My S90 cannot be engaged. A friend of mine suggested that the solenoid set or solenoid connection has problem. Can somebody tell me how to check the solenoids and/or solenoid connections through the wriing harness? Thanks.

The best, and easiest, way to check the transmission is with an OBD11 scanner. Checking solenoids through the wiring harness could cause more damage if you were to probe into the wrong place.

Thanks for the advice. I scanned the code and got P0740, which suggested that TCC solenoid may
have problem. I was told that we can open the tranny pan and measure the resistance of solenoid.
But the other way to measure the resistance is through wiring harness without opening the pan.
So where is the solenoid connector and where is the wiring harness? Thanks.