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Hey guys,
1991 Mercury Sable 3.0 V6 - Car shudders (a forward-to-backward shake, not side-to-side or up and down) at 35 MPH. It's completely independant of motor speed, as it happens consistently at 35 MPH no matter what gear and RPM. It felt to me like some sort of harmonic vibration so I had all four wheels balanced with no change. Always starts right up, idles just fine, never stalls, runs beautifully at any other speed, but from 34 to 36 MPH it sure does shake. I feel it most through the gas pedal all though my leg (although passengers mention it, so I know the whole car is shaking) Timing it, seems like it shakes 2.5 times a second. Uphill, straight-away, doesn't change and I never get cheap gas. Even accelerating through from say, 25 to 40 MPH, I can always tell when it passes over the 35 mark because it shakes for those moments. I've tried waiting it out, driving at 35 for a solid minute or so to see if it might pass but it doesn't - I'm forced to avoid that speed, which really bites. I should add that this is also independant of how warmed up it is, as it happens from a cold start as well as after an hour of driving. I've checked to make sure it's not transmission-related by resting my hand on the floor shifter and the shake is not coming from my AT. It shakes the whole car and I feel it most from the gas pedal.
What could be causing this?

Thanks for any and all help!
 

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Wow, thats a pretty thorough explanation of the problem.
When I usually get a car with a problem they just tell me..... its broken, fix it.
Hmm... Try putting it in neutral while coasting at that speed to see if it still does it or not. It's an odd problem no doubt. I'd make a note of the RPM's at the speed of the problem and take it to that rpm range in neutral or at a higher speed at that rpm. I'm sure the other guys will jump in and give their ideas. Hope this helps!

EDIT: Also...... perhaps the egr is opening at 35mph and getting stuck open, open too much, isnt supposed to open (yet)? Hmm...
 

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WOW! Really good description, usually takes 3 or 4 posts to get all that info. I don't believe that it would be the egr because that would merely cause the engine to start firing less fuel and performance would go. If it was stuck open it would be really "stuck" like jammed open and would cause performance issues at all speeds and RPMs. It would mostly just cause a loss of performance, not a shaking issue like you have. I would say test the car in different gears, but you seem like you tried that.

My suggestion to finding the problem would be to accelerate to just over 35 (about 37) and get the car shaking. As soon as it starts to shake, get it into neutral AND TURN THE ENGINE COMPLETELY OFF. WARNING!!!!</span> You will lose ALL power steering. If you do this make sure you do it on a long straight away. You will be able to make minor course corrections, but even a lane change will be unbelievable difficult, but no problem to keep it straight ahead. If the car is still shaking at 35mph, then you can totally rule out the engine (it's not running, just sitting there, literally). However, if the car stops shaking when you turn off the engine, then you know without a doubt if it's the engine or not. This will instantly let you rule out the engine or whether it's the problem.

Now because you said that it's a front to back motion and not a side-to-side or a generalized shake, it definitely stands to reason that it is a problem of rapid acceleration and deceleration, which of course implies a drive train problem. Here's my 2 Theories:

1) Harmonic Balancer is shot: It's a LOOOOOONG shot, but it's possible that at 35mph, wheel rotational force is somehow imparting additional force upon the rest of the drivetrain, somewhat making the drivetrain fight itself and additional vibration in the drivetrain that the harmonic balancer cannot compensate for. This is a big *I think* as I'm not positive what the harmonic balancer does.

2) This is my better idea, and I bet 1 no one's though of it yet.
<span style="font-family:Optima">TORQUE CONVERTER CLUTCH LOCK-UP FAILURE

Oh damn I'm good!!! What I believe is happening is that the PCM has gone somewhat berzerk in the sense that it cannot determine when the best time to lock-up the TC is. What I think is happening is that it will lock-up, and then detect a slowdown of the turbine (tranny side of the TC) and so unlocks the TC to provide added torque response. It basically isn't making up it's mind about whether to lock-up or not. NOW, through the computer in the mix and it's 15 million cycles (15MHz processor) of calculations per second, and you get it pulsing the TC lock-up solenoid (what actually locks up the TC clutches) This rapid pulsing can cause a front-to-back shape. It's exactly as though in a manual car, you push the clutch in a little bit so you have slippage (exactly as though the TC unlocked) and then let go and let it snap back together and they clamp together (like the TC has just locked up). NOW, do this even 20 times per second and you get a front-to-back shake. And for sure, the computer can do it ALOT faster than 20 times per second.

Now, WHY? At somewhere between 30mph and 45 mph, the engine RPM and the car's speed through the transmission get close together, and at one point the engine side of the TC and the tranny side of the TC will be spinning at the same speed. At this time the TC *should* lock up to stop slipage and increase gas mileage while cruising. And so there is a sensor on the turbine that reads turbine speed and then there is the engine RPM. When they get close to each other, the PCM locks up the TC if it thinks you are cruising.

AND SO: I believe that MAYBE the turbine speed sensor has failed, so the PCM cannot detect the turbine speed, and the programing default is to lock up the PCM at 35 and above and to not lock it up below 35. Yet, when you accelerate past 35 it makes up it's mind and keeps the TC locked up. So then when you stay at 35, it doesn't know if your going to accelerate past or slow down. And, when it locks up, you lose torque and will slow down a tiny bit, but enough to drop below 35, so it unlocks, and you have more torque, so you accelerate past 35 and it locks, and everything starts all over again. The only way to test it though would be to wire in a relay on the TC lock up solenoid and connect it to an LED. The idea being when the TC locks up the light should be on, when unlocked its off. And when it does it's crazy bucking lock/unlock crap, it should flash really fast. Be sure to use a relay though, because you don't want to burn up the PCM or the solenoid. Using a relay will keep the LED and it's power source out of the engine's wiring harness and using a small relay on the solenoid line will not be any difficulty for the PCM to set of it and the solenoid. Have fun. I think I've answered my theory on the problem. Let me know and have fun!!

