Just bought a 99 - Noise in 4WD, Help!

[jaserlet]

I recently bought a 1999 Expedition XLT from its original owner. 150,000 miles in very nice cosmetic condition. Spark plugs were recently changed but everything else is original, including shocks and wheel bearings. When I did my test drive, 4H and 4L (it has the A4WD feature) seemed to work fine. However when I tried 4H again today I noticed some additional noise that worries me!

Going 40 mph down the highway, I turned the knob to 4H. The "4x4" light on the dash came on and almost immediately I could hear some additional road noise, mostly from the front, sounded sort of like I was going over rougher pavement. The noise was pretty consistent from 40 - 65 mph and was the loudest when I took my foot off the gas and coasted down between 55 and 50 mph. Again it sounded a lot like really tough pavement road noise. At 40 mph I turned the knob back to A4WD and the "4x4" light turned off but the noise continued. I had to pull over and come to a complete stop and then start driving again for the noise to go away. I was able to repeat this same situation again a few miles down the road.

Any ideas as to what might be causing this noise in 4WD? And why doesn't it go away when I put it back into A4WD? I didn't experience this with a 2005 F-150 XLT 4x4... though I realize isn't the same as my 1999 Expedition!

 

[Thermo]

jaserlet, when you say additional road noise, is this more like a U-joint whine, a flat spot in a tire (thump, thump, thump), or something else.

Two things are coming to mind. The first is that there was a TSB concerning the front differential that involved some springs. There was a problem with noise from the differential that was resolved by installing these springs. The other could be either a bad U-joint or a bad wheel bearing. By putting the truck into 4Hi, you shift the power sharing from 10% front/90% rear (AWD) to 50/50 (4HI). This change in power split can cause what you are seeing.

Granted, I have to ask, did you shift to 4HI on dry asphault? If so, that alone could be your problem as you don't have a means to relieve the driveline stresses if you have a 1 or 2 tires that are slightly different sizes. This will cause the tires to spin at different rates and what you are feeling is effectively these smaller tires getting skipped down the road due to being a different diameter. This would also account for the 4HI not wanting to release as the transfer case is bound up due to the driveshafts attempting to spin at different speeds.

If you want me to go into more detail about the binding up, let me know.

 

[thebeast1190]

check this out

http://www.4x4abc.com/4WD101/def_turnpart.html

 

[jaserlet]

Thank you for explaining how 4WD truly works! I was not aware of this and had heard from my misinformed buddies that it's ok to use 4WD on dry pavement! These explanations make total sense. Thank you for setting me straight!

 

[josh1095]

nice read, thanks beast