I'm new to this site and not sure where this question should be posted. I have a 99 Expedition, E B, with AWD, 5.4 L and 3.73 rear.
My problem is I have chatter or skip in the rear when accelerating but more pronounced when towing. It feals like Im on snow or ice and the power is being shifted from one whell to the other. The 4x4 light has come on the dash and blinks at a consistent rate. I just had a 60K check and had all fluids changed and an additive to the rear end to help with the internal clutch? The diagnosis yesterday was the reason this is happening is my tires have a ½ inch difference in circumference from front to rear and the computer is picking it up as a change and terrain and initiating the AWD / Control Track 4x4 system!! Has anyone experience this or heard of it. The tires have only 15k on them (replaced under recall). If anyone has any advice I’m all ears.

 

A 1/2" difference in size between the tires would probably cause this because the transfer case sends power to each axle based on the rotation of the two drive shafts. If there is a difference, it sends power to the front. When the difference is gone it starts to unlock again.

Does this only happen in the AWD position or in 4H as well? If this is the problem, it would go away when you engage 4H.

Ian

 

I concur with Ian.

Why do you have different sized tires/rims from front to back on a 4wd?

 

Thanks for your thoughts.
I haven't tried running it in 4wd on pavment but I'll check it out on the boat ramp this weekend.
The 1/2 " difference is in the circumfrance of the tires as a result of not rotating sooner than the 15K (which was just done). A 1/2 inch variance in circumpronce is a small amount on the radious or tread height. I'm guessing but it cant be more than a 1/16" difference in tread height. I can't believe the computer or design of this system would be so sensitive to pick up on this. The service department recomending rotation every 6k. I din't have this problem when i was running the origional Firstones which were replaced at 45k miles under the recall. The service department thought my only option at this point is to replace all four tires with new and start over with regular rotation.
I say its to expensive and I'll run it the way it is.
Will the computer recognize if the tire wear back to a close height of each other.

 

i had to have my ride hieght adjusted not too long ago..

one of the sensors was bad and was making the truck sit 1" lower on the passenger side.....

i never noticed any slip though in the AWD

 

A tire shop that deals with racing tires should be able to shave the taller tires to make them all the same circumference but I can't see normal wear making the tires 1/2" different!

Ian

 

A couple good burn-outs ought to take care of the problem.

 

To get a half inch difference in the diameter, you would need a 1/4" difference in tread thickness, not 1/16" - that would only give a 1/8" difference in overall diameter.

Ian

 

see that all the time. diff types of tires. but same size. will have a diff runout. you think that baby is gonna blow. most folks get pissed when you tell them its there tires doing it. until they get new tires. and then there glad we didnt sell them a tranny like they thought.

 

There was a defective sensor in the transfer case in 99 Expy's. There is a special service number for the Fix, but I no longer have the number (sorry). I do however have the name and phone number of the engineer that made the fix at Ford.

What happens is, the sensor senses slippage when there really isn't any slippage and transfers the load to the front wheels. Once the load is transferred the sensor decides that there really isn't any slippage and transfers the load back to the rear wheels. This all happens so fast, it feels more like a bad spark plug miss under load.

One test you can run, is to lock into four wheel drive. If you do this, the transfer case is locked and can't transfer the load back and forth. And you won't feel any slippage. But once you go back to AWD then it will happen under light to medium load. You won't feel it under full load, because the transfer case will automatically lock.

Hope this info is helpful!
**** Murphy