69 Dart dash volt regulator
Just the IVR? doubt it. You would have to simulate the load of the gauges.
Here is some of what goes wrong..
Obviously the IVR itself
If only one gauge works or seems inaccurate, the gauge unit
gauge sender
wiring between
ANY connections, including the wire end to the sender
At the dash, poor connections at the cluster connector and broken / loose pins on the circuit board
Also, bad connections between the IVR contact fingers and the printed circuit
And loose / corroded nuts on the dash instruments.
You can see, that after 40 plus years, "it adds up."