Screech then fluttering sound

[DesmondD]

Bighorn 1993, V6 petrol, no turbo, auto trans: Several months ago I had a screech (yes a real screech) that lasted for some minutes, and then it stopped and has never come back. But since then there has been a sort of "fluttering sound" at most vehicle speeds, but never when the vehicle is stationary at any number of engine revs. The "fluttering" sound also happens even when gliding in neutral with the engine off. The speedo and odometers also stopped working at very much, or possibly exactly, the same time. I thought, OK, the speedo cable must have jammed and twisted and broken and is now whirring around inside its sheathing. But on my last WOF check the instrument cluster was replaced as my guy said the speedo itself was broken, as well as the speed sensor, that we also replaced. And I have been told by more than one person that the speedo is controlled electronically, and not by a mechanical cable, even though I have the older analogue instrument cluster. The fluttering sound seems far too regular and persistent to be anything like a leaf in the climate control air intake vents, but I mention this to help to explain what it sounds like. Any ideas??

[geeves]

Most ubs69 bighorns have an electronic speedo. Easy way to tell is to look at the rear output of the transfer case where you will find a cable or module with wires. You say the sensor was replaced so I would take it as read that its electronic. The auto takes its speed reading from the speedo so if the speedo fails its quite possible the auto is not changing correctly ie late changes and no lock up. The rev counter is a separate circuit so the only place a common failure could occur is in the cluster or power supply to it. I cant see how its related to the noise.
The noise sounds like a stone in a brake caliper or something caught round the driveshaft maybe

[DesmondD]

Thanks Jeeves. Today I jacked up one wheel at a time and on rotating each I couldn't hear anything strange. I also looked around the drive shaft (not when it was on the jack!) and nothing was hung up on it. I then confirmed a suspicion by running it donwhill, in neutral with the engine off, both forwards and backwards, and sure enough the periodic flutter-scrape sound is there, and a bit speed-dependent, when going forwards but not when going backwards. Can the noise be coming from whatever turns the electronic speed-sensor signal into something mechanical for the analogue speedo (I recall speedo and odometers don't work when a vehicle is reversing)? Although I must note that the whole instrument cluster was replaced, not just the speedometer dial. Also, it remains possible that I simply couldn't turn the wheels by hand fast enough to produce the nosie when it was on the jack - is there any tricky way to turn them faster? eg by wrapping my recovery strop around the tyre and hauling on the free end?? or by getting a four-arm X-type of lug-nut wrench and turning it when on a lug-nut?? Any thoughts appreciated.

[geeves]

Dont try the strop thing. You would have to pull it with a vehicle to get it faster than you can turn by hand and you could pull the car off the jack.
Try the same rolling test again but first remove the driveshafts. You will need a friend to tow you back home. This will eliminate everything except the axles and wheels if the noise is still there.
My money is on a stone stuck in the brake still although a wheel bearing makes the same noise. Wheel bearing makes the noise both ways. Brake often does not

[DesmondD]

Dear Geeves - a very late thank-you for your advice, and the sound went away by itself - I guess the stone kept wearing until it got smaller and fell out!. But I did discover something about speed sensors (their threading) that I will post in the appropraite Forum. Cheers, Des