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  #1  
Old 08-29-2003, 12:21 AM
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Please help diagnose this sound

It's regarding my 87 300D. Today I started hearing a single thump from the rear every time I quickly release the throttle while driving at above 2000 rpm or so. If I release it slowly or at lower rpm I don't hear it. What could it be?

By the way I had new tires installed today and I started hearing this sound shortly afterward, but not immediately. I'm thinking this is a coincidence or could it be related? Lug nuts are all tight.
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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL
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  #2  
Old 08-29-2003, 01:46 AM
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Flex joint (name may not be right but it's what M-B uses instead of U-joints) on the driveshaft, maybe?

Just a thought...good luck!
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  #3  
Old 08-29-2003, 12:56 PM
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Torque link in the right rear is gone, get it and the one one the left replaced, or do it yourself.

Not difficult, but a PITA -- you need two 19mm wrenches, no really good way to get a ratchet on anything.

New links are from the W210 chassis, so you need a triple square 12mm driver and a "rebuild" kit and bushing kit for each side along with the link. All this is cheap.

To change:

Jack up rear of car and put on jack stands. Allow wheels to hang. Remove wheel, and find the torque link -- its the diagonal one from the upper side of the wheel carrier to the subframe. Thrust link is from bottom to front of wheel carrier.

You can tell easily, it's the one you can't get any wrenches to...

Unbolt both ends and remove link. You will have to pull it off the sleeve on the wheel carrier. Pull sleeve out of wheel carrier.

Install new short sleeve in wheel carrier hole, put bolt in link, and weasel the bolt into the wheel carrier. Tight fit, as the new bolt gives less wriggle in the link. install new bolt on the subframe side. Don't tighten either end.

Jack wheel carrier up level, then tighten bolts to proper torque (50 nM). You MUST have the links in the level position as the steel sleeve in the center is bolted down tight. All movement of the link is by the flex of the rubber bushing, so if you tighten it with the carrier hanging down, you will overstress the rubber.

Do both sides and have the alignment checked.

Sadly, you will also probably find cracked subframe mounts, bad diff mounts, and signs of wear in the rest of the links, too.

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 08-29-2003, 01:36 PM
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psfred, thanks a lot for the extensive description. I'm sort of mechanically inclined, but probably not to this extent so I'll see how much the labor would be. What does the torque link do and how can it go bad? Any chance it could've been caused by reckless handling at the tire shop?
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2004 VW Jetta TDI (manual)

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  #5  
Old 08-29-2003, 01:37 PM
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psfred, thanks a lot for the extensive description. I'm sort of mechanically inclined, but probably not to this extent so I'll see how much they charge for labor. What does the torque link do and how can it go bad? Any chance it could've been caused by reckless handling at the tire shop?
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2004 VW Jetta TDI (manual)

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  #6  
Old 08-29-2003, 04:30 PM
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MB's current rear suspension is a five link design -- spring link on the bottom perpendicular to the centerline holds the wheel carrier (hub with bearing and brakes) on a bushing. The spring sits here, top end is on the subframe. Camber link is the same on the top. In the center of the wheel carrier, front edge, there is a tie rod with normal tie rod end and an eccentric bolt on the other. Front edge top to middle of the subframe is the torque link, from the bottom edge of the wheel carrier to the front of the subframe at about 25 degrees is the thrust link.

These five links allow the wheel to move nearly straight up and down but not side to side or to turn in or out. There is some camber change with height to compensate for body roll in turns.


The torque link prevents the wheel carrier from laying over at the top toward the inside under torque loads (acceleration), and when the rubber bushing in it goes bad, the wheel carrier DOES move. This causes torque steer -- car pulls to the right under acceleration, and the wheel carrier will flop over the other way (tilted out at the top front) when yoyu lift your foot of the pedal. When the bushing gets pretty bad, the thumps when you do this. It will get fairly dramatic, with the body shifting on the suspension eventually.

No way the tire shop did anything. Probably the rubber finally tore while the car was on the lift with the suspension hanging down for a while. You will see shreds of rubber from the bushing hanging around the bolt and nut if they are bad.

While not exactly difficult, replacement is a PITA due to limited room -- you have to screw a nut off about a quarter inch of fine thread one flat at a time, and it's a locker nut, won't spin freely until it's nearly off. You also must have a floor jack and substanital jack stands.

Also, as I said, if one is bad, the rest aren't far behind, I'm fairly sure part of the thump (if not all of it, for that matter) is bad differential mounts (do you have loud shifts, with a bump in the read on gear changes?) and the subframe mounts are probably going. You can check the diff mounts by watching the bolt while you jack up the rear of the car by using a floor jack under the diff -- if the bolts move very much in the hole, they are shot. Diff mounts are on the rear of the diff, about centerline, plus one vertical one in the front.

I've got all this work scheduled on the 87 myself later this fall, plan to remove the subframe to get better access since I need to do all four mounts, the rest of the links, and diff mounts.

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #7  
Old 09-01-2003, 01:16 PM
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Peter, the diff mounts do seem to be shot. I can tell just by looking at the rubber which is torn and pieces are coming out. That's the first thing I'll have my shop replace. I'll also tell them to inspect the entire suspension. I already looked at the rear right suspension and couldn't see anything else wrong. I'll look at the left rear today.
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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL
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  #8  
Old 09-01-2003, 04:04 PM
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Look at the inside end of the torque link, it's the one that goes for some reason. If there are shreds of rubber, it's shot.

However, unless you have torque steer on acceleration, it is probably just the diff mounts, for now.

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #9  
Old 09-02-2003, 03:45 PM
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I don't think I have torque steer on acceleration, however strangely my alignment is off again even though I did front-wheel alignment last week and it was fine initially (judging by the steering wheel). But it's consistently off and not related to throttle input.

How difficult is it to do the diff mounts at home? So far the lowest quote I got was 3.5 hours of labor.
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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL
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  #10  
Old 09-03-2003, 01:14 AM
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Diff mount replacement requires dropping the diff to get clearance and a puller to remove and replace them. Usually seriously stuck, I have been reliably informed. Alignment is important on the right side one, there is a "special tool" for this -- measure the location fore and aft before removing it, make sure it's in the correct position or you will have problems getting the front diff mount back on.

I'm going to drop the entire subframe on mine as I have to do links, subframe mounts, and diff mounts, and it will be much easier on the floor rather than under the car.

dying links in the rear suspension will make the car pull one way or the other. When they get loose, you will get torque steer -- will pull one way on acceleration, the other on deceleration, usually with a nice clomp from the rear and eventually the sensation that the rear suspension is moving around under the car. It is.

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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