I have a 2007f550 auto trans with a flashing tow/haul light and a code of po791 coming up. Flashing light will clear in k/o position and remain off until placing selector in drive,after two feet or so of travel,an audible clunk and the trans acts like it is in third gear.reverse is no change and highway will not shift into o.d. The iss "A" is listed as the culprit but ford doesnt list a iss a. Any help would be greatley appreciated.

 

That code is Intermediate Shaft Speed Sensor "A" Circuit. There are three speed sensors on the TorqShift trans in your truck. From front to back there is a turbine speed sensor, intermediate speed sensor, and output speed sensor.

You have either a failed intermediate speed sensor OR a broken wire between the speed sensor and the computer. The intermediate speed sensor is on top of the trans in the middle of the transmission.

 

A broken wire at the weathertight connector. You were dead on mark,thanks for the heads up.