Tire Size Lessons Learned

[ssfrebo]

I inquired a couple of months ago on this forum about the effect on my 99 SOHC drivetrain if running different size (OD) tires in 2WD only. The replies were quick but mixed. Anyway, running down the highway with big tires on the back and smaller on the front I occasionally heard alarming noises and felt vibration coming from the drivetrain (felt like prolonged speed bumps) and pulling to one side. Apparently there are some speed sensors that were offended at the tire size difference. After a couple of days of this I had the right size tires put on, problem solved. However, I have noticed now when in OD and cruise control, the drivetrain will really rattle when starting up a steeper grade.

Now my question is....what could I have damaged and should I have it checked. For normal driving, everything seems ok, just hear the loud drivetrain rattling going up a hill in OD and cruise. This rattling is absent when not in OD and cruise, and going up the same hill.....Scott

 

[Brock94]

Speed sensors should affect two systems-- ABS and shifting in your tranny. Maybe the two computers have saved incorrect driving parameters in their memory from the time you had different size tires on-- that could make the computer shift incorrectly/ at the wrong time. Try disconnecting your battery for a while to clear the memory and see if that fixes it. Can't hurt and it's worth a try.

 

[jimcom]

I agree, disconnecting the batt was the first thing that popped in my head when I read your post. I've had bad gas before that made the X run like crap and even when I switched gas the thing still ran poorly, I figured it couldn't hurt to do a disconnect and let the computer relearn everything.
I'm no genius at this stuff but it did the trick, might work for you too.

 

[GJarrett]

I searched for the original thread you referenced to here and went and read it. You should not have done this. You were informed in that thread that your computer might kick in front drive anyway and that you would have to do a "brown wire mod" to be able to run different size tires.

You don't have 2wd. You have "4auto" and yes the tcase will sense slippage and kick some power to your front drive at speeds up to 40mph or so and anytime you hit the gas hard. As mentioned, you would have to do a "brown wire mod" (disconnect/cut the brown wire that powers your tcase shift motor) or your vehicle will be sending power to your front driveaxle whether you want it to or not.

Your tcase is driven by a magnetic clutch that locks in the power to the front. Since it is a magnetic clutch that can slip under extreme duress instead of gear driven that will snap gear teeth, you've probably been boogering up your tcase in the magnetic clutch while doing this. The "alarming noises and felt vibration coming from the drivetrain" was your tcase providing power to your front with different size tires on it and "pulling to one side" was the tire that was receiving that power. Since you really do not have 2wd, you were exerting tremendous stress on your drivetrain and forcing the magnetic clutch to slip and wear. With your front drivetrain spinning at a different speed than your rear drivetrain, somehing or another had to give. Being the weakest link, the magnetic clutch was slipping - if it wasn't, then you'd either be screaming tire rubber from the tires spinning at a different speed, or grenading the CV joints or axleshafts if the tires grabbed high enough traction.

I'm betting that resetting the computer by disconnecting the battery won't fix this, and you've done some kind of real damage to the innards of your tcase (not just experiencing a computer setting problem); ie, the magnetic clutch assembly will always slip now from the damage it experienced while being forced to slip.

If resetting the computer doesn't work for you, you can test my theory by removing your front driveshaft and taking it for a drive. If the problem is in your tranny or some other area you will still experience the problem; if disconnecting the power from the tcase to the front removes the problem, that will confirm that the tcase and its delivery of power to the front is the problem.

 

[Hartman]

Perhaps he did the brown wire mod.

 

[ExplorerSportNu]

May I ask why in the world you have/had small tires up front and bigger in the rear ???

 

[ssfrebo]

Thanks everyone for the feedback. In response to some of the infomation provided in the feedback...one clarification....since returning the tires to the correct size, all noises have gone away and have put several thousand miles on the vehicle with no problem with the exception of loading up the overdrive and starting to hear and feel some rumbling. I'll probably have that checked out at some point. Why different size tires? The old set had a right front with a badly pulled belt so I had the local station put on a set I had in the garage that were low mileage, but different OD, front to back. I did not think this would matter in 2WD but posted a question anyway just to make sure. One of the responses indicated should not be a problem, the other did mention a brown wire mod, but I did not follow up, which turned out to be a mistake. Anyway, the forum did give me the information I should have heeded, so good job to those who tried to steer me in the right direction.