66-67 Dodge Charger Source Guide

When the instrumentation for our 60s-70s MoPars was designed, electromechanical devices were still far less expensive than semiconductor devices. That is why mechanical voltage regulators were used. Today, we can buy a semiconductor device for pennies (I remember paying DOLLARS for what is now a crude transistor). It makes no sense to stay with a mechanical regulator for your dash gauges, especially when you've updated to the MoPar Performance electronic alternator voltage regulator. To my knowledge, no one (yet) manufactures a retrofit regulator for the instrument clusters. But, with modern integrated circuits, it's relatively easy to fabricate one for yourself!

Before getting into the modern era, it might help to understand how the old type worked. There were two physical versions used on MoPars (they work identically). The A-body version was part of the fuel gauge; the others used a separate unit that plugged into the instrument cluster's printed circuit board.

The nominal 12 volts of the car's charging system is regulated down to (approximately) 5 volts by a thermal "bimetallic" regulator, similar to that used in a home heating system thermostat except that, in this case, the bimetallic strip is heated by a nichrome (nickel-chromium) wire element. As the heater element heats up, the bimetallic strip bends. When it bends enough, it opens a set of contacts - interrupting the output (and heater) current. The bimetallic strip then cools and closes the contacts, beginning a new cycle. The up-and-down voltage variations are smoothed by a capacitor (condenser) connected to the output. Over time, the contacts wear, and the heating element can go open. This is when your temperature, fuel, and oil pressure gauges stop working. Even when they are working, they tend to fluctuate up-and-down. It was a crude system but, at the time, it was the best thing available.

The New Regulator

The new circuit...

LM317T Connection Diagram (front view)

	V OUT	R2
	3.61	510 ohms
	3.84	560 ohms
	4.12	620 ohms
	4.40	680 ohms
	4.72	750 ohms
	5.05	820 ohms
	5.46	910 ohms
	5.88	1000 ohms
	6.34	1100 ohms
	6.81	1200 ohms
	7.27	1300 ohms
	8.19	1500 ohms
	8.66	1600 ohms
	9.58	1800 ohms
	10.51	2000 ohms
	11.44	2200 ohms

Bill Of Materials

Building the circuit...

The National Semiconductor data book specifies 240 ohms for R1 but that is not as easy to come by as a 270 ohm resistor so, the chart (above) shows the calculated output voltage based on the latter. If you'd like to be able to have a variable rather than a "selected" voltage adjustment, you can use (say) a 510 ohm resistor in series with a 1000 ohm potentiometer in place of R2. That will give you an adjustment range of 3.61 to about 8.2 volts. It's only a few pennies more.

IMPORTANT NOTES:

How it works:

Summary