1991 may have an OD button to turn OverDrive off and on
A4LD was Fords first computer controlled transmission
In '91 it will have 2 computer activated solenoids inside the trans, TCC solenoid locked Torque Converter once vehicle was moving, this gives better MPG since torque converter is now not slipping like older versions.
Then a 3-4 shift solenoid, OD solenoid, computer uses speed and engine RPMs to shift into OD(4th)
On the shifter side of the transmission just in front of shifter(not above it) there will be a 3 wire connector.
Wiring is like this:
http://www.explorerforum.com/photopost/data/503/A4LD_specification_chart_from_Transgo_.JPG
Lower right of image
You will need to remove this connector and then test the OHMS of each of the 2 solenoids inside the transmission.
The center pin(12v) is shared by each solenoid, so you measure OHMS from center pin to each outside pin, separately, you want to see under 50ohms, high ohms or N/C(no connection) means bad solenoid or internal wire.
26-40 ohms is the "good range", 0 OHMS is bad as well
Then with connector still unhooked turn on the key and make sure 12volt wire to truck has 12volts, if not then there would most likely be other issues than just no OD as there is not 1 fuse for TCC/OD wiring, it just comes thru ignition switch.
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