I'm experiencing soft shifts under a med-hard throttle condition on a '90 NA and was thinking who I could ask that owned a '90 NA automatic this question. I suspect I have a stuck line pressure solenoid. Under light throttle it shifts fine. No codes are present and A/T diagnostics checks ok. Solenoid has continuity through the coil which makes me think it is stuck in the closed position.
With the key to the on position, when the throttle is depressed is a chattering sound heard. This sound is heard on a '93 and when our A/T controllers are swapped the '93 still chatters. No sound is heard from mine which makes me think it is stuck or the 90 models are different.
