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        General >> Problems >> "Bumping" noise from rear offside wheel
        
(Message started by: KeithCEng on Mar 19^th, 2011, 1:49pm)

Title: "Bumping" noise from rear offside wheel
Post by KeithCEng on Mar 19^th, 2011, 1:49pm

My 1996 Cossie Saloon has recently developed a regular “bumping” noise from the rear offside wheel which occurs at roughly 2 “bumps” per second. These “bumping” sounds do not become more frequent with increasing road speed but they do become louder. Curiously, the “bumping” virtually disappears when driving uphill and when coasting downhill. Today, my local mobile mechanic replaced the inner and outer CV joints on the offside half-shaft – unfortunately, it made absolutely no difference (except to my wallet, which is now £105 lighter!).

I am beginning to suspect that perhaps this is something to do with the ABS/TCS (although neither system is showing an alarm light, apart from the normal “self-test” for 3 seconds when first starting). Am I right in assuming that the rear hub incorporates a toothed ring for sensing wheel speed and if so, would missing “teeth” on this ring fool the ABS/TCS into causing this problem by initiating “cadence” braking on the road wheel?

Any thoughts please?

Cheers

KeithCEng

Title: Re: "Bumping" noise from rear offside wh
Post by Highlander on Mar 19^th, 2011, 2:21pm

Any ABS or TCS operation should show on the dash warning light, anything wrong with the system would not be affected by downhill or uphill driving.

Is it a constant noise or irregular?

Shocks?

Spring?

Subframe bushes?

diff bushes?

antiroll bar bushes

Get underneath it with a crowbar and see if you can get anything to move ;)

Title: Re: "Bumping" noise from rear offside wh
Post by Snoopy on Mar 19^th, 2011, 2:40pm

If it was me I would look very carefully at the rear wheel bearings. When they start tp fail they can generate some random noises. Mine did when they started to fail. As well as checking those other things.
In my experience the only sure way of checking them is to disassemble or at least partially do so, but I am sure others will have more ideas and experiences

Title: Re: "Bumping" noise from rear offside wh
Post by KeithCEng on Mar 20^th, 2011, 8:32pm

Thanks to Highlander & Snoopy for their prompt responses.

Highlander asked whether the noise was constant or irregular - It is a constant "bump-bump-bump-bump...." whose frequency does not change with differing road speed; It seems to occur mainly during driving on the flat.

Snoopy suggested the wheel bearing as a possibility - The mechanic did check this when replacing the CV joints and said it was OK (although he didn't disassemble it).

How does the wheel speed sensor operate?

Cheers

KeithCEng

Title: Re: "Bumping" noise from rear offside wh
Post by Highlander on Mar 22^nd, 2011, 4:56pm

http://www.fordscorpio.co.uk/abssystem.htm

I think you're up the wrong tree with the ABS theory though ;)

I meant constant as in continuously whilst driving? right from moving off? or does it come and go?

on 03/19/11 at 13:49:10, KeithCEng wrote:

Curiously, the “bumping” virtually disappears when driving uphill and when coasting downhill.

That alone would suggest to me a worn bush of some type, the large bush at the front of the subframe or the diff mount bushes possibly? but there are many bushes in the subframe that could be at fault.

What about the opposite? coasting uphill, driving downhill?

Does it do it under acceleration or braking?

My theory is that its not so much the incline making the bump go away as in the tension being placed on whatever is making the "bump"

ie. hard acceleration (or climbing at lower revs) will place more tension on a worn suspension bush.

coasting may result in the same tension being placed but in an opposite movement.

therefore driving along normally may be alternating between the two giving you the "bump"

Big subframe bush is no.2 but I believe these are no longer available?

http://www.fordscorpio.co.uk/images/rearsuspension.gif

Title: Re: "Bumping" noise from rear offside wh
Post by KeithCEng on Mar 24^th, 2011, 7:56pm

Thanks to Highlander for that input - it is an interesting theory and I shall investigate further!

Cheers

KeithCEng

Title: Re: "Bumping" noise from rear offside wh
Post by Mike H on Apr 3^rd, 2011, 4:58pm

Not changing frequency with speed seems to negate wheel bearings et al (?)