Mechanics

99 SHAKES AT IDLE IN DRIVE W

1999 Pontiac Grand Am

Shakes or Wobbles problem
1999 Pontiac Grand Am 4 cyl Front Wheel Drive Automatic 124000 miles

I have a 1999 Grand Am with a 2.4L 4 cylinder, automatic. When idling in drive with the a/c on and sitting still at a light, the whole car shakes. The steering wheel vibrates, the floor vibrates, and the dash shakes up and down. The engine is at 600 rpm under these conditions, and if raised slightly to around 750 or so, the shakes go away. If you put the trans into neutral or reverse, the shake will also go away. If you turn the a/c off, the shake is still there, but much less so.

I checked the motor and trans mounts, and found the front trans mount cracked. I replaced it, but the shake is still there. I also noticed that when I drive into the driveway, which has a slight uphill slope, and park with the trans in drive the shake is there. Put it into neutral, or reverse, it is gone. WHEN IN PARK IT IS GONE, UNTIL TOU TAKE YOUR FOOT OFF THE BRAKE PEDAL. At that point the car coasts back slightly until the transmission is holding the car, and the vibration comes back. This had me convinced that the front trans mount was the culprit, but as I said, replacing it made no difference.

The engine runs fine otherwise, and performs well under acceleration. There is no check engine light or codes. It is not a problem in the tires, wheel bearings, suspension, or any of that, because it only does it sitting still.

Is it possible that the rear trans mount has gotten weak with age? It looks OK, and does not have any cracks that I can see, but is the only logical answer that I can come up with. If it was anything in the engine itself causing the shakes, why would it only do it in park once the transmission is holding the car? The driveway only has a slight incline, but it is enough for the car to roll back if put in neutral. Putting it in park does put slight pressure on the trans mount, but not much.

I would just change the rear mount, but it is $60.00, and does not look to be any fun to get to either. I am sure that this is not a normal condition, and that someone else has already had the same problem. Hopefully that person will be willing and able to help me. Thanks in advance.
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Imgumby
May 7, 2010.



Mount may look good, but put a large screwdriver between mount and frame, see how much play there is.

It is also possible your idle is too low. Idle specs should be on label in engine compartment.

Mmprince4000
May 8, 2010.
" PostPosted: Sat May 08, 2010 5: 50 am Post subject: Mount may look good, but put a large screwdriver between mount and frame, see how much play there is.
It is also possible your idle is too low. Idle specs should be on label in engine compartment.&Quot; Thanks, mmprince4000, for the response. The rear mount has exactly the same movement as the new one that I just installed on the front, so that does not appear to be the culprit. I pulled out the mount on the right side of the engine near the strut tower, since it is relatively easy to do, but it appears to be fine also. The mount on the left side is a lot more difficult to remove, so I gave it a close examination with a mirror and light, and it too looks in great shape. No signs of cracking or excessive movement.
As for the idle speed, that is controlled by the engine ECM, and is not adjustable. The IAC (idle air controller) on the throttle body which is used by the ECM to set idle speed is working fine, so that is not causing it either. Any other ideas? This thing is making me pull out the few remaining hairs on my head.

Tiny
Imgumby
May 11, 2010.
If you rule out engine then you have to look at trans.

Excessive hydraulic pressure will cause a shake, but a trans shop can measure hydraulic pressure.

Mmprince4000
May 12, 2010.
" Excessive hydraulic pressure will cause a shake, but a trans shop can measure hydraulic pressure.&Quot; Excessive hydraulic pressure may cause the shake in drive, but that would not account for the shake in Park when I take my foot off the brake pedal, and the car comes to rest against the parking pawl inside the transmission. (Please refer to paragraph 2 of my original posting.) The fact that the trans shifts perfectly, and smoothly, and drives completely normal otherwise leads me to believe that hydraulic pressure is not causing this. Excessive pressure should cause harsh shifting, like a shift kit does. The clutch pack is applied in drive, even when you are stopped at a light. At that point, the torque convertor slips, effectively disconnecting the engine from the transmission. As long as the engine is at idle, it really does not have much forward force on the vehicle. If the lock up clutch in the convertor was staying applied at the stop light, the engine should stall, and this is not the case either. If the convertor clutch was applying and releasing rapidly while at the light, that could cause the shaking, but then it should have convertor clutch chatter when it engages while driving. Plus the convertor clutch is not used when in Park. Loosing still more hair.

Tiny
Imgumby
May 12, 2010.
TCC is possible cause, if locked in park, you would get some shaking, but if fully locked it would die when put into drive, so it may be the solenoid is partially locked.

Try disconnecting the electrical connector to it.

Mmprince4000
May 13, 2010.
Disconnecting the connector to TCC made no difference at all. Still shaking like a vibrating bed in a cheap motel.

Tiny
Imgumby
May 13, 2010.
Then the TCC will have to be changed, although I would have a trans shop or dealer test the solenoid.

They can isolate the circuit and see if it is working.

Mmprince4000
May 14, 2010.
Mmprince4000 wrote: " Then the TCC will have to be changed, although I would have a trans shop or dealer test the solenoid.
They can isolate the circuit and see if it is working.&Quot; I'm not sure that I understand. I thought that disconnecting the TCC connector eliminated the TCC as the cause of the shaking, because it made no difference. If it would have been the cause, the shake would have gone away. Even if the TCC was locked in park, how would this cause shaking?
The TCC eliminates the fluid coupling between the engine and transmission
by engaging a clutch inside the torque convertor, making a direct mechanical link between the engine and transmission. It should not matter if the TCC clutch is engaged when in Park, since all the other clutches are disengaged. In effect, the transmission is in Neutral, except that the output shaft is not allowed to rotate. The car does not shake in Neutral, and only shakes in Park once the transmission is holding the car.
If the TCC was engaging and disengaging at a rapid rate, why would it not shake when the car is rolling? It only shakes while sitting still. If the car is moving, even at a very slow rate, the shake is not there.

Tiny
Imgumby
May 19, 2010.
TCC is a solenoid, so even though power is off, solenoid could be stuck " on".
Solenoid shaft could be binding in " on" position.

Mmprince4000
May 20, 2010.
I got it solved. Just in case anyone else has this problem, here is what I did. I replaced the rear motor mount, even though it looked fine. I found that the local auto parts store has an insert for only 17.00 rather than the whole mount for over 60.00. Not exactly a fun do it yourself project, but it can be done. Replacing the insert was the easy part, getting the mount out of the car was the hard part. This still did not fix the problem though. It still had the shakes in Drive while sitting still, and in Park when the trans was holding the car.

I could see that the inner part of the mount was very close to hitting the pad inside the mount, and that a very small amount of pressure on the mount would cause it to contact. This would transfer the natural vibrations of the engine to the chassis, like a drumstick hitting a drum. I unbolted the RIGHT side motor mount, and inserted 2 extra thick flat washers between the studs and mount. (You could use 2 or 3 standard 1/2" flat washers on each stud) This effectively raised the engine just high enough so that it would no longer contact the pad inside the mount when the transmission was in drive and the car was sitting still, or when in park holding the car. The vibration is completely gone!

Tiny
Imgumby
Jun 5, 2010.
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