[Alan Roehrich]

Do not put Sea Foam in a transmission. Sea Foam is a good product, used in your fuel system. You might try a can of Trans Medic, followed by a complete fluid and filter change (if you do that, please make sure the old filter seal is removed before you install the new filter).

Have the computer scanned to see exactly what codes it has.

If the rest of the shifts have a normal feel and occur at the correct time, the TV cable is adjusted close enough that the cable is not causing a problem with OD and lock up.

Fourth gear, AKA over drive, should occur around 38-48 MPH under normal conditions on a level surface, followed shortly afterward by lock up, there are two separate events. Fourth is normally supposed to happen first, then lock up at around 45 MPH. Sometimes, on certain vehicles, lock up can happen first, leading to a somewhat harsh 3-4 shift. They also sometimes happen at the same time, leading to a stacked shift feeling.

Fourth gear is strictly a mechanical and hydraulic function, there is no electrical signal involved. Fourth gear is a combination of the 3-4 clutches and the 2-4 band, so if you have 2nd gear and 3rd gear, you have the friction materials to have 4th gear. If the vehicle will take off from a complete stop with the gear selector in D or OD, the over run sprag is good. So, if you have 2nd gear, 3rd gear, and do not need manual low (L1) to take off from a standing stop, you have what is necessary for OD or 4th gear. At that point, 4th gear becomes a valvebody problem, or possibly a servo problem. The servo is on the passenger side of the case, behind the filler tube, in front of the TV cable. It's the large round piece. I would not advise you fool with it if you are not well versed in transmission work.

It is possible that, the TCC solenoid is bad, the o-ring on the end of the solenoid is bad, one of the pressure or temperature switches that is in line with the TCC solenoid is bad, the brake light switch is bad, a fuse is blown, or you have a bad ground.

I would start with the fuse panel, then check the brake light switch (the part that controls the lock up solenoid is on the end, not on the side, there are two, 2 wire connectors). Any slight pressure on the brake pedal, or adjustment problem, can open that switch and stop lock up from happening. Then look for a vacuum switch on the firewall. Check power to the vacuum switch, and then check vacuum.