Solid State Instrument Voltage Stabilizer

The fuel gauge and the coolant temperature gauge on the MGB (and many other british cars) are powered by a mechanical instrument voltage stabilizer (voltage regulator). The output of the mechanical stabilizer is a series of +12 pulses or pulse width modulation that approximates a +10V dc output. The stabilizer can be replaced with a solid state voltage regulator. You can buy such a device or build it yourself. This is a description for the construction of such a replacement.

The solid state instrument voltage stabilizer is constructed inside the old instrument stabilizer case using an LM317T voltage regulator, and two resistors; R1= 1050 ohms, R2 = 150 ohms. The above drawing shows the connections. Other values can be used if the ratio of R1/R2 = 7 and R1 is 1000 to 5000 ohms. Using a 1100 ohm resistor for R1 will give only a 0.5V error at the output. The closer you get to the ratio of 7, the more accurate the output will be.

First dissassemble the old regulator by prying open the case. Remove the female connector by drillling out the rivet. Save the ground strap from the old mechanical device. Reinstall the connector using a #4-40 screw. The #4-40 screw will hold the LM317T. The metal part of the LM317T must not touch the metal case. Cut the corners off the LM317 to make room and to insure it will not touch the case. Solder the nut to the LM317T heat sink. Drill a new hole for the connector if required to allow room for the LM317T heat sink. The connector and lead serve as a heat sink for the LM317T.

Solder the ground strap to R1. The ground strap is in the upper left above, and the lower left below. Solder the other resistors, R2, to pin 1 and pin 2. Solder pin 3 to the male connector. Wrap the leads to provide a mechanically secure connection. Reassemble the device making sure the ground strap has a good connection to the case. After assembly check each connector to chassis to make sure there are no shorts.

A good chassis connection is required through the mounting screw of the case just as the original device.