Overdrive not engaging
My E39 (1998 528i AT with OD) with 65k mi is not engaging the overdrive. Check Engine light comes on and off (mostly on). Experiencing no problem driving local, but bothers me to run at 3,000 rpm doing 68-70 mph. The rpm should be around 2300-2500, no? Is there anything I can do before replacing the overdrive unit? Any idea on the cost of replacing the OD unit? Thanks a bunch!