subject: Oil cooled Servo stabilizer manufacturer [print this page] Are you using original fuel level and coolant temperature gauges on your 1968 or later MGB? Unlike modern aftermarket gauges, these two old-fashioned gauges are thermal ("bimetallic") devices. Their indicator needle is connected to a short beam constructed of two dissimilar metals. The beam is wrapped in resistance wire, which warms-up proportionally to the amount of electrical current passing through it. The gauge needle's position is determined by flexion of the beam due to the differing thermal-expansion rates of the two metals. To work properly, the MG fuel level and coolant temperature gauges need to be used with an external Voltage Stabilizer. (Note: the 1968-1972 MGB oil pressure gauge is also a thermal device. However, because of the unique nature of its sensor, it doesn't require a Voltage Stabilizer.)
The schematic drawing below, "Stock MGB Voltage Stabilizer Circuit" illustrates how the fuel and temperature gauges are wired. In summary, the gauges are designed to receive a constant supply voltage of 10 Volts and it's the job of the Voltage Stabilizer to provide it. The fuel level and coolant temperature sensors, downstream of their respective gauges, are connected through their mountings to chassis/ground. The sensors act like variable resistors; their resistance changes with fuel level or coolant temperature respectively.
The Servo Voltage Stabilizer itself is nominally provided 12 Volts. However, we know from experience that the "voltage in" isn't so stable or predictable! A number of factors affect the supply voltage available, including: the battery's state of charge, the output of the generator or alternator at any given moment (which may be affected by engine RPM), and whether various loads on the system are "on" or "off".
