven't compared it to 2Hi to 4Hi, but I could see where it might be more noticeable. That's going to require a moment of sync.

 

[Sneezy]

2Hi to 4A is completely seemless

I haven't compared it to 2Hi to 4Hi, but I could see where it might be more noticeable. That's going to require a moment of sync.

Makes me wonder if all of the basic transfer cases that don’t have 4A built in as an option clunk when engaging. I think they are different internally, but I’m not sure in what ways they differ

Edit: wow I’m not having a good day apparently, if I had re-read the OP’s first post he explains that 4A transfer cases keep 4H engaged all the time and uses a clutch to engage the front end when needed. That and not having IWE’s may have something to do with it as well

 

[Snakebitten]

I might be misunderstanding...
But a 4A transfer case acts no differently than a non 4A transfer case in 4Hi. Right?

The clutch wouldn't be in play.

4A uses the clutch and there are multiple parameters that the pcm is measuring and the clutch responds by altering the % of torque that is transferred to the front differential.

Unless I got this wrong?

 

[Sneezy]

I might be misunderstanding...
But a 4A transfer case acts no differently than a non 4A transfer case in 4Hi. Right?

The clutch wouldn't be in play.

4A uses the clutch and there are multiple parameters that the pcm is measuring and the clutch responds by altering the % of torque that is transferred to the front differential.

Unless I got this wrong?

I’m not sure, when 4A is engaged can you feel drivetrain bind up when doing sharp turns? I could feel it in 4H parking lot maneuvers in the snow. My only thinking was if they were different in how they actuated (clutch in the 4A t-case and shift forks in the non-4A t-case) the PCM might allow for a nominal amount of clutch slip to keep the drivetrain from binding if one or more wheels was on a high traction surface while others were slipping. Thus essentially acting like any other AWD vehicle while it’s in 4A. Then if you manually select 4H rather than 4A it’s locked no matter what. This is totally just speculation, but would be an interesting theory to test.

 

[Snakebitten]

In 4A, which is about 50% of my winter driving because of rain, rain, rain,..... I never feel any binding, but I definitely can feel the steering weighted a bit, as well as increased feedback through the wheel.
It's really rather refined considering it's a pick-up truck.

4Hi, which is NOT recommended for non-slip conditions would probably bind, but I don't try it on the road since it's not what I believe it is intended.

My driveway is a 1/4 long gravel road. I will intentionally try out 4Hi and compare.

 

[amschind]

I just spoke with a really good Ford tech, and he wasn't aware of the 4A Hi-Lock transfer case; I am pretty sure that exists, there just aren't many of them where he is. A guy on the other forum installed the 4A hi-lock case on his XLT, so I believe that it exists but is rare.

Options:
1) ESOF: fork that locks with 2H/4H/4L
2) Standard 4A: clutch driven 2H/4A/4H/4L; CANNOT lock 4H or 4L, just uses the clutch
3) Hi-Lock 4A: Clutch driven 4A, but 4H and 4L modes lock like the ESOF.

1 is XLT/XL, 2 is Lariat -Platinum and 3 is Raptor and MAYBE Limited. Option 2 CANNOT produce a clunk because it never locks anything, it just applies as much pressure as it can on the 4A clutch.

 

[DT400]

I just spoke with a really good Ford tech, and he wasn't aware of the 4A Hi-Lock transfer case; I am pretty sure that exists, there just aren't many of them where he is. A guy on the other forum installed the 4A hi-lock case on his XLT, so I believe that it exists but is rare.

Options:
1) ESOF: fork that locks with 2H/4H/4L
2) Standard 4A: clutch driven 2H/4A/4H/4L; CANNOT lock 4H or 4L, just uses the clutch
3) Hi-Lock 4A: Clutch driven 4A, but 4H and 4L modes lock like the ESOF.

1 is XLT/XL, 2 is Lariat -Platinum and 3 is Raptor and MAYBE Limited. Option 2 CANNOT produce a clunk because it never locks anything, it just applies as much pressure as it can on the 4A clutch.

So option 2 with 4A isn't a "true" 4wd then?
Its like when people refer to a limited slip as a locker, they are similar but definitely not the same.

Darrell

 

[amschind]

Discussion Thread

That's where I'm getting a lot of my info from.

 

[hotrodmex]

1 is XLT/XL, 2 is Lariat -Platinum Limited plus Tremor 400-401A and 3 is Raptor and MAYBE Limited Tremor 402A.

FTFY, for the GEN14.

IIRC GEN13 Limiteds did get the Hi-lock.

So option 2 with 4A isn't a "true" 4wd then?
Its like when people refer to a limited slip as a locker, they are similar but definitely not the same.

In this case the clutch pack can provide complete lock up with the electromagnetic clutch assisted by ball-ramp plates. So it doesn't lock with dog teeth, but it's nowhere near a limited slip diff in locking limitation.

 

[amschind]

FTFY, for the GEN14.

IIRC GEN13 Limiteds did get the Hi-lock.



In this case the clutch pack can provide complete lock up with the electromagnetic clutch assisted by ball-ramp plates. So it doesn't lock with dog teeth, but it's nowhere near a limited slip diff in locking limitation.

Thanks!

Just so that it's in easily searchable format for posterity:
2H/4H/4L Lockup: XL-XLT
2H/4A/4H/4L with magnetic clutch is Lariat-Limited
2H/4A/4H/4L with gear/chain lockup is Raptor and 402A Tremor

 

[John861]

The clunk from 2H to 4H on the ESOF is typically just the splines (really more of a dog clutch) finally falling into mesh. When stopped, the shift motor attempts to move the shift fork, but if the teeth aren't lined up, the shift fork stops and the shift motor compresses a spring until the motor reaches the desired position. Once the truck moves, the minor differences in rotation of the front and rear drivelines allow the teeth to align, and it drops into place with the force of the spring mentioned earlier. In general, it is smoother to shift to 4H at a roll.

Another reason for the clunking can be if the truck is shifted at speed and the synchronizer doesn't get the front driveline up to speed fast enough. In most cases this will cause the IWE to ratchet a bit, but I suppose it could also clunk. The last I knew, the IWE ingagement was time based (the controller assumes the driveline will be synchronized within x seconds of the transfer case reaching the 4H position), not based on any particular feedback that it is indeed synchronized.


As noted, 4A is completely different. . In general, the TOD systems (Torque On Demand, Borg Warner's trademark name for the system) have the front axle connected at the hub all the time, so the driveline is always synchronized. The clutch in the transfer case can then modulate the friction between the front and rear drivelines to manage torque, or in the case of 4H mode, pretty much keep the clutch engaged all the time. But more to the point, with the front axle engaged all the time, there is nothing to synchronize so no clunk. And no spines to get aligned, so again no clunk. But of course, there are trade-offs, and you can get clunks from the ball ramps in the clutch and other stuff.

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