JessicaR

I have no clue what it is, but when I release the breaks on my car while going in reverse I hear a loud "clanking" sound. Anyone have any ideas?

EvilJester

Backlash on ring and pinion, or bad u-joints?

bliffle

Most likely it's UJoints. The shop manual says replace the whole propshaft ($450 new, $250 aftermarket from mazdatrix), but it's possible to replace just the worn UJoint ($25?) if you know how. Check the Haynes manual. I'm getting ready to replace a propshaft now. Fortunately I had one ready-to-go that came with a good used SE transmission I bought a couple years ago. UJoints, like many other items, are just designed to go about 120k MTBF.

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JessicaR

Thanks guys, at least I have an idea now.

bliffle

UJoints are one of those things that's just like any other rear-wheel-drive car, so it has nothing to do with the rotary engine, so any neighborhood mechanic should be able to do it. All he has to do is put it up on the rack, immobilize the shaft with a long screwdriver thru the cross, undo the 4 little bolts holding it at the back and drop the shaft out. A gorilla can do it. Then he just puts that $250 aftermarket shaft from m'trix in OR he replaces the UJoint cups, needles and cross, which is only $20 but takes some skill and brains. I'd probably drive around to mechanics and try to get one to put in just the new $20 UJoint for $200 total, or if that doesn't work put in the $250 driveshaft (which I'd buy and walkin) for $100 or less.

Here's another way: if you're hardup for money and don't want to do the work, call the junkyard or PicnNPull that has Mazdas and ask them how much they'll charge for a propeller shaft that you'll pickup at the front counter. I would guess $50 or less. Go over and pick it up: take along a BF or brother or whatever, but usually there are some Good Guys around that will help you (it only weighs about 10 lbs and is 3 feet long). Then call around to mechanics and ask them to put it in for $50. Youi'll find someone. Walk that prop shaft in and 15 minutes later you'll be out with a decent propshaft in place for a total less than $100 AND you'll know how to do it next time. Very empowering.

All us 7 guys are cheap as hell, and most of us were Poor Students at one time or another.

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JessicaR

Thanks bliffle! One thing that strikes me odd is, why is it the Ujoint if what's causing the noise is releasing the brakes in reverse? It makes a noise even if I just lightly tap on the brake, meaning that I'm not applying nealy enough pressure to slow me down one bit. And the noise is always up front. My car is an 85 GSL if that helps any. I just want to be sure to give yall all the information I can. Oh, and when I say "clank" I don't necessarily mean "CLANK", it's not overwhelmingly loud, but you can hear it.

bliffle

Doesn't require much change in rotational speed between diff and propshaft to cause the clank. A mechanic can check the UJoint when he has it up on the rack.

There is another clank you can get while backing up: the rear brakes have a ratchet to take up slack as rear brake shoes wear in use. That ratchet is designed to operate when you stop while reversing. But that's a softer bigger clunk, not the metallic clink you get from a UJoint.

Truth is, there just aren't many alternatives to the UJoint for that symptom. Differentials are big and tough with case-hardened gears operating in a closed dirt-free environment bathed in a bucket of specially-designed lube that slops all over everything. But the UJoint is hanging out there right in the dirty atmosphere 6 inches above the gritty sand on the road and is lubed by a thin film of cheap grease that was put in 20 years ago, and then the poor thing has to bend thru an angle as the suspension goes up and down. No wonder they poop out after 100k! It's a wonder they last THAT long!

Be glad you don't have a front-drive car because that replacement is more expensive to buy and more expensive to install.

There seems to be an epidemic of UJoint failures this week:

https://www.rx7club.com/1st-generation-specific-1979-1985-18/how-can-you-tell-your-u-joint-bad-479788/

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JessicaR

Thanks for the information Bliffle. I truly appreciate all the help I get here!

bliffle

Breaking news! Film at 11! More than you ever wanted to know about UJoints.

https://www.rx7club.com/1st-generation-specific-1979-1985-18/how-can-you-tell-your-u-joint-bad-479788/page2/