1973 Duster - Fuel and Temp Gauge issue

It may be the VR but there are other causes and I've written about them here a LOT. One big problem is that the power path from the battery to the VR can suffer voltage drop, and so the VR power terminal sees low voltage, and 'tries' to ramp up charging voltage until the VR setpoint is reached. So by the time the setpoint is reached at the VR, the battery is boiling LOL

Here is how to check

VERY FIRST thing to try is with the engine running, pull the connector off the voltage regulator It should stop charging. This simply checks that the field wire is not shorted, and that the green wire brush of the alternator is not grounded

Turn key to run with engine stopped. Probe the ballast resistor to find the IGN feed from the bulkhead. This should be the terminals on one end which are jumpered together. Now with one probe on that terminal, either compare that point (to ground) to battery voltage, or put the remaining probe onto battery POS post. This will give you the voltage drop directly. You should read a VERY small voltage, not over perhaps .3V (3/10 of one volt). ANY drop there is added to the charging voltage of "normally" 14V So say, you read 1.5V there, the thing will charge at 15.5. [14V (vr setpoint)+ 1.5V (the voltage drop)]

Also the VR MUST be grounded and the ground point "at" battery NEG potential. You can check this, too. Make this test first with all acessories off, and again with heater, lights, etc powered on. Start the engine and run and fast idle to simulate "low to medium cruise" RPM. Stick one probe into the VR mounting flange, and the other onto the battery NEG post. As before, you should read a very low voltage, the lower the better. Zero is perfect Again, any voltage reading is added to the set point.

If these two points check OK, if the VR is grounded, and the battery seems good, replace the VR