2008 4.6L LTD AWD question and noise (clunk)

[pjw73nh]

Greetings,
I've done a search here and while there is a lot of info on this subject, I haven't seen anything conclusive. There seems to be a lot of disagreement as to how the AWD actually functions.

Would any of you knowledgable folks help me understand how the AWD in my 2008 4.6L Explorer LTD functions? Just a "functional" explanation would do. I have a pretty good mechanical understanding and ability, I just don't have any experience with AWD or "auto AWD". I am OK with RWD, FWD & 4WD.

Buttons: I have 4x4 Auto, 4x4 high and 4x4 low. I also have traction control.

I presume in 4x4 high and 4x4 low both axles are locked up as is the transfer case? In 4x4 Auto, is it basically a RWD vehicle until the rears start to spin/slip? And then the fronts become driven?

The other night I was driving in a snowstorm. There was about 5 inches of snow, packed down to about 3 on the highway. I had to pull over under an overpass every 10-15 miles to de-ice my wipers. When I would start back up, I had to get into the travel lane quickly. As soon as my driven wheels (not sure which ones front or rear) would break traction, I would hear/feel a pretty heavy clunk (not a slam) from the back, and it felt like something engaged.

A few days later I was parallel parking and in order to get far enough off the street, I had to go up on the snow bank a little bit. As I backed up the snow bank and the driven wheels started to break traction, I felt the clunk again and it took me by surprise. Each time I gave it some gas, and the wheels would slip, I'd hear/feel the clunk again. Kind of harsh.

Does this sound normal?

Thanks.

 

[Town]

You have the automatic AWD system that includes a transfer case that has a planetary differential and a clutch. The rear wheels are driven directly through the transfer case and are always engaged. The clutch controls the differential to put a variable amount of torque through a chain to the front axle. The clutch is controlled by an AWD module that can vary the torque applied to the front axle by varying the duty cycle (drive time) from a very low value up to 40% (high duty cycle) of torque to the front axle. The AWD module does this by varying the duty cycle to control the amount of drive according to wheel slip. The low duty cycle on the clutch does not constantly drive the transfer case differential but applies short bursts to allow "slippage" of the front axle to allow front and back axles to be "driven" at different speeds such as when turning a corner. This prevents drive line wind-up, but keeps the smooth drive going without noticeable engagement issues.

The AWD system is quite sensitive and needs tires within 2/32" in radius (front axle to rear axle particularly) to work smoothly.

When you select 4x4 the differential is locked mechanically. (EDIT: that is for 4x4 low, the 4x4 high uses the clutch on full duty cycle to lock the drive to the front axle).

In my limited Explorer experience the AWD and 4x4 systems are very smooth in operation. There are a lot of parts that could cause clunking. First ensure tire sizes and wear are the same. Take off the load from the drive shafts and check the universal joints for looseness since this can cause a clunk (but also vibration at certain speeds). Then you are into half shafts, CV joints (snap when turning slowly at full lock), and differentials.

Good luck.

 

[pjw73nh]

Town, thank you for the explanation. I appreciate it.

 

[pzy3339]

no one has yet to comment on how rough the auto 4x4 engagement should be. I bought a 06' v8 limited about a month ago and we got some snow and i noticed this as well. my 99 explorer did not engage nearly as hardshly as this newer one. it makes a kinda gross sound sometimes too. not sure about this truck.....miss the 99.

 

[Town]

no one has yet to comment on how rough the auto 4x4 engagement should be. I bought a 06' v8 limited about a month ago and we got some snow and i noticed this as well. my 99 explorer did not engage nearly as hardshly as this newer one. it makes a kinda gross sound sometimes too. not sure about this truck.....miss the 99.

For a properly operating vehicle there is no engagement issue, all 4 wheels are driven all the time, it is seemless. The clutch just changes the torque applied to the front drive shaft in response to wheel/axle slip. My son's 2007 Explorer V8 is very smooth and he is very pleased with it. How does your vehicle feel in 4x4 low? Do you have the same issues?

Did you check the transfer case fluid level and condition? The 2006 and 2007 use Mercon that is no longer available. Mercon V usually replaces Mercon but not apparently in this transfer case. The 2008 and later models use transfer case fluid, but when I checked with my dealer on what to use for a 2007 model he initially said Mercon V but when I queried this he said he needed the VIN to know for sure. The wrong fluid may not allow the transfer case (or the clutch) to work properly.

Good luck.

 

[pzy3339]

mine seems like it is 2wd until slip then it grabs onto the front driveshaft for 4x4. 4x4 high and 4x4 low work great. the only issue is when the rears slip in 4auto and the clutch engages. the engagment is fairly harsh and often accompanied by a nasty torquing sound.

 

[Town]

mine seems like it is 2wd until slip then it grabs onto the front driveshaft for 4x4. 4x4 high and 4x4 low work great. the only issue is when the rears slip in 4auto and the clutch engages. the engagment is fairly harsh and often accompanied by a nasty torquing sound.

Since you do not have a problem in 4x4 high and low the problem seems to be in the transfer case clutch or related variable clutch apply strategy. There are no diagnostics that cover such a condition that I can find in the manual. There is a single wire to transmit the duty cycle to the clutch coil (and a return sensor wire) and you know the wire and connections are not a problem since it operates fine in 4x4 high.

To increase/decrease the drive torque through the clutch to the front drive shaft:

1. the 4x4 module must be working correctly to vary the duty cycle; a dealer can check for DTC's indicating a problem; the 4x4 module can be replaced easily (by you) without a dealer having to do a programmable module install.

2. the clutch and actuating coil must be working, and this is inside the transfer case and a replacement part does not seem to be available on RockAuto. They do list a reman transfer case for about $1400.

3. the fluid in the transfer case must be correct or it may allow clutch slip. As noted previously the fluid for your transfer case has changed and the safest approach is to provide your VIN to the dealer for the correct fluid. It is easy to change (drain plug and a fill plug) and it takes 1.5 quarts approx to the bottom of the fill hole. The transfer case fluid for later models that is probably what you need is about $10 per quart).

I would change the transfer case oil as a first step.

Good luck.