A similar question was addressed brilliantly by mstern01. However, my problem is slightly different. Was driving down highway and the car started making noises and sounded like something was moving around (broken belt, flat tire, metal in trans). As I got to a hill, it labored a bit to get up the hill. Made it up and down several hills. Once parked at destination for 2 hours, it started again and got me home. Next day started and ran with loud screeching sound but would not shift into gear. A few days later, same sound but would go into gear and move in reverse and forward but I didn’t drive it.
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Thanks for the compliment, I hope the earlier comment helped. As to your question, there are two very mechanical possibilities that I can think of right off the mark. First, it is very possible that the torque converter has taken some serious damage.
I don’t know how often you change the transmission fluid, but, I think every 30,000 to 50,000 miles is about right — about every two to three years, or sooner, if you do some heavy duty driving. (Heavy duty driving is either lots of short trips where the engine/transmission never heat up correctly; or in hot, dusty conditions where the temps can run up to 95 or more and stay there. Either type of driving really stresses the entire driveline.)
If you leave the fluid in there longer the chances are good that sediments and other contaminants will form almost rock-like deposits on just about any moving part in the tranny. If the buildup is big enough, it can cause all sorts of damage. In this case, I think the impeller or converter turbine, may have had a large enough buildup so that it went “out of true” or became imbalanced and, one thing lead to another, the parts were damaged.
The damage isn’t enough to cause the transmission to stop working, but it is enough to let you know that something happened. In this case, since you still have forward and reverse, I think your transmission has gone into limp-home mode which means that you have either one or two gears forward and reverse. The only reason they still work is to allow you to either get home and then to a service area or so you can drive directly to the service area.
The other possibility is that there was a buildup on one of the planetary gears that is causing it to come directly into contact with another gear and you are hearing the resulting gear-on-gear clashing.
In either case, you should get you Impala to a service area as soon as possible. It’s possible that you may be able to keep the damage to a minimum so that you will only have to have either the converter or the planetary gearing repaired which will cost you roughly $1,400 — depending on labor rates and location. You shouldn’t have to drop in a rebuilt tranny which will definitely save you money. Let me know what happens.