My car does not know when to shift gears sometimes ... high revs but still in first ... shift to second at 40 ...squeals the tires . ..has been suggested the speed sensor is no good ... however it may also be that the transmission ECM is no good yet, you can not tell this unless you install a new speed sensor ... Is this a correct approach ... the car has 130 k on it ...

There could be several issues that could cause your GMC Envoy's transmission to not shift gears properly and the RPMs to rev high. Here are a few potential problems that you may want to consider:

1. Low transmission fluid: If the transmission fluid is low, it could cause problems with the shifting mechanism. You'll want to check the transmission fluid level and add more if needed.
2. Faulty shift solenoid: The shift solenoid is responsible for directing transmission fluid to the appropriate gear. If the solenoid is faulty, it could cause shifting problems.
3. Worn transmission bands or gears: If the transmission bands or gears are worn, it could cause the transmission to not shift properly and the RPMs to rev high.
4. Clogged transmission filter: A clogged transmission filter can restrict the flow of transmission fluid, causing shifting issues.
5. Faulty transmission control module (TCM): The TCM is responsible for controlling the transmission. If it's not functioning properly, it could cause shifting problems.

I would recommend having a mechanic diagnose and fix the problem for you. It's important to address transmission issues promptly, as they can lead to further damage if left unchecked.

No not the whole transmission. Could be a band or servo. It still has to come out and be tore down to fix.

Define idles high like its going high rpms before shifting or the engine itself is high reving if its high rpming when shifting its mostly you tranny going out you can try tranny fix oil I used it on my truck when it had shifting problems and ive had no problems since it takes a few days to work though and if its the motor itself it could be a lot of things but I dont know much of the problem based on what you told us

your shift silinoid is gone there is 2 inside the transmission

Without getting into transmission diagnostics that would require special tools, you can check the transmission fluid level. You could also drain and replace the trans fluid to see if this solves the problem. Changing the fluid is considered regular maintence, and will not hurt a thing. The trans will benifit from fresh fluids in the long run also.

As it should. Lower gear means higher engine revs.

Warning - downshift at too high a speed will cause damage to your engine and transmission.

Why are you dong this anyway? Saving the brakes? Pads are about $30.00 and should last 30 - 40 thousand miles. Downshifting puts unnecessary strain on your entire vehicle, even though it might be fun. It also increases fuel consumption and is hard on the tires.

Thanks for your question at FixYa.com

Hi,
Does it rev ok? If your motor revs high but still doesn't go fast, you could be having a transmission issue, where it's not shifting into high gear... I'm assuming it's an automatic.
I hope this gets you on track... Good luck.

sond like the torque converter came apart trans will need to come out highly suggest torque converter from tci

theres many things that can be the cause,instead of having people tell you to try this and that,change this and that,scan codes it will point you at problem area and start from there

Check transmission fluid levels,,replace lost fluid as needed,,,if this does not work,,take the car to a mechanic,,,sorry to say,,this kind of problem almost alway's requires a transmission mechanic to repair.