'66 Mustang GT - Constant Voltage Unit
 

Having difficulty getting y gauges and dash lights to work - anyone know how to check the "Constant Voltage Unit" to see if it's OK?

With a volt meter. Just check for +5 Vdc on the output side of it.

It is a regulator, changes the input 12V to 5V for the cluster. When it goes bad,
all your gauges will give erroneous readings except the fuel gauge; and ampmeter
if equipped.

Sample 5Vdc Regulator:
https://cimg3.ibsrv.net/gimg/mustang...f0c7cdc173.jpg
10.0µF 50V Axial-Lead Non-Polarized Capacitors.
+5V Fixed-Voltage Regulator 7805.

It puts out a constant 4.98 volts DC all the way up to a 1 amp load.
Put an inline fuse (2 amps) in the circuit in case there is a short.

A new unit will be like $26.00+ on-line. That diagram is in case you want to build your own
and put it inside the shell of the old one. Heat shrink tube all the leads...Voltage Regulator Circuit; 12V to 5V for older mustang gauge clusters.

CVU problem
 

Mustang Dude...thanks 4 the reply & diagram...and yes, fuel gauge and ammeter work and they others don't - HAS 2 BE the answer...getting a new CVU

The fuel gauge runs off the voltage regulator along with the oil pressure gauge and the temperature gauge. Unfortunately the wires are all Blk-Green. Make sure it is connected correctly. The Wire from the big connector under the dash (12V positive) is by itself. There are two wires on the other side.
https://cimg6.ibsrv.net/gimg/mustang...215e513569.jpg