Question about 4WD Auto characteristics

[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.