How To Replace Your MGB's Lucas Thermal Voltage Stabilizer
Upper left: the resistance-wire wrapped bimetallic element of an old-fashioned thermal voltage stabilizer.
Lower right: a solid-state electronic voltage regulator that can be used to replace it.

How To:   Replace Your MGB's Lucas Thermal Voltage Stabilizer

Install a Solid State Regulator Inside the Lucas Case

as published in British V8 Magazine, Volume XVI Issue 2, September 2008

text by: Curtis Jacobson
photos by: Jim Miller

Notes: this article references MGB equipment, schematics, etc., and the information may or may not be transferable to other vintage British sports cars.

Background Information

Are you still using your MGB's original fuel level and coolant temperature gauges? Unlike aftermarket gauges, these two old-fashioned gauges are simple "single-coil" devices. To work properly, they need to be used with an external "voltage stabilizer". Please refer to the schematic drawing below ("Stock MGB Voltage Stabilizer Circuit") for a quick illustration of how these instruments are wired into their respective circuits. According to the schematic, the gauges are designed to recieve a supply voltage of 10V (constant) from a device called a voltage stabilizer. 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" - the resistance changes with fuel level or coolant temperature respectively.

Reading from left to right, the first thing that we notice is that the voltage stabilizer itself is nominally provided 12V power. However, we know from experience that the voltage "in" isn't so stable or predictable! Any number of things could 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.

Stock MGB Voltage Stabilizer Circuit Diagram

Regardless of how unstable or unpredictable the 12V power supply is, the voltage stabilizer's purpose is to iron things out. One conclusion you might reasonably take from the schematic is that: "If the voltage stabilizer provides more than 10V both gauges will read high, and if the voltage stabilizer provides less than 10V both gauges will read low." So, the voltage stabilizer provides the gauges a stable, accurate 10V input to work with, right?

Not exactly! The original MG voltage stabilizer doesn't actually regulate voltage at all. Instead, it cycles power "on" and "off" to achieve an average voltage over time of about 10V. If you take an old-fashioned voltage stabilizer apart, you find a bimetallic element with a resistance wire wrapped around it, much like an "automatic reset circuit breaker". Heat in the resistance wire causes the bimetallic element to bend, rapidly opening and closing a set of contact points and thus switching between the nominal supply voltage of 12V and zero volts. That suits your old MGB gauges fine because your gauges have a lot of dampening built into them. If your gauges are sufficiently damped, you may not see their needles constantly wiggling as the voltage stabilizer chatters on and off; you'll only percieve the "average" readings that they're showing you.

By modern standards, thermal voltage stabilizers aren't particularly accurate or consistent. (For example, their output varies a lot between summertime and wintertime because their accuracy is affected by ambient temperature. Their output also varies as the bimetallic element fatigues and as their internal contacts grow dirty.) If your voltage stabilizer needs replacement anyway, or if you just want to dramatically improve the accuracy and consistancy of your fuel level and coolant temperature gauges, you may want to consider converting to a modern solid-state electronic voltage regulator.

Important note: if you replace your original MGB gauges with aftermarket gauges, you need to remove the old-fashioned thermal voltage stablizer. Modern gauges have a dual-coil design which provides accurate information regardless of supply voltage, but which may not be able to accomodate the abrupt on/off cycling of a thermal voltage stabilizer.

Shopping List

Solid state voltage regulators are inexpensive, but they may be difficult to find locally. The easiest places to purchase them are online. "Google" the part numbers to identify potential suppliers.

National Semiconductor's "LM2940T-10.0" and NTE Electronics' "NTE1953" are low dropout (LDO) solid-state voltage regulators. If you supply either of them with a DC voltage between 10.5V and about 30V, they will provide a constant output voltage of 10.0V. Similar to an old-fashioned bimetallic voltage stabilizer, they can't boost voltage: so if the supply voltage drops to below about 10.5V, these LDO's will "dropout" and simply pass through whatever supply voltage is available.

Note: there are other alternative voltage regulators that could also be used. One example is Texas Instruments' part number "UA7810CKCS" (a.k.a. "7810" or "LM7810") which frankly you're probably more likely to find at your local Radio Shack. The main advantage of the National Semiconductor or NTE Electronics devices is their somewhat lower dropout specification.

Optional: a small LED lamp and a 1000 ohm resistor.

Also needed: just a few basics including a soldering iron, solder, about six inches of insulated wire, heat shrink tubing (or possibly electrical tape), etc.

Solid State LDO Voltage Regulator Pin-Outs

Wiring in a Voltage Regulator

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1. Disconnect the car's battery and remove the original voltage stabilizer from the car. (Note: the voltage stabilizer is mounted on the drivers-side firewall just above the steering column.)

2. Open the voltage regulator by carefully prying back the tabs that clamp the metal cover to the plastic base.

3. Cut and remove the old bimetallic regulator mechanism, being careful to leave enough of the two terminals for soldering wires onto them later.

4. Prepare your solid-state voltage regulator by cutting off the center of its three terminals. (This terminal is nominally a "ground" connection, and it would be redundant with the mounting tab in our installation. They're connected internally...)

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5. Using a short length of wire, jumper between the solid-state voltage regulator's VIN (12V in) terminal to the "B" terminal on the plastic base. Carefully solder both connections.

6. Using a short length of wire, jumper between the solid-state voltage regulator's VOUT (10V out) terminal to the "I" terminal on the plastic base. Carefully solder both connections.

7. The LED indicator is optional. It is connected between the 10V OUT terminal and ground through a 1000 Ohm resister.

8. To function properly, the solid-state regulator must be electrically well-grounded to the rest of the vehicle. Accomplish this by (first) connecting it to the voltage regulator's metal cover with a machine screw and nut. Note: the voltage regulator itself must in turn be grounded to the car by its mounting. If the fasteners are corroded or dirty, they should be cleaned at this time.

Voltage Stabilizer base and connections

LED lamp

Disclaimer: This page was researched and written by Curtis Jacobson and Jim Miller. Views expressed are those of the authors, and are provided without warrantee or guarantee. Apply at your own risk.

Photos by Jim Miller for British V8 Magazine. All rights reserved.

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