I am looking for some advice on my 1990 Acura Integra. It shifts hard sometimes, and when I put it in park the S3 light stays lit, and wont show that it is in park, even though it is. While the S3 light is lit, it wont let me turn the ignition fully off to remove the key. I have to cycle the key a few times tell it lights up the P, then I can remove the key from the ignition. It starts up just fine every time.

I am not sure how to test or see if there is any codes, or not. I cant seem to find the TCU under the dash either, where is it?

Doing some research online, it looks as if it could be one of two things. The TCU could be bad, which also would explain the hard shifting, and the S3 light being on. It could also be the Shift Solenoid, which would light up the S3 light as well.

I dont know if anyone is familiar with this issue or not. All help and advice would be greatly appreciated!

Also, I did change the ATF fluid thinking it would fix the hard shift, but it didn't.