Strange metallic clunk in transmission...

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Strange metallic clunk in transmission...

Sometimes when my car shifts into 3rd gear while cruising at very low speeds(say 50km/h), it emits a loud, metallic "thunk" from somewhere along the drivetrain.

It's a 3-speed auto, so 3rd is a very big gear for such low speed, but I'm not entirely sure that has anything to do with it.

This generally only happens when I deliberately back off the gas to stay at a constant speed.

The car doesn't 'jerk' as a resul of this or anything, so I don't think it's a serious issue- just a very annoying, loud noise.

Anyone ever experienced this before or could maybe give me some idea of what's happening?

astro_rob's picture
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Hmmm...

What OS you running?

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Surley you meant

What nOS are you running Wink

Eudimorphodon's picture
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I don't know what your car is, but...

Assuming it's rear wheel drive you could have a bad universal joint. When they get worn out the changes tension along the driveshaft when downshifting can allow the shaft to, well, thunk. (I know there's some technical term for what happens, but it escapes me.)

Anyway. That happened in my old Toyota Celica, which for some reason had a two-part driveshaft with three U-joints, rather then a single shaft with two. The middle one wents bad, getting "thunkier and thunkier", until one fine day when the thunks actually managed to destroy the collar bearing supporting the center. Then it started making *really bad* noises. ;^>

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Motor mount?

A Ford Tempo I had years ago did this very thing. I never figured out what it was - as the rest of the car fell apart before the transmission did (it leaked everything.

I did do a bit of reading after the fact, though, and one -possibility- is a bad motor mount.

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Re: Motor mount?

eeun wrote:
I did do a bit of reading after the fact, though, and one -possibility- is a bad motor mount.

That's a good guess in a front-wheel drive car, yep.

It *could* also just be an overdrive clutch in the transmission kicking in and out. I've experienced cars that don't do that particularly smoothly. But if it's really a *thunk* I'd rule out other possiblities first.

Jon
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My wife's 94 Town & Country d

My wife's 94 Town & Country does some thunking I'm thinking is the motor or tranny mounts. Happens when making a laft turn, when revving too much in reverse, sometime when in drive... A nice solid thunk. Not always though. As for Disco, with only 3 gears I'd guess that the sudden change in torque might lift the motor/tranny might be enough to move the something around. Does the car make any other noises when driving? How is the motor positioned, ie. crankshaft in line with center of car or transverse w/ crankshaft in line between the front wheels? Also, often you can lift the hood and rev the enginein D or R with the parking brake set and your other foot on the brakes to see if it moves around much.

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OK

It's an '83 Volvo 240(rear-wheel drive).

The engine does move a fair bit under the torque load, but Volvo designed the engine mounts to compensate for this... and the sound seems to be coming from the mid-to-rear area of the car.

Hmm... off it goes to a mechanic.

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