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Chris (aka- Moose)
Chris (aka- Moose), Mercury Technician
Category: Mercury
Satisfied Customers: 48368
Experience:  16 years experience with Mercury
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I have a 2002 Mercury Mountaineer 4.0L AWD that the casing

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I have a 2002 Mercury Mountaineer 4.0L AWD that the casing cracked on the front differential. we replace the front differential with a used part and right after the part was put on it started making a loud clunk when turning hard left or right but not all the time, almost always when turning after backing out of a parking spot. Relpaced outside tie rod and checked the oil in the transfer case it was normal color. Did not make this noise before part was changed.

Hello Customer

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Did you just replace the differential or the complete axle assembly?

Customer: replied 8 years ago.
Just the differential

Either the differntial was not set up correct meaning the backlash and bearing preload is not correct.

Or all 4 tires are not the same brand and size and tread depth and the AWD is fighting itselff front to rear.

Or you need to install a S spring between the spider gears to prevent tehm from being loose in turns.

Customer: replied 8 years ago.
The tire on the front match and the rear tires match, but it did not make this noise until the part was changed, it did fight itself but never made the clunk noise. Could the part be bad or possibly not installed correctly, plus it came out of a 2002 ford explorer with 17,000 miles on it could that make a difference?
If you just replaced the diff and nothing else then the diff should not be a faulty part. Since it was used it sounds like you may have installed a ring gear as well. Are you sure the gear ratios are the same.
Customer: replied 8 years ago.
Yes the gear ratios are the same we checked with the dealership using the VIN, only the used front diff was installed
Then you need the S spring or recheck the ring gear backlash. This is assuming there is not another concern not even connected to the front axle where as the transfer case viscous coupling has failed and is grabbing to much.
Customer: replied 8 years ago.
What would it do if the used part dealer gave me the wrong gear ratio, and how would you check the ring gear backlash?

You check backlash witha dial indicator.

If you have the wrong gear aratio then hte front and rear axles are not turning teh same speed and this will cause numerous part fialures i nthe driveline.

Customer: replied 8 years ago.
so if gear ratio was off it would do more than an occasional clunk right?
The clunk it will and can do. It will overtime cause axle wear front and rear and driveshaft wear and transfer case wear.
Customer: replied 8 years ago.

the dealership told us our gear ratio was 355 how can I find out what the gear ratio is on the front differential we installed. Where is the tag and what should it say to be a 355.

There should be a stamped number on the end of the pinion and on the back of the ring gear. You count the teeth on both to get the ratio. 1 pinion tooth equals 3.55 ring gear teeth.
Chris (aka- Moose) and other Mercury Specialists are ready to help you
Customer: replied 8 years ago.

We are pretty sure we got the right ratio but we will check that and the s spring and the rear ring backlash. Thanks for all your help! It's been driving us nuts!

You are welcome