What is involved with switching the original 84 thunderbird speedometer cluster with on out of a 84 turbo coupe?
The tach is the only difference but it probably won't read right since it's calibrated to read 4 impulses per revolution, if you use it on a V8 for example it will probably read twice as high.
I believe the pinout is different. The plug is the same, but you'll need to swap some wires around if I recall correctly. Obviously the speedometer side is mechanical and will work fine though. The V6/V8 cars weren't wired for the tach, so there won't be a signal there by default.
I don't know if anybody figured out how to make the tach work with a non-4-cylinder or not.
I would do what we did to solve the charge circuit on my son's sport.
I would purchase an aftermarket tach
remove the internals of the new TC tach
replace the internals of the old tach with the aftermarket tach keeping the housing & face & needle
basically you insert the new stuff inside the tc tach housing so pick the brand wisely by its internal guts size to make sure it fits inside.
wire the tach up like the instructions say basically not using the cars supplied harness at all
I tried to make an aftermarket tach fit in my housing... didn't work for a relatively simple job.
good luck