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Need a little advice from the expert diagnosticians:  My '40 Century has begun to exhibit a "thrumming noise" whose rhythm is directly related to road speed and has nothing to do with engine rpm.  I suspected a bad tire or wheel bearing, but they checked out good, and my local mechanic thinks it is probably the u-joint.  With the rear wheels off the ground and the car in gear to turn the driveshaft, he said there was noise from that area.  Since it's a hard teardown, does anyone have an idea of how to confirm the diagnosis before beginning?

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Rod, u joint is unlikely in my experience.   Where is "that area" ?  the u joint is in the back of the transmission/torque ball. 

   These noises are FRUSTRATING to say the least.  Wild ass guess is more likely to be pinion related.

 

  Ben

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Yes, the noise he heard was from the torque ball area.  Tomorrow I think I'll get her up on stands, idling in high, and see what I hear.  If I can get help, I'll have someone else rev it to about 20mph and simultaneously cut the ignition and throw in the clutch, just to take engine noises out of the equation.

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Good news and bad.  Good that we found the noise was from the front right outside bearing, that had at least 3 bad balls.  There was no noise spinning the wheel by hand and there was no discernible play, but the bearing was shot.  The bad news is that during the process of diagnosis I checked the fluid level in the differential and found it to be overfull a complete quart.  Now I think I know where my transmission oil has been disappearing to.  I didn't think the small leaks I have should have accounted for the constant need to top up.  So it looks like a big job in the near future:  pulling the driveline to get at the torque ball and universal joint, since the book says the likely cause is a scored universal joint or bushing, a gasket plugging the return hole, too much clearance between the u-joint and bushing, loose adjustment of ball joint or run-out of front end of the propeller shaft.  I've seen that the u-joint is available, but can anyone recommend sources for gaskets, bushings, seals, etc.  I've never had a ball joint assembly apart so don't really know what I'll be dealing with here.  Any advice will be greatly appreciated.

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My advice is "Don't"  If the differential is quite, I would let it be for awhile. See if it over fills again. My '50 shop manual actually instructs to turn the back cover one bolt, thereby raising the fill hole somewhat.    What some have done on later years with auto trans is to drill a small [ 1/4 in ? ] hole in the bottom of the torque tube just in front of the flange at the back. Install a plug. Remove plug occasionally to check/drain any thing that gets that far back.

 My advice and I am sticking to it.

 

  Ben

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Sounds like pretty good advice.....especially the new drain hole....I really don't want to overfill the diff.  After I drained the extra quart out I drove the car about 30 miles, let it cool, and checked again.  Another 9 oz.

 

Tomorrow I will fill the tranny, make sure the diff's still full, and drive another 30 miles and check again.  Losing a quart of tranny oil every hundred miles I don't think I can live with.  It''s a bitch to refill anyway, without a lift.

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Too much oil in the diff. is likely to endanger the rear brakes too, if the inner wheel bearing seals are not the best.

 

Should there be a pinion seal on the differential?

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I'm sorry to say I don't know about the pinion seal....maybe I can find a schematic.  There's still the problem of losing oil from the transmission....that's my biggest worry in a touring car.

 

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A very frequent reason a seal is damaged is the bearing behind it is bad and allowing sideways movement of the shaft. This may be the case in your car. The rear bearing in the gear box may need looking at. Does that area get very hot in running?

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The mystery continues.  Today I filled the tranny (8 oz.), verified the diff was filled correctly, and took a 50 mile drive.  Let everything cool off a couple of hours and checked the diff, which was 3 oz. overfull.  Quite a difference from 9 oz. after 30 miles!  Guess I'll drive another 50 and check it again, just for the heck of it.  Since the tranny only holds 40 oz. I don't want to run it down too far.

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