My 1989 makes a knocking sound from the rear of the car when the automatic transmission shifts up or down into 2nd gear. It's not the transmission. I had a transmission mechanic take it for a ride and we ruled that out. The sound is a single knock at the shift point(!?) and is much more pronounced when moving slowly. Tromp down on the gas and the car doesn't seem to make the sound. I also have a shimmy or shake beginning at 60mph. Not sure if they're related. We had it on a lift and ruled out universal joints. The mechanic is also doubtful its the rear end. What else is there to check? Anyone else ever have a similar problem? -Geo.
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Re: C4 Knocking Sound
George,
It could be a result of normal lash in the ring/pinion. If the transmission has any delay on the upshift, the rear will exhibit this because the ring/pinion will momentarily be on the "coast" rather than the "pull" if there is the slightest delay in your upshift. This won't normally happen on a hard upshift because there is little or no lag time on an upshift.
The noise that you describe can be induced by slowly accelerating to just before the shift point, and then releasing the throttle. The transmission will upshift, normally, with a slight "clang".
Joe -
Re: C4 Knocking Sound
Geo
I don't know what the noise is but my 88 does the same thing. It only has 12k on it so I have never worried about it, and nothing has fallen off! TomComment
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Re: C4 Knocking Sound
Joe,
Is this something to worry about? Will there be eventual problems with the rear end? And most important...do I have to live with this annoying sound or is there a solution? -Geo.Comment
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Re: C4 Knocking Sound
Georgie boy!!!
I'm very happy to see you posting anf getting some value out of youe membership. As you have already learned, this is a great bunch who welcome the challenge of solving problems. My suggestion ofr the 89 is to trade up for a solid axle car (a 61 perhaps???)
JimComment
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Re: C4 Knocking Sound
George,
As you know, sounds are very tough to describe, and certain noises are better diagnosed in "person".
It looks to me that a low mileage, well maintained car such as yours might be exhibiting normal rear axle ring/pinion lash "take-up" during light load shifting.
If you hear a moderately loud metallic "clang" during upshift, after you lift at or near the shift point, then this is normal "take-up". My low mileage '85 does it, and has done so since "mile 1". It sounds like Tom S's '88 does it, too. Most cars with auto trans do this, but the timing of the lift must be perfect. It should not do this during normal upshift, whether light or full load.
If the phenomenon is as described, above, then rest assured, and enjoy your new car. If it is not, and occurs on a normal upshift or downshift, then the first place I would look, is at the transmission. How are the shifts? They should be positive, with no lag and no slipping. A healthy 700R4 has nice, crisp shifts; moderate throttle shifts with L98 should be very positive, with a definite "surge" at the shift points. Full throttle 1-2 shifts should chirp the rear tires. Is the TV cable adjusted properly? Has the trans ever been disturbed/rebuilt/serviced/new screen/new fluid, etc etc. Fluid should not be brownish, but red, with no acrid odor.
If the transmission is operating as it should, with correct shift points, then I would look at all U-joints. Next, examine rear suspension and attachment points.
JoeComment
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Re: C4 Knocking Sound
Joe,
Thanks for the insight. I'll bring you suggestions to my mechanic.
-GeorgeComment
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Re: C4 Knocking Sound
George,
The "shimmy" may be the tires. If they are the originals, they may have flat spotted. They may also not be balanced well. The wheels, if original, are also not easy to balance.Comment
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Re: C4 Knocking Sound
Rick I have been driving my 85 since 1992. I always disliked the way the torque converter would "lock up" at slow speeds. There was a later modification by a chip change by GM, but I don't know what year this begun. I would get the similiar symptons with low speed driving, I would drive around town with the car in 2nd gear, especially in areas with the speed limit 30-35 on city streets with stop and go driving. I eliminated the problem with a on/off switch for the torque converter "lock-up" function. Very easy to install. There are two ways to do it, either buy the kit from a Corvette parts supplier Mid-America or others or just take two pieces of electrical wiring and buy a electrical on/off switch. The wiring is connected to wiring that goes to the torque converter lockup wiring that is under the dash. When ever you apply the brakes, a switch or even part of the brake light switch (don't remember at the moment) disenguages the torque converter lockup. What the switch does is inhibit the torque converter lockup, you decide when you want it. My switch is placed on the right side of my right knee where the drivers dash meets the center dash, no drilling and with easy reach. It is a different operating car in city driving, no drive-train noise. My 85 would actually do torque converter "lockup" in second gear when it was only used.Comment
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Re: C4 Knocking Sound
Jim.
Thanks for the insight on the torque converter...would have never thought of that one.Comment
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