Mismatched Tires and 4x4 Auto-Switching
Thanks, explrr and n2epv!
I've been trying to get to the bottom of this issue for months, and finally happened to google "rhythmic thumping" plus the usual search terms.
My symptoms, with my 2003 Explorer XLS V6.
When getting up to 65 or 70 MPH, then letting off the gas and coasting, the truck would pull to the right (felt like driving through a puddle only on the right side), and would generally stay like that. Once I got down below, say 40 MPH, I would get that rhythmic thumping (a little faster than once per second, and it never varied--it was never faster or never slower, not even down at 5 MPH). If I applied brakes during the thumping, it was exacerbated.
The problem never happened if I coasted in neutral. Also, the problem often went away when I stepped on the gas again.
I thought it might be worn suspension components. I inspected the front end, and I did notice that I had bad upper ball joints and outer tie rod ends. I replaced the upper control arms, outer tie rod ends, and for good measure, the inner tie rods and lower ball joints. I also replaced the right front hub bearing. (It had no play yet, but it was getting slightly rough, and I already had a new one on hand.)
However, the problem persisted...
People told me that my rotors were warped, or my tires were faulty. I knew they weren't because the thumping would have changed frequency with speed, so I didn't bother pursuing those.
I notice that a the right rear coil spring was cracked, but since the rest of the rear suspension components looked good, I couldn't make sense of how the coil spring would contribute to a flat-ground, straight-ahead problem, I didn't bother with it. (I'll save that for another day.)
Others said it was binding calipers, which I thought sounded reasonable, but I couldn't make sense of why it would be intermittent. I inspected the right-side calipers, and did a full brake fluid change while I was at it. While I was in there, I cleaned out the right parking brake drum and backed off the tension on those shoes a tad, just in case they were playing a part.
Still, the problem persisted...
Others folks told me it was my CV joints, but they looked fine and I had no symptoms when turning in circles at minimum radius.
The interwebs (referencing different vehicles) told me it might be bad engine mounts. I inspected mine, and yes, they were bad, but I couldn't fathom how they could make the truck pull to the right. I'll also save *that* job for another day.
I do have a set of unmatched tires on the truck right now (because I'm broke). Some folks pointed to those as the source of the problem, but didn't elaborate on why that might be the cause, so I didn't really have a plan of action on that one.
Finally I found this thread today, and it was explained that unmatched tires can cause 4x4-Auto to switch on and off (because it *thinks* it detects slipping conditions). Finally, something that made sense! I didn't completely understand why that would cause a pull to one side while in 4x4-high, but I figured that all bets are off if you're driving at highway speeds on dry pavement with 4x4 engaged.
Today, I took the truck out and pulled fuses
#17 and 18 (those are v1 and v2 4x4 fuses--maybe 4x4 low vs. high, I don't know). I tried to reproduce the problem with those fuses removed, and couldn't! Then I repeated the tests, both in and out, and all signs point to automatic 4x4 switching, and that's probably due to the unmatched tires.
So, I'm really happy to have found this thread. Now that I know I don't have any problems that would give intermittent alignment problems, I can finally get this sucker aligned!
Now, on to some questions of my own:
We're done with winter, here in VA, and I'm not going to need 4x4 (except in known, specific cases). Is there a difference between the "brown wire mod" switched "off" and having these fuses removed? What I'd like to do is keep the fuses out, get a new set of tires in the fall, and *then* put the fuses back in (like explrr's last post).
Does that sound like a plan, or will this have other repercussions?
Thanks,
Jamie