Good day all,
I fear I have an issue that is beyond my comfort level of repairability. I have an early '89 XJS Coupe that was a middle-of-the-field kind of find that I've been working on for over a year now. It's a well running car that needed a lot of suspension work. While up on the lift at my home doing forward and rear suspension overhauls, I noted a leak from between the driveshaft output flange and differential input flange. Cursory searches led me to believe the leak was from the splines of the pinion rather than the seal, but I went ahead and bought the pinion seals as a "while I'm in here" situation.

I went to remove the driveshaft and was surprised to find the 4 bolts that connect the flanges were loose. Not as though someone removed the bolts and put the lock nuts back on by had, but just touching the mating surfaces, still needing wrenches. Odd. With the driveshaft out of the way, I mark the pinion and nut location as to reinstall and match the original position. I get my 1-1/8" socket and the big breaker for the nut, along with my "big red" breaker bar setup on the flange. I brace up and, much to my dismay, the pinion nut took minimal effort. I can spin it by hand with the socket...With my torque practiced arm, I'd say it took 10lbft or less.

I've gone over a lot of the forums and websites, including Bernard's website to find ways of increasing preload to remove a whine, but they all had starting torque. I'm beginning to resign to the idea that it may be best to overhaul this unit, in which case a swap from Welsh may be in order, but I'm hoping there may be some knowledge out there for guidance so I can get the car driving for a bit. Less preload is favorable than too much, but near correct preload is better still.

Thanks in advance!

 

l have successfully re torqued pinion flange a few times on jags, you have nothing to lose trying that first off, even without a current reading.
pinion nut should be torqued to 120 lbs.ft and then flange rotation torque checked. Keep tightening flang nut until figure obtained is between 25 to 30 lb.ins. This rotation check is only through the backlash movement of pinion and is lb.ins not lb.ft.
If flange nut to 120 lb.ft results in rotation torque above value stated rebuild is needed.

 

Forgive my ignorance here: "the rotation is only through the backlash movement of pinion..." Could you explain that? Is the measurement taken when rotating the input clockwise? Before every time I take the lb.in. measurement, I should rotate the pinion counter-clockwise first? Or am I misunderstanding that entirely?

Probably another foolish question here, to achieve the 25-30 lb.in measurement, I assume I need to disconnect all other forms of drag? Such as the output driveshafts and brake pads? Even typing that out makes me realize it's a dumb question, but I want to be perfectly clear. I have lb.in. torque wrenches from my job, so the tooling is not an issue.

 

Yes, you need to rotate counter-clockwise each time before testing the rotation torque. Does not matter what else connected since within the backlash movement it is only the pinion moving.

 

I think a more logical way would be to back off the pads as best you can and measure the torque to rotate the pinion; do it a number of times and average the readings. Then torque the nut gradually (with a little blue Loctite on the threads) until the rotating torque is 25 lb in more than initially measured. If the nut torque feels less than 100 lb ft, you will need to overhaul. If it is tight, drive it and see.

 

pinion bearing preload can NOT be set if anything more than the pinion is being turned. Turning through backlash movement window is the only way that is achieved with the crownwheel in place. The procedure has been in the Jag manuals and used successfully for years.
l don't think OP recorded his initial preload prior to removing flange nut.

 

Quite correct, the figure of 20-25 inch-lbs for old bearings and 30-40 inch-lbs for new bearings is for the pinion in its bearings only. There should be nothing else in the case when this measurement is taken.