driveline lurches when coasting

[dadzh0me]

i did try to find similar problem on other threads but did not.
when on the frwy or at speeds above 40 when i let of the gas it seems as though the entire driveline engages and disengages (bad spelling) quietly not neccessarily a bad sound just noticeable. you can feel it some might not i do. if this is redundant i apoligize .

 

[Nevea'sExplorer]

Is it more like the transfer case is transferring the power to the front wheels? I know on my truck when it does that it's almost like a large clunk like a universal joint is bad or something. But it really doesn't engaged that often. I did notice today on the highway it did it for some unknown reason. I don't think I ran over a slippery spot on the road either?

 

[dadzh0me]

no clunks

its very smooth like if it was manual trans winding out , thanks for your reply.

 

[Nevea'sExplorer]

its very smooth like if it was manual trans winding out , thanks for your reply.

Like the transmission hanging in gear longer then it should before it finally shifts? If so mine used to do that until I added a CAI and now it shifts very smooth.

 

[dadzh0me]

not the tranny winding out more like transfering power from side to side and front to rear.

 

[beavis195]

mine kinda seems like it does it when I have the cruise control on and I'm going downhill. So I think I understand what your talking about but maybe not...

 

[1969scott]

Did anyone find the problem associated with this? Mine just started doing it, almost like the 4x4 is engaging then disengaging over and over while coasting.

 

[IZwack]

Did anyone find the problem associated with this? Mine just started doing it, almost like the 4x4 is engaging then disengaging over and over while coasting.

Disconnect the transfer case's electrical connectors and drive on. If the symptoms go away, then yes its the 4wd engaging when its not supposed to.

 

[aspitaletto]

probably those stupid speed sensors acting up... ive had mine replaced several times. cause the 4Wd to engage anytime it wants

 

[1969scott]

4x4 engaging while coasting

How do the speed sensors input into the auto 4x4 logic? I'm trying to get a good understanding of the whole 4x4 system before I poke and hope changing out parts. By any chance, does anyone know of a source for electrical schematics for an 02 4x4 xlt? That would help greatly. I'm sure everything is processed in the cpu, but there's always the possibility of a direct contact to the relay which engages the xfer case. Thanks.

 

[IZwack]

Its like this.. there are two cool guys, one at the back and another at the front. Both of them are counting their associated driveshaft RPM and the rear one is like 1,2,3,4.. and so on. But the front one is like 1,2 <pause> 3 <pause> 4. So the 4wd computer is like, "LOL!! quit playing you front sensor *******. I've had enough of your BS! You count slower than snail so I'm going to lock you with the rear!!". So thats when 4wd is locked. And of course the slower counting is due to the front axle rotating slower (or not rotating at all as in the vehicle is stuck) when compared to the rear axle.

 

[1969scott]

So why would this only be a problem when coasting, not always? I still have more troubleshooting to do, but thanks so far.

 

[IZwack]

Well assuming the vehicle is indeed going into 4wd when the vehicle is coasting, then the only reason that I can come up with as to why the vehicle is engaging 4wd during coast is that the gears are driving on the "coast" side.

This image will probably confuse you more than help but just look at the two upper-left pair:

Ring and pinion gears dont mesh "100%" per say, they have what is called "backlash" which allows some "play" in the gear mesh.

When your car is accelerating, the gears are meshing on the "drive" side of the teeth. But when the car is coasting, the gears are meshing on the "coast" side of the gear. Obviously, the distance the gears are able to rotate between "coast" and "drive" is measurable and that is called "backlash".

This backlash, allows the driveshaft to rotate X number of degrees between "drive" and "coast". This X number of degrees is also measurable by the speed sensors at the transfer case and when the rear axle finally switches from "drive" to "coast", that number of degree is what I believe is causing an early 4wd engagement problem.

See, during normal conditions (not in 4wd), the front axle never rides on its "drive" side because it is technically always coasting -- as in its not pulling the vehicle forward because the rear axle is doing all the pushing. So you see, with the front axle never being on the "drive" side of the gear, and the rear axle going from "drive" to "coast", then obviously we can see a small bit of rotational RPM speed between the two driveshafts which might be kicking in the 4wd prematurely.

But of course this should not be happening so what could be causing this issue? Well maybe a rear pinion that needs to be looked at -- which is definitely not unheard of in the 2002+ Explorers with the independent rear suspension (IRS for short). A loose pinion (due to worn bearings or whatever) has A LOT of backlash as it wobbles around in the axle housing. This could also be due to a faulty sensor at the transfer case. Another reason could also be a set of tires that are not the same diameter -- in other words, you've been taking that right turn out of the grocery store parking lot way too quick and way too many times buddy boy!

 

[1969scott]

Well, I actually do understand what you're talking about (former MM in the Navy), and I could see the excess backlash causing 4wd to engage only when I first took my foot off the gas. I can definately see a problem with a tire larger than the other, too. I haven't fully ruled out the module due to I can hear it click when I engage 4-lo, but no click going into or out of 4-hi/4 auto. It seems to not surge when I shift from 4-lo to 4 auto. What is a way to ckeck the speed sensors?

 

[ummduh]

My 03 was doing this. Problem on mine was 2 fold:

FR wheel bearing was going bad, putting a new bearing also means a new speed sensor (bearing noise may have been interfering with the speed sensor?) I included this because it definitely was getting worse the more noise the bearing was making.

And 2, I don't have equal tires front and back. At that time I had the half worn tires in the back and the new ones up front (bought the Ex this way, and never having a AWD vehicle before it just never dawned on me).

What I did to isolate it was to unplug all related fuses. This stopped the surging, but for some reason also KILLDED (yes killded, it was that bad) my gas mileage.

Put the fuses back in and swapped the larger tires to the back.

It doesn't surge anymore and my mileage seems to have returned to mostly normal.

Still gunna yank the front d-shaft and see what happens. Seems to be frowned upon on here but WTH, why not try it. I'll never use 4wd again in CA, not afraid of killing the transfer case. If there's no ill effects I may even go so far as removing the whole front driveline and 2wd converting it. (don't say to sell it and buy a 2wd, I'm going to drive this thing into the ground! The wife is FINALLY happy with a vehicle for the first time!)

 

[Pontisteve]

I have also heard that the speed sensors (VSS) can cause this. Best way to test it, in my opinion, would be to hook a scan tool up, check for VSS codes, and then drive the car at various speeds. Have someone keep an eye on the live data in the scantool, to see if the VSS reads anything other than the mph that the speedometer is indicating.

It's also possible that the speedometer may have some random screwups. That could be a sign of a bad VSS too, since the speedometer gets its info from the VSS originally.