-mobiuslogic

Note: I just realized just how f****** long this is. Sorry to steal the thread, just think this is like 4 or 5 posts all put together.

EDIT: Also try the neutral test as by putting it in neutral with the engine on at 35mph, should tell the PCM to not lock up the TC. Oh and have the codes read, it should through a code if it thinks the TC sensor has failed.

So here's your test procedure:

1) Get to 35mph
2) Shift to Neutral
a) If shake continues, problem is likely engine. Shut off engine to confirm. If shake stops with engine, engine is the problem.
B ) If shake stops with neutral, then problem lies in the tranny. However you've ruled out the tranny itself so the problem should be the TC lockup I stated earlier.
3) Have codes read and confirm the problem.
4) REPORT BACK AND LET US KNOW!

Do that and you should nail the problem down and know what to fix. Also, I don't think that the TC itself as a whole is FUBAR (as in it got eaten up) as it would not be working at any speed, thus suggesting the lockup clutch problem.
 

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It could also be your inner CV joints causing the problem. If the problem goes away when shifted to neutral as mentioned earlier, this could be another possibility.
 

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Originally posted by dIESEL600@Jan 4 2004, 01:00 PM
It could also be your inner CV joints causing the problem. If the problem goes away when shifted to neutral as mentioned earlier, this could be another possibility.
Negative, this would cause problems at all speeds with a more profound problem to the wheel on the outside of the turn since it has to turn faster than the inside wheel, and hence make more noise if that side is the one going. Same goes for wheel bearings.

-mobiuslogic
 

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Discussion Starter #6
Thanks for all of the responses so far, and thank you mobiuslogic for such details. Sometime in the next few days I will bring it in to get the OBDI scan and see what it spits out. In the meantime, I'm going to coast in neutral some more and run through the gears to see if anything might change. I'll keep everyone posted!
 

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Originally posted by mobiuslogic+Jan 4 2004, 04:08 PM-->QUOTE (mobiuslogic @ Jan 4 2004, 04:08 PM)
<!--QuoteBegin-dIESEL600
@Jan 4 2004, 01:00 PM
It could also be your inner CV joints causing the problem. If the problem goes away when shifted to neutral as mentioned earlier, this could be another possibility.
Negative, this would cause problems at all speeds with a more profound problem to the wheel on the outside of the turn since it has to turn faster than the inside wheel, and hence make more noise if that side is the one going. Same goes for wheel bearings.

-mobiuslogic [/b]
Negative. I said INNER CV JOINTS, not outer. You would be correct had I said outer cv joints. Worn inner joints often present themselves as a vibration or shudder only at a specific speed or speed range, regardless of whether you are turning or not.
 
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A few things come to mind, and actually make sense.

- Don't shut your engine off while you're driving. Putting it in neutral will decrease the engine speed, and if it is engine related (which I doubt it is), the vibration will go away.

- I'll start with simple stuff. It is possible that the tires have separated belts, or flat spots. Does the car sit for long periods of time? How old are the tires? A tire can be balanced, even with flat spots and separated belts, but the symptoms of the faulty tires will still be there.

- Has the check engine light come on at all?

- Have you had any other recent work done to the car? What was it?

- Not sure of the Gen 1 shifter setup, but if it's possible to leave it in second gear while driving, do that. Accelerate to the speed that is usually vibrates at, and see if it still does it. My thought here is torque converter lockup. It will lock up in third and fourth, but not second. If it's a lockup issue, you should be able to tell this way.

- The INNER CV joints could cause this same problem, as mentioned earlier. Check the simple things, and we'll go from there.
 

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I would suspect something in the back end. If it were in the front end it would be shaking the steering wheel.
 

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Originally posted by dIESEL600@Jan 4 2004, 06:57 PM
Negative. I said INNER CV JOINTS, not outer. You would be correct had I said outer cv joints. Worn inner joints often present themselves as a vibration or shudder only at a specific speed or speed range, regardless of whether you are turning or not.
my bad, I was in a rush to get to work. My bad, my bad. My money is still on my torque converter lock-up theory.

-mobiuslogic
 

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Discussion Starter #11
I still haven't gotten my OBDI scanned. Maybe this week I'll get around to it. It's been very cold this past week and I noticed this evening that is was shaking much less. What could this be about? What could the cold have to do with it? Is the car just generally stiffer and more compressed from the cold? Doesn't make any sense to me.
Are the inner CV joints still be a possibility even though the shake persists out of gear?
Lastly, (my car is a '91) will an OBDI scan alert me if this is a TC problem?

Thanks!
 

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Its not my car but I have the side to side shake at 45 to 47mph under load uphill only.
Can it be the motor mounts? I had two replaced, mechanic is a great guy, came with the car! He pressed the brake and the motor wobbled, so we replaced two motor mounts. He feels the CV is ok, but I'm still noticing a wobble.
 

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check your brakes, rotars and all, and your antilock brake system, if equipped. Also pull your wheels off, clean the rims as well as possible, then replace them, making sure to have them on there correctly and torqued evenly. I had a tempo that acted that way under certain speeds, it was just from a warped rotar. My parents car acts that way sometimes or they lose their brakes all together at other times because of a bad wheel speed sensor and brake modulator
 
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