Interesting Rearend Problem

17Mark71

Registered User
Here's what was done:
A brand new set of FRPP 4.10's were added along with a brand new tracloc unit.
Speedo gear was not changed. Friction modifier was used. All new bearings, washers, shims, etc...

Prior set up was 3.55 gears with open rear end.

Here's the symptoms:
At speeds above 65 when you let off the gas just a fraction a vibration that sounds like "running over rumble strips" occurs. It does not happen below 65. The sound/vibration is definitely coming from under or the rear of the car. There's no clunking noises when put into or taken out of gear.

The guys at L&R Service took it apart and checked everything. The fluid's clean, doesn't smell burnt. Any ideas?
 
17Mark71 said:
Here's what was done:
A brand new set of FRPP 4.10's were added along with a brand new tracloc unit.
Speedo gear was not changed. Friction modifier was used. All new bearings, washers, shims, etc...

Prior set up was 3.55 gears with open rear end.

Here's the symptoms:
At speeds above 65 when you let off the gas just a fraction a vibration that sounds like "running over rumble strips" occurs. It does not happen below 65. The sound/vibration is definitely coming from under or the rear of the car. There's no clunking noises when put into or taken out of gear.

The guys at L&R Service took it apart and checked everything. The fluid's clean, doesn't smell burnt. Any ideas?

Who installed the gears and where they setup correctly?
 
I trust the mechs. They've done good work for me in the past. As I stated they took it all apart and everything looked good. U joints are relatively new. Car has recently balanced Mk VIII drive shaft as well.
 
Check the bolts that hold the shaft into the ujoint by the IRS

Mine came loose once and had similiar problems
 
fwiw, a respected gear guy around here hates FRPP gears and will only use OEM Ford gears. I have no idea what the difference is, but I know he won't use the FRPP gears.
 
You also might want to check your halfshafts for play, I have a similar issue and I can visually see the halfsaft moving around when it is up on jackstands under power. I have not had a chance to put a new one in yet but I believe that will solve the problem.
 
Another thing to try is replacing your stock rubber front diff mounts with a set of poly ones (or even new rubber ones). I had the exact same symptoms in one of my personal cars. As it turned out, the stock front mounts were allowing the pinion angle to change quite a bit when letting off the throttle at highway speeds.

-Rod
 
yes,def sounds like the backlash is not correct,im not sure how you "measure" it though....ive never seen a rear end done and its been right,always seems to take a time or to,ive also seen them in specs on the table,but afte a mile or 2 theres an issue with the backlash or the dredded whining from the rear.....this is just my experiance anyway
 
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