Typically the cause of a window not rolling up or rolling up slowly is a bad motor, especially if the car is old and the motor has never been replaced.
However, there are several other possibilities such as faulty wring, blown fuse, or a bad switch.
Faulty wiring is easily checked with an ohm meter.
A blown fuse is also easily checked by checking the fuse controlling the windows on your fuse panel.
Another possibility, which is often the case in 90's Ford cars (though perhaps not for the 86 T-Bird), is the passenger side window switch is dead. If it is then the window will not roll down AND it will not roll down if you are using a switch for that window on the drivers side (if your car is equipped with two switches for the passenger window). On the 85 T-Bird both switches are on the same connection. If this is the case for the 86, then this is not the problem. However, you might want to check the switch anyway, because taking the door panel off for the 80's t-birds to get at the window motor is a pain.
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Yes. They do sell a window motor GEAR KIT.
Window regulators go out in these cars often. They are available at certain auto parts stores for around $30 each.
The fuse for the window controls on a 1986 Jaguar XJ6 is in the passenger compartment fuse box. It is in the second row down and the fourth fuse from the right in the fuse panel.
motor is mounted inside the door
1985 - 1988 Nissan Maxima is the only other Nissan model with power window motors that will fit a 1986 300ZX.
I would suspect an intermittent short in the wiring to the power window motor in the section between the door and the door jam.
at the back of the car
you are poor and have no money?