I´ve had the error codes scanned on my 95 Riviera and they indicated error 640: QDM 1 Fault.
I found out this is a quad driver module, but where is this part, so I can check it?

Thanks, Lutz

quad driver built inside PCM which control several componant such as boost control solenoid,evap purge valve,and tcc solenoid inside transmission

Thank you for the detailed information!
Could the tcc solenoid inside transmission report an error if the transmission oillevel gets low?
I had a supplemental oil cooler installed befopre the error occured...