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MG MGB Technical - Diff or wheel bearings?

The back end of my 74 GT developed a high pitched whine that within a few seconds became loud howl, like a really bad opera diva, soon after a motorway run.

UJs were shot, diff was low on oil. UJs and oil have been replaced, the diva is still singing.

I know it's a long shot, but before I order an axle is there any rule of thumb for deciding whether a wheel bearing could be the culprit? Also, any bets on how many miles I can run for if I keep the speed low enough the keep the whine in the background? The timing is lousy.
SteveP

maybe jack the rear up and hold onto the wheel and see if there is any play, if there is i would guess the bearing is shot.
A P New

Of course, that makes sense! I'll try that this afternoon. Thanks.
SteveP

Wheel bearings usually make a low pitched droning noise.

regards John
john wright

If you weave from side to side while driving, this loads and unloads wheel bearings and the noise should change. If not, it sounds like your axle. Does the noise change on the overrun?
Art Pearse

Thanks for that. No noticeable change swinging the car from side to side, but volume is mid level at cruise, loudest immedaitely after lift off and quietest just after that, off load.
SteveP

Steve,

Your differntial is whining due to low oil and the resulting wear. Probably won't go away, but probably won't hurt anything either.

Sorry,
dave
Dave Braun

Dave, "won't hurt anything either" to me is great news! If I can keep running for a couple of weeks I'll then have time to fix it. I'm feeling almost optimistic.
SteveP

It's not likely to fail and leave you stranded. These units can go many, many miles while singing the blues. RAY
rjm RAY

Hi Steve,

You could try some diff / gearbox additive that may stop or reduce the whine.

Herb
H J Adler

The whine is due to the case hardening of the cwp teeth breaking down - my V8 has whined exactly the same as yours for the last 5/6 years and gets no worse or better - don't waste time or money on additives. The only reason I haven't sorted it is that mine is a very rare 3.3 axle which I don't want to change for a new 3.07 cwp.
Chris at Octarine Services

I've been running a growling 3.31 diff in one of my MGCs since 2003....including runs to North Carolina and to Colorado just this year.

I now use Lucas 85-130 hypoid in it. I've even got accustomed to the howl.

It smooths out at about 65mph...any lower or higher it gets pretty loud.

Not exchanged out for the same reason Chris gives.
Rick Ingram

If the noise changes as you go slightly between overrun and acceleration then it is definitely diff and not wheel bearings, which make a relatively constant rythmic growling. My roadster ran out and started whining, I ran it for many years like that (with oil, of course) and if anything it got quieter over time. Eventually I changed the axle for other reasons.
P Hunt

The reassurances have proved spot on, thank you. I've run the car for the last couple of days (~75 miles), albeit sedately, and if anything the whine has quietened a bit, only becoming noticeable over 50mph or on the overrun. I wholeheartedly recommend oil to anyone with a diff in their car!

Having shifted myself into overdrive, expecting the worst, I bought a supposedley good axle over the weekend but will take my time over things now, perhaps fitting it over the Christmas break.

I love having a B as a daily drive; it's always an adventure.

Thanks again to all.

Steve
SteveP

This thread was discussed between 29/11/2009 and 01/12/2009

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