[cabundis]

I have a 89 mustang and as soon as it shifts into third gear it starts to vibrate fluid looks smells fine.
would it be tourq converter?

[dmorris1200]

Ford transmissions are notorious for doing this when the torque converter lock-up. I would recommend draining the fluid and installing a new filter and fluid. There should also be a drain plug in the torque converter so you can drain the converter too. They usually put the drain plug behind the inspection plate (behind the oil pan) or behind a round rubber plug on the bottom of the bellhousing. You have to rotate the converter till you find the drain plug. I usually rotate the engine with a socket and 1/2" drive ratchet on the crank bolt. It is important to get all the fluid out of the system (converter and trans). This may not fix her but it's worth a shot before getting into major trans work.

[KULTULZ]

dmorris is correct. If you are experiencing torque convertor shudder, a complete fluid change is recommended along with a -Friction Modifier-

If you will notice, they also have an aerosol flush to clean the oil cooler. You want to get as much out as you can.