Question about 4WD Auto characteristics

[Lghtspeedz]

I have a 2020 Limited 4WD with the Heavy Duty Tow Package. It’s been cold and rainy here, so in some conditions traction is not as good. I don’t yet have winter tires on, so I’ve been utilizing 4A mode for extra grip I’m a few situations. Is it normal for the driveline to feel a little more coarse when accelerating in 4A mode? I’m not accelerating aggressively, but I can feel a little roughness through the engine and steering wheel. My previous vehicles were AWD, so it was pretty seemless. My one experience with 4WD is from an old ‘98 Explorer. I seem to recall a similar coarseness from that too but I guess I expected that to be refined out. Is this slight sensation a little normal?

Thanks all!

 

[BMW2FORD]

I do feel a difference in the return of the steering wheel on tight turns when accelerating (torque steer) and hear a little but more noise in 4wd. There are some discussions on this board that a slight ringing or metallic noise from the transfer case (around the center floor of the car) can also be heard in 4wd mode which all of these are pretty normal. I’ve driven a lot of 4wd cars and have to say this one has the most Jekyll and Hyde feel and sound between 2wd and 4wd. The one feel I don’t like that I’m still trying to figure out is that only in Sport 4wd in 3rd and 4th gears with about 1/3rd gas pedal load, there is a skipping feel from the driveline. It’s not there in normal 4wd or sport 2wd and no other transmission gears.

 

[99WhiteC5Coupe]

The 4A mode in my 2015 Expedtion sounds louder and coarser, than the 2H mode (dry or wet pavement).

 

[scottdm]

In 4A, the front hubs are locked and the front driveline is spinning, so it's normal in the Expy to feel the "coarse" feeling that is being described. This allows the transfer case to engage as needed to engage the front wheels. I almost always drive in 2H to avoid this issue unless the roads are snow covered.

 

[Lghtspeedz]

Thanks everyone. I suspected it’s normal. There are definitely some differences. Were it not for that coarse feeling, I’d probably prefer this mode. The front wheel engagement really makes the car feel more nimble to me. Ah well, it’s still a nice drive in 2WD.

Thanks again for putting my mind at ease.

 

[mr_dave]

In 4A, the front hubs are locked and the front driveline is spinning, so it's normal in the Expy to feel the "coarse" feeling that is being described. This allows the transfer case to engage as needed to engage the front wheels. I almost always drive in 2H to avoid this issue unless the roads are snow covered.

This is correct. We discussed this a little while ago and reached the same conclusion:

- 2H = rear wheels only (obviously) however front hubs will be engaged with engine off, or if there's a vacuum issue
- 4A = entire front drivetrain always engaged (hubs, diff, driveshaft) with the transfer case automatically applying power to the front as needed
- 4H = all 4 wheels driven all the time

Mechanic says not to use 4a all the time

I can spin the wheel in when off it just spins the shafts and driveshaft with it. I don’t think I’ve ever tried it running.

www.expeditionforum.com

 

[chuck s]

Not sure how the 2020 dash works but on my 2017 I can put one panel in the "off road" display mode and the dash will indicate where the power is going. Just a tiny goes to the front axle in normal driving with more as needed. There's also an inclinometer in that same display which is interesting in the mountains -- just don't get dashboard fixation.

I normally only engage 4A in the rain or other potentially slippery road conditions. (Or when my family borrows "the truck" for whatever reason they think they need it.) I can feel the added drag of the front axle immediately when I engage 4A.

-- Chuck

 

[mhExped]

I experienced a problem awhile back where my '18 Limited w/ HD Tow Pkg began making a grinding/rubbing sensation. Initially it was limited to 4A and when under load (driving uphill and/or towing), but got worse over time to the point where the symptoms could be felt/heard even in 2H and on level ground and w/o towing.

Turns out I had a bad front diff at that time. After that got replaced, the problem was "better" but I continue to feel a slight rubbing/grinding while in 4A under load. I do a decent amount of towing and drive on hills frequently and don't recall experiencing this when the car was new. The dealer tells me it's "normal" when in 4A due to the front hubs being locked.

Here's a repeatable scenario, I'm curious if anyone else wants to give it a try and report back...Find a hill in a quiet, less-trafficked area. Put the car in 4A and drive up the hill at a steady speed 10-15 mph to minimize road/wind/engine noise/vibrations - you can even drop into manual to take the trans out of the equation. Do you feel/hear the same regular/rhythmic rubbing/grinding? Do we all agree this is normal/expected?

@Lghtspeedz, I wonder if what I've described could at all be related to what you've described? Is the "roughness" you describe steady or rhythmic/pulsing? What if you repeat the test described above? Does the problem get worse or change in anyway?

 

[ROBERT BONNER]

Unexpected noises/vibrations are always something to be concerned about. That being said, with a vehicle equipped as yours (and mine) are, there is the eLSD in back and a viscous clutch transfer case amidships that are constantly varying torque transfer between the front and rear axles as well as between the two rear wheels while in any of the A4WD modes. As those clutches tighten/loosen and allow slip they make a lot of noise. The moaning and groaning coming out of those two clutch systems scared me half to death the first few times I took mine through mud holes off road with the windows down. Once you get used to it...you start to expect it. There is no doubt that any steering angle change in combination with any slip causes differences in clutch pressure and changes in the sounds. Additionally, if you are towing on anything other than perfectly dry, flat sand and gravel free pavement, you will experience enough slippage somewhere to change the pressure in the clutch(es).

