I assume the instrument panel has been unscrewed, but is still electrically attached to the dash harness.
To test the CVR you need to make sure the instrument case ground is present on the base screw of the CVR. This wire is black. (This is also the case ground for the panel lights).
The CVR needs a ground to work. You should make sure this ground is good either by using a test light or your VOM. Using your VOM place the red probe on a known good 12V source with the black probe on the CVR ground terminal. You can use this ground in further tests. If the ground is not good you cannot continue with these test.
Ford used the same color going into and coming out out of the CVR, so we will use number of wires.
The single black/green wire going into the constant voltage regulator (CVR) is 12V (switched power).
The double black/green wire coming out is regulated power to the gauges.
The CVR pulses the 12V coming in to create an output voltage that averages about 5V. On an analog voltmeter/ohm/meter (VOM) the needle constantly swings around 5V.
To test input to the CVR, red probe on single wire on CVR; black to chassis ground = 12V on VOM.
To test output from CVR, red probe on double black/green wire on CVR; black to chassis ground = about 5V average on needle swings. You can also test for the 5V average at the gauge itself.
For the gauges the "ground" is each sender. Each of the senders are just variable resistors to ground.
To test (assuming the CVR works) simply apply power to the CVR via the Ignition key, ground the sender side of each gauge and the gauge will climb. If each climbs, then the gauge works. This doesn't mean it's accurate -- it just means it functions and if there are issues with the gauge not reading at all with the sender wire attached it is either in the circuit to the sender or the sender itself.
These are the sender colors on each gauge. Remove the colored wire (index below) going to each gauge stud in turn and connect that gauge stud to chassis ground. Then, watch for gauge movement.
Oil=white/red.
Water Temp=red/white.
Gas=yellow/white.
Don't leave the ground attached or you will burn out the gauge.
You only attach it for a few seconds to see if the gauge needle moves.
As you might surmise, you can test each gauge while the instrument panel is in place. Simply ground the plug going into each of the senders by unplugging them and attaching a short test lead and grounding the wire. Turn on the key for a few seconds while watching the gauge being tested.
By doing both tests you can isolate the issue to either the instrument panel side, wiring or sender.
