I drive a 1999 Pontiac Grand Prix FWD with 4-wheel disk brakes. All 4 wheels have had their brakes done in the past 6 months.
Starting yesterday I heard a loud thump coming from the rear whenever I hit a bump. It sounded like something heavy and loose in the trunk, like maybe a can of paint or something similar bouncing around. Every time I hit a bump I would hear a quick succession of THUPTHUMthumthump… and it would be gone. After listening to it for a while I figured it was coming from the left rear wheel.
Today I noticed that whenever I put my foot on the brakes while it reverse the left rear wheel made a single loud clunk / clang sound each time I pushed the pedal. It wasn’t a grinding sound or rubbing sound that comes with worn-out pads. FWIW it does not make that noise when moving forward, only in reverse.
I don’t even know where to begin looking. I figure brakes, but the bouncing / thumping sound whenever I hit a bump doesn’t sound like brakes.
I’m not supposed to do car repair in my driveway, so if I tear this apart I want to have a good idea of what I’m looking for.
Is braking normal? It can be hard to tell with rears, as they don’t contribute much to braking effort under most conditions. I would definitely find a way to get a look under the rear before you do much more driving.
Is there a park or closed store somewhere nearby where you can drive up onto or back out over a square curb? Use great care and be sure to set the tranny in a high gear or Park and stomp on the parking brake, but having the tail of the car hang out from a 4-6 inch curb can be enough to lay down and get a look up into the brake assemblies. If it’s a mechanical fault, one of those things should not be like the other, and you should be able to see a loose, broken or missing part.
That or drive carefully to a garage you can trust enough to look and give you an honest answer.
ETA: On rereading your post, it definitely sounds like a loose or broken shock. Anything else that would make that much noise would have noticeable driving issues. Try giving that corner of the car a good hard shove down, and compare it to the other side. A good shock will stop the rebound almost instantly; a bad one will allow it to bounce more freely, oscillate or make your favorite bang-clunk sound.
Bent wheel, or piece of trim, or something hitting the wheel. It’s possible you hear it only in reverse, because you are going slow in reverse, or because the way the thing is bent, it gets stuck in only one direction.
I would also lean towards the loose shock theory, but one other possibility is that something might have come loose in the parking brake. On this car you’ve got little mini drum brakes tucked up inside the rear brake rotors. If there’s nothing obviously wrong with the shocks, you may want to pull the rotors off and check that all is well. Maybe first just double check that the parking brake is working if you’re one of those people who never uses it.
Oh, yeah, totally. This is now my first choice. A corroded cable can make a thud-thud noise. I drive manual, so I put on my parking brake all the time, whenever the car is stopped, and sometimes at long lights, especially on a hill, when I’m in neutral. I forget that people who have automatics may go years without using the parking brake.
Sometimes in an older car, a little of the cover over the cable wears away, and water gets inside, then it gets rusted inside the cover. It makes it hang up, and you get the thud.
You may have a broken ratchet in your self-adjusting brake system. Self-adjusting brakes recalibrate each wheel separately every time you back up and apply the brakes, and pieces of that mechanism can come loose or break off, and roll around inside your hubs.
That’s the loud single clank when you brake while reversing.
So I pulled the left wheel off, and…
… the goddamn brake caliper had came off!!!
Well, not fully came off. The lower mounting bolt was gone completely, and the upper mounting bolt was loose. The whole caliper was swinging back and forth.
Holy shit. I had my kids in my car today!
I’m hoping that is the source of the noise. Obviously, the big thunk when backing up makes sense, the bouncing noise when going over a bump is less obvious. I’ll find a new mounting bolt and after putting everything back together I’ll see. The shock/strut assembly looks fine, although covered in dirt so a small fracture may not be obvious.
And yes . . . I realize the Irony of someone calling himself Lancia driving a GM. My dream is to own a Lancia, although I suspect if I did I would have these type of problems every morning just getting out of the driveway.
Holy crap! just be glad it was the rear-- that’s why nothing bad happened. I worked in a shop for a little while, and worked on vehicles in the Army. The rule I was told was that the front brakes take about 80% of the braking load.
Ooh, yeah, I’ve had that happen to me before, although on the front of an old SUV I had. On mine the caliper looked fine to casual inspection but made the most goddawful grinding noise when braking. Turns out when I hit the brakes the thing would swing out and grind against the inside of the wheel, but would go right back into place when I let off. Very weird! Double check that there aren’t any nasty wear marks anywhere before just re-securing the caliper and calling it good.
After 20 minutes fondling Google I’m not having any luck finding the caliper mounting bolts. I hope this isn’t a dealer-only part. I don’t really want to go down to Ace Hardware and get a regular bolt. I will, however, invest in some Locktite.
If you are putting a manual transmission in gear with the objective of holding a vehicle in place you need to place it in LOW gear.
Lancia posts,
You would not want to substitute any braking system bolts with a hardware store bolt. Almost any auto parts store will have all the parts needed and then some.
I do have a torque wrench, but it is one of those old beam-style wrenches. I have no idea how accurate it is, but I’ll be borrowing a click-type when I put this thing back together. I had a professional do the front, as I had the front bearings replaced in a shop so I had them do the brakes at the same time. One day a month or two later the backs started grinding with no warning, so it was kinda a rush job. In the dirt driveway, in the rain, getting dark… I confess I may have been less than patient and thorough with them. I’ll be taking both sides apart this weekend and re-checking the torque on all the bolts. Lesson learned
I ran down to the auto parts store, and they didn’t have the mounting bolts. The dude behind the counter said they didn’t even have an entry for them in their computer. That surprises me. I’ll go to the other stores in town tomorrow.
Incidentally, the mounting bolts aren’t the same as the pins. The mounting bolts are short, less than an inch long, and quite thick. The pins that the caliper slides on are different. Longer, skinnier.
