I own a 1987 Volvo 240 DL wagon with an automatic transmission and I am having a problem with the overdrive. Occasionally the overdrive will not engage and the car will stay in 3rd gear at highway speeds. It acts like lockout the button on the shifter has been pushed, but there is no light on the dash indicating the lockout is engaged. If I hit the lockout button the light does come on and I hear a click from the relay, but it makes no difference in the overdrive status when I disengage the button. The overdrive does engage some of the time, and when it engages the lockout switch on the shifter will work and cause it to disengage. I have been told that most problems with overdrives are due to either the relay or the solenoid. Does the fact that the lockout switch appears to work (light goes on and off) when the problem occurs point towards the solenoid? Is there a way to determine for sure if it is the solenoid or switch that is to blame?