 

[mhExped]

...constantly varying torque transfer between the front and rear axles as well as between the two rear wheels while in any of the A4WD modes. As those clutches tighten/loosen and allow slip they make a lot of noise...

That's the interesting thing about the uphill test I outline above...I hear/feel these vibrations/grinding even with constant speed, constant angle of ascent, steady steering (i.e., driving straight), dry smooth pavement, constant gear, etc. And, previously those symptoms (albeit more pronounced) indicated a trashed front diff.

Although these modern power delivery systems are capable of adjusting various aspects on-the-fly as conditions change, I wouldn't expect any changes to power delivery systems under constant conditions. Am I wrong here?

 

[vincentrose]

I have a 2014 Expy, and I have switched between 2H and 4A many times at speed, and for the life of me, I can't feel or hear any difference between the two. I have crawled under it and the front axles are disengaged in 2H and engaged while in 4A, also when I punch it on gravel in 2H, the rear wheels spin very easily, not so much in 4A mode, so I know its working like it should, but on the road, I don't feel or hear any difference between 2H and 4A

 

[Lghtspeedz]

That's the interesting thing about the uphill test I outline above...I hear/feel these vibrations/grinding even with constant speed, constant angle of ascent, steady steering (i.e., driving straight), dry smooth pavement, constant gear, etc. And, previously those symptoms (albeit more pronounced) indicated a trashed front diff.

Although these modern power delivery systems are capable of adjusting various aspects on-the-fly as conditions change, I wouldn't expect any changes to power delivery systems under constant conditions. Am I wrong here?

I'll have to give that a try. For me it seems like I notice it more in the 20-35mph range and specifically when the engine is around 2,500-3,250 rpm. I will say this, the power delivery tab on the center screen tells me it's working, even at "steady state" speeds in the range you cited. It engages the front wheels until 45mph or more where it seems to switch shift power only to the rear. I will say, that if you punch the gas a little at 55mph it re-engages all four wheels.

 

[Anerbe]

The center display shows power distribution in 4A in my 2019 Platinum. I can feel the difference in driving dynamics vs. RWD only, especially in wet turns with throttle application. The front power application is shown all the time with acceleration, although the amount varies depending on which drive program you choose. The only time i may see the car only put power to the rear wheel only with 4A activated is if on the highway at higher speeds.

My old 97 XLT seemed to be very reactive in 4Auto, where it would start RWD, then when traction was problematic in rear, it would shift power to the front.

 

[Soliyou]

That's the interesting thing about the uphill test I outline above...I hear/feel these vibrations/grinding even with constant speed, constant angle of ascent, steady steering (i.e., driving straight), dry smooth pavement, constant gear, etc. And, previously those symptoms (albeit more pronounced) indicated a trashed front diff.

Although these modern power delivery systems are capable of adjusting various aspects on-the-fly as conditions change, I wouldn't expect any changes to power delivery systems under constant conditions. Am I wrong here?

I too have the grind at 15-20 mph. I also have a pretty loud whine that occurs when the front driveline gets warm. It mainly occurs right before the system disengages the front axle. Of course the dealer can’t re-create.

 

[ROBERT BONNER]

Your scenario didn't discuss whether you tried the same test in 2wd to see if the noise went away. Clearly if there is a difference, it's front drive axle train related, if there isn't a difference you can eliminate the front axle system from the equation. Since they've already replaced the front axle once....and the noise is the same (??) it would seem that the replaced parts are not part of the problem. Other sources of front noise would include brakes, bearings and hub actuators. There is a known late model hub issue affecting all 150's and Expeditions that is a low vacuum related issue where vacuum leaks cause one or both hubs to drag on the wheel bearing cartridge(s) under certain conditions (both 2wd and A4wd)....the current TSB instructs dealers to disconnect the vacuum to the system to permanently engage the hubs to check the system....If you disconnect the vacuum to the hubs and the noise (described as a grinding) goes away, it's a hub system vacuum problem (actuators, lines, vacuum reservoir, check valve, actuator valve, etc.).

 

[mhExped]

Your scenario didn't discuss whether you tried the same test in 2wd to see if the noise went away.

Initially, my problem was in 4WD and under stress/load only (going uphill and/or towing). Over time, the symptoms grew more noticeable and were present in 2WD as well, even on flat ground and without any stress/load. It was that pattern that ultimately led to the front diff being replaced.

Since then, the test scenario I described results in the felt vibrations in 4WD and under stress only (uphill is the easiest test for me to reproduce). That is, the vibrations are NOT noticeable in 2WD, nor when on flat ground and/or w/o stress/load.

To be clear, the noise/pattern is NOT the same as _immediately_ before the front diff replacement. However, it's still noticeable. Dealer says this is "normal" across all Expeditions, but this being the only Expedition I have experience with, I am unable to agree/disagree without hearing from others who can relay their experiences.

I do not recall noticing these vibrations in the first 2-1/2-ish years of ownership, but maybe I wasn't paying attention until they grew so noticeable I couldn't ignore them and now I'm overly sensitive.

Again, if more Expy owners tell me theirs exhibit the same pattern than those who don't, I can feel better that this pattern is indeed "normal." Without that corroboration, though, I can't help but wonder if something else has failed or is on its way to failing. Who knows.

Getting back to the OP and his concerns...I mentioned my situation because I wonder if what he's noticing falls into the "normal" category or the "something like the front diff has failed" category.

 

[ROBERT BONNER]

Well, if it's coming from the front end while in 2WD, I would try disconnecting and plugging the vacuum supply to the front hub clutches and see if it goes away. If it does, it's a hub clutch issue as discussed above. If it doesn't go away, I'm at a loss.