Put a non lockup 518 in my 74 road runner last summer. Wired up the OD solenoid with a toggle switch. Once I got driving the car, I found that the transmission will shift into 4th without the switch, and then slips if I try to get it to downshift by accelerating. I completely unhooked the power and the ground, so that there is no power going to the solenoid. The transmission will still shift into 4th by itself. Is this the OD solenoid, or a problem with something in the transmission? I thought if its just a valve body fix, I might try to fix it and drive it this summer. If I have to pull the transmission, I will probably just get it rebuilt. Engine is a 400/451, hughes 3k converter, jvx adapter, new 3.73 gears. Transmission is unknown condition; although I put a transgo kit in it before the install. Thanks for any help!