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  #1  
Old 12th June 2012
mphilleo mphilleo is offline
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Default Unusual Vibration When Stopped in Gear

At this rate, I should make a project thread with all the niggling problems I have on my '93 900S auto vert. I have another problem that I've noticed since I bought it and finally deciding to try to address it. Just to clarify, I've cleaned the filter, changed the fluid multiple times to freshen the ATF in the torque converter and it shifts really nicely, actually. The motor is smooth as silk, too. Here's my problem: with the shifter in "D" at a stop sign or cross walk, and my foot on the brake pedal, the car vibrates strongly. In fact, it's even popped my hood open on a couple occasions. However, if it's idling in "P" there's no vibrations that are out of the ordinary and it holds nicely to 900 RPM or so (I have recently cleaned the IAC valve).

Suspecting my brakes weren't grabbing well, I recently replaced the bomb on my ABS system and had the fluid flushed and refilled at a trusted independent import shop. It brakes great, so I don't suspect it's the brakes. Since the transmission, motor and brakes are great, I can only suspect the motor or transmission mounts, but that doesn't explain to me why the problem would occur at specific times only. Has anyone had a similar experience or have suggestions on what to check? Thanks in advance, everyone!
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  #2  
Old 12th June 2012
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Your observation that it does it in Drive, but not in Park, and only at pretty much idle speeds, might help to narrow it down some. My "guess" would be mounts, or possible part of the suspension?
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  #3  
Old 12th June 2012
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Thanks for the input - any guess why it'd be so selective about when it rears its ugly head? And would you suspect motor mounts, tranny mounts or both?
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Old 12th June 2012
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I had that issue too when I owned an auto. Worse when the AC was on.

With the transmission in D & your foot on the brake, the transmission/engine block is under strain. Specifically, the block wants to nose up. If you step on the brake when you're or R, it ought to strain nose down.

Either way, if your front mount is shot, that'll make the transmission casing touch the mounting bracket or crossmember - that's where the vibrations are transmitted.

I suggest changing the front mount first. Of the other 2, the left one is a PITA to replace.
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Old 12th June 2012
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Even the tiniest amount of play between the Drivers and the Tripod joint can cause that.
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Old 12th June 2012
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Quote:
Originally Posted by Jim Mesthene View Post
Even the tiniest amount of play between the Drivers and the Tripod joint can cause that.
How to measure that? I take it also applies to the manual transmissions, right?
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Old 12th June 2012
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Vibrates in D but not in P...As others have said, sounds like a bad mount, especially if it will be "violent" enough to unlatch the hood.
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Old 12th June 2012
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Quote:
Originally Posted by supremedalek View Post
How to measure that? I take it also applies to the manual transmissions, right?
There's no procedure to measure it that I know of, but Driver wear is Driver wear. Round up the usual suspects.
Manuals will not do the same thing. The vibration I'm describing is transmitted through the axles at idle. They're always under power in an Automatic, but not with a Manual.
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Old 12th June 2012
mphilleo mphilleo is offline
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Thanks for the confirmation, everyone. I wasn't sure if the mount(s) could be the source of the problem or not. So based on what I'm reading, the front mount is the most likely culprit. Two questions. Is that a hydraulic or solid mount? Second, are there any other mounts I would need to have changed? I may just have my shop do it in one fell swoop since I've never done anything like that and don't have the tools.
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Old 12th June 2012
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Quote:
Originally Posted by mphilleo View Post
Thanks for the confirmation, everyone. I wasn't sure if the mount(s) could be the source of the problem or not. So based on what I'm reading, the front mount is the most likely culprit. Two questions. Is that a hydraulic or solid mount? Second, are there any other mounts I would need to have changed? I may just have my shop do it in one fell swoop since I've never done anything like that and don't have the tools.
They picked one of the two mount types depending on the barometric pressure of the day...

Chances are that it's solid. With this type, you can visually see the condition. Usually the rubber parts are sheered.



I took the opportunity to change the other 2 (left and right) as well. That's it - there are only 3.
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Old 13th June 2012
mphilleo mphilleo is offline
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Thanks for the clarification, James. So there's just the one solid transmission mount (front) and the two (side) hydraulic motor mounts? I will probably get the transmission motor mount from eeuroparts, but it looks like they only have Febi hydraulic side mounts for $113.19, whereas Rockauto has the Beck/Arnley version for $103.79. I'm guessing for the price difference, it's better to go through eeuroparts? While I was there, I noticed Rockauto has what appears to be a front mount under the motor mount section for $99.79...not really sure what that would be, if there's just the solid front trans mount and the two side hydraulic motor mounts. Heh, maybe I'm just splitting hairs here.
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Old 13th June 2012
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Some call all 3 as motor mounts. As it happens, the front is attached to the transmission, so it's strictly a transmission mount.

What's important is the type. There doesn't seem to be a pattern to what mount type went where - I've heard of all sorts of combinations before. And as some transmission front covers (to which the front mount is attached) are interchangeable, you might get mid-life switches. I had a solid front mount on my auto, then the solid mount when I converted it to a manual, and a hydraulic one when I replaced the manual gearbox. I couldn't reuse the transmission front cover & hence the solid mount as the covers were not interchangeable. Turns out one has an extra input shaft bearing on the cover...

Best to eyeball it before you order.
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Old 13th June 2012
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Have a professional inspect the mounts before you spend $500 having them replaced......Or not.
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Old 13th June 2012
mphilleo mphilleo is offline
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I'm not opposed to taking good advice. I might just do that. Though I admit I'm not crazy about going back to the drawing board if it's not them.
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Old 13th June 2012
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In my opinion, not having inspected your car, it's statistically unlikely that motor mounts are causing your problem. If it were motor mounts, why would it only do it in Drive at idle? Wouldn't it be worse under load?
If it is a motor mount, there's a 90% chance it's just the front one. If a motor mount were causing your problem, and I could inspect your car, I could confirm the diagnosis without replacing the mounts (before spending the $500). I'm nothing special, any competent professional should be able to do the same.
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Old 13th June 2012
mphilleo mphilleo is offline
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You make a good point. I may have to check some variables and take better notes. Part of me wonders if there's a vacuum problem or possibly the idle control valve is making trouble (is this possible or likely)? Part of me hopes it is the idle control valve, but then one would think that my idle wouldn't just be poor at stops, like I had mentioned. Honestly, I only have a semi-functional knowledge of automotive systems that I'm sort of grasping at straws to an extent. If the transmission and brakes didn't work so well otherwise, I'd suspect them. I'm sure there's more to this than I'm aware of, but maybe I need to see if I can narrow down the symptoms further. Barring that, I can always take it in to my mechanic. If nothing else, I suppose I can at least do a visual inspection of the mounts.
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Old 13th June 2012
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Just my .02...and certainly Jim has more experience on these cars than any number of us added together...

You feel the vibration in drive but not park or neutral. If it were a vacuum leak I don't think you would have such a violent vibration...enough to unlatch the hood. The left and right engine mounts, if fine, would provide rotational stability under load (after an initial engine/transmission lurch at takeoff). In drive, the torque force on the package would be "up." In reverse, it would be "down" which would be why you may not feel vibration in any other gear selection but drive. With your foot on the brake and in drive the car is fighting these two opposing forces. When you are driving, it's not. You could try to bury the skinny pedal from a stop to see what that does to everything...you might feel a very strong thunk as everything lifts at the front (or back if you will) of the engine/transmission
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So what you're saying is that I need to find a way to make my rod longer?
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Old 13th June 2012
mphilleo mphilleo is offline
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scantar, what you say makes sense to me (as I understand it, anyway) and points to the single front transmission mount as the likely culprit, unless maybe I'm not quite understanding you. Again, I'll give it a visual inspection, for what that's worth, but at least the mount is only about $30, so it's not a big financial burden. How easy or difficult is it for your average DIYer to change one of those?
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Old 13th June 2012
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Once you get things out of the way, the front is pretty easy to change. For sure remove the hood...2 12mm bolts. It's helpful to have a second poir of hands to lift the hood but I've done plenty by myself. Also the cooling fans will be in the way so it's easier if you can move those too. Once that's done it will be self explanatory. In fact, Banman and I removed a front mount in a junkyard with me on a pry bar taking the weight off the front of the engine and him turning the mount to where it could be angled out. 'Course it was a junkyard car so we weren't that concerned about what happened after we walked away from it
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So what you're saying is that I need to find a way to make my rod longer?
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  #20  
Old 13th June 2012
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I'm inclined to think Jim may be right on this one with vibration from the inner drivers. It was one of my opinions. Don't personally think it's the mounts. We'll see.
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