One of the bugaboos of the late Series IIA and Series III Land Rovers lies in the electrical temperature and fuel gauges. These gauges are readily recognized by a needle that slowly rises to the indicated position. This is especially apparent in the fuel gauge at engine start-up. These gauges slowly rise because they are controlled by a bi-metallic strip that bends as is becomes heated by voltage traveling through it to ground via the sending unit. These gauges are also recognizable by a straight needle versus a needle with an arrow head.
To understand the mysteries of the gauge, one must understand how a bi-metallic strip works.
Bi-metallic strips have two metals, with different coefficients of thermal expansion, bonded together. When the temperature changes, the strip bends. The gauge needle is attached to a bi-metallic strip, and indicates the degree of heat at the strip. Voltage traveling through a small copper wire wrapped around the strip heats it in our application. In a perfect world, this would also indicate what the temperature or fuel sensor was sending to the gauge. Alas, however, the world is not perfect. The changing voltage of the charging system on an automobile introduces variable amounts of voltage into the mix, changing the reading. The ambient temperature behind the dash can also change the reading. For the gauges to read accurately, they must receive a sustained voltage that is affected only by the sensor. This requirement is usually around 10 volts.
Enter the voltage stabilizer. This device looks like a small metal box behind the dash with three, sometimes four wires going to it. One set of terminals is powered by the ignition switch. This should be switched power. The other set of terminals has wires leading to the fuel and temperature gauges. The circuit is completed by a wire from the other side of the gauge to the sending unit and then to ground. Please do not ask me wire colors, I rewired my Rovers and really do not have any idea of the original colors. There are several Lucas color code charts on the internet for reference. Because of the electrical path, it does not matter if you swap the wires on the gauge. It does however, matter if you swap them on the voltage stabilizer, as indicated by the male and female connectors there. The voltage stabilizer itself must be securely grounded.
Many people consider the voltage stabilizer to be a consumable item. I, myself, priced a replacement at $26, and decided I had nothing to lose by opening the old one up. I used a pair of end cutting dikes. Inside, I found yet another bi-metallic strip, and contacts much like points, to complete a circuit. In function, as the voltage decreases, the circuit is completed, allowing power to the gauge's bi-metallic strip, and then to ground via the sensor. When the voltage increases, the stabilizer's bi-metallic strip bends, opening the contacts and interupting the circuit. This opening and closing circuit can adjust several times a second. Because the bi-metallic strip in the gauge takes time to adjust, however, it does not move, instead averaging out the tiny bursts of voltage from the stabilizer as the required amount to sustain the needle in one position.
More interesting, however was my discovery of a tiny thumbscrew to adjust the gap of the contacts. By adjusting this gap, one can effectively increase or decrease the average voltage fed to the gauge, changing a given needle position. I also noted that the contacts of my thirty five year old voltage stabilizer were dirty. I cleaned them with 220 grit wet/dry sandpaper. There is a ground tab that must have good contact with the canister. I cleaned this as well. I then reassembled the stabilizer, carefully crimping over the lips of the canister it fits in with pliers. I screwed it back to the bulkhead behind the gauge panel, and hooked up the wires. I started the Rover, and watched as my indicators rose slowly. I reinstalled the gauge panel with the center screw.
To adjust out the temperature gauge, I had to know that the engine was not overheating and that it was not cold. I loosened the radiator cap to prevent pressure from building up, and drove the Rover for about 15 minutes. Once the engine was warm, and