'95 Suburban thumps after braking

Hi,
First let me say thanks for all the assistance I've received on this board. It has been invaluable and I hope others learn from my experiences/mistakes also... :-)
So the Suburban, she thumps, but only when I let off the brake (medium-hard braking). I've taken it to the local shops but no help. Everyone feels it, but no one can pin point it. First they speculated drive shaft, then tranny mount. I couldn't figure why they'd be looking there but "they're the experts"... In the end, no true source of movement was found.
I then looked at the suspension. I found a bolt missing from the leaf springs (it's a U-bracket that surrounds the leafs), replaced it, no help. Figured I'd live with it for a while to see if something breaks or becomes more obvious.
Then I noticed my tires were worn and needed replacing. Didn't give it much thought as they looked like dry rot had set in but I did take notice that they only had 10k miles on them. They were purchased from tirerack so I figured they just weren't a good place to buy tires. The next set I bought from Discount Tires and then had it aligned. Now 2k miles later, the right front is showing signs of obvious feathering. The left could also be but it's definitely not as noticeable. I can't help but think that the thump and the feathering are related. I checked the toe-in and it's set at 3/4". Seems close enough so my theory is that something is moving and causing the alignment to change but then resets such that the alignment goes back to normal after letting off the brake. I don't feel the steering wheel move when it happens though.
I've lifted the front and tried to shake things about but nothing gives. I only weigh 150lbs so I'm probably not big enough to do any meaningful shaking but I tried... ;-) Using a tie-strap w/ rachet I was able to get the steering links to move about 1/4" front to back but again I'm not finding any true source of movement.
Any thoughts? Should I just sell the damn thing? I can't afford to be going through tires every 10k miles...
Thanks, Omar
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The thump you are hearing when you apply the brake, is it in the front or the rear of the vehicle?
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twins0203
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Sounds like "drive line clunk" to me. Very common.
http://www.misterfixit.com/clunk.htm
JB
odelrio wrote:

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Sounds like a suspension conponent. I actually have the same sympton, and am going to look at it tomorrow. It feels like bind up, and goes thump when the break is released. Its not the drive shaft clunk. Worn bushings or ball joints are my first guess. Does the suspension feel like its bouncing more than usuall ?
Cheers

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Yeah, I'd say check the balljoints.
Very unlikely here, but on my parent's astro van, when you pressed the brakes on it the front did a clunk kinda like the one you are experiencing, we checked it out and it ended up being the upper control arm thumping back and forth when using the brakes because when the front end was aligned, apparently they didn't tighten the bolts and all of the front end shims had fallen out, giving the a-arm alot of room to move in.
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Could be that one of your rear axle seals is leaking EP80/90 oil onto the brakes and the oil is making the brake shoe momentarily " hang" onto the drum , you feel the clunk as the brake shoe springs pull the shoes free of the drum as you start to move again. The problem only manifests itself if the timing between brake application and restart is within the time it takes the brake springs to unstick the shoes from the drums. The solution is to replace the seal and possibly the axle bearing, degrease the brake assembly and install new shoes

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You need to grease the splines on the driveshaft Larry

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odelrio wrote:

If in fact the toe is actually 3/4" (toe-in)....that's way out to lunch. It's not close enough. You need to be more like 0-1/8" toed-in. 3/4" will burn up tires at the rate that you are describing.
Ian
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I agree with Ian. 3/4" is crazy out of spec. Also check (Very Carefully) the rear driveshaft U-joint at the rear diff. I had a problem on my '89 Jimmy(FS) that cause all sorts of thumping and shuttering during medium brakeing. I was out of town, went to a brake shop in Atlanta. Rebuild the brakes for $400.....NO Help. Went to Firestone and they suspected bad drums...I just paid $400 for brake job!!!. Well they replaced the drums, no help so they removed them. Then they said the dear was shotdue to metal in the lube. This rear was rebuild 8 mo. ago with a new Auburn diff. So I told them forget it. Actually they were really trying and refused any payment as they had not fixed the problem even though they worked on it for hours. They are on Cobb Highway near WindHill. Since I had to drive home to NY I did more extensive pattern testing and found it would not occur if I placed the trans in Neutral(auto). So I suspected driveline lockup. Asked PepBoys if they could replace the driveshaft U-joints...too busy to get a lift but if I just gave them the shafts they'd do it. I keep tools in the truck so I dropped the rear shaft in their parking lot and found one cup on the cross completely toasted...just metal dust but the other 3 were fine. That was the problem...seems when you brake the rear pitchs down due to the wheels forcing the brake shoe/ rear asm in the direction of rotation. This INCREASES the angle at the rear drive shaft and pinion. This extra angle caused the bad joint to bind up and jam. Never had that problem again. So all it took was a $15 ujoint to do it. None of the professionals I took it to had noticed it either. Sometimes it's the least likely that causes it.
On Sat, 03 Jul 2004 16:28:05 GMT, "shiden_Kai"

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I've got an Express Van that has started to do something similar. It only does it when I come to a complete stop after moderate to hard braking. As soon as I release the brake pedal and start to accelerate "thump". I have some thoughts on this. 1. Moderate to hard braking will cause the rear axle springs to wrap. 2. If you come to a full stop under these conditions, the wrap is held as a preload on the springs while stopped. 3. One of the other conditions while this happens is that the nose of the diff drops which can extend the spline/yoke connection by an inch or more. The longer the driveshaft, the less this distance is. 4. When the brakes are released, the preloaded springs release, at the same time, the acceleration loads the spline/yoke connection which is in the process of closing up due to the brake release. 5. The "thump" is the binding followed by the sudden release of the slipyoke.
This is just my theory, and would only apply to a thump after a moderate to hard FULL STOP followed by a moderate acceleration, such as when you realize you can't make the light and put on the binders, then hold them while the light is red. Light turns green, you release the pedal, and immediately accelerate, then "thump".
KenG
odelrio wrote:

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Jack it up and check the lower control arm bushings
KENG wrote:

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Thanks for all the suggestions. Here's my plan (now that the 'burb is running...)
First, I have an appt to have the alignment rechecked in the morning.
Next I'll remove the drive shaft and check the spline and u-joints (front and rear). Hopefully I'll find something.
I should be able to take care of this tomorrow and I'll report the findings.
Thanks again!
-Omar
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snipped-for-privacy@yahoo.com (odelrio) wrote in message >

Murphy strikes... On the way over to the alignment shop, the 'burb didn't start after a brief pit stop at the office. The battery sat too long after being out of commission for 3 months. Once I got the battery straightened out, the shop was too busy so we've rescheduled for tomorrow.
-Omar
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snipped-for-privacy@yahoo.com (odelrio) wrote in message

This had to wait and it seems this was not related to the thump anyways...

I removed the drive shaft and everything looked normal. The u-joints front and rear were not dry and did not bind in any way. Removing two of the caps still showed greased needle bearings, nothing unusual. On the ones I removed, I added a little more grease before putting them back. I did notice the absense of grease fittings. Not sure how these are supposed to be maintained but perhaps it's not needed.
The spline also looked normal. There were no sharp edges or evidence of wear of any sort. This too I added more grease just in case but I did notice the presence of ATF so I guess that's what keeps this lubricated.
I went for a test drive, attempted a few hard stops but didn't notice the thump. Maybe I'll have the wife drive it, she seems to bring out the personalities in our vehicles... ;-)
-Omar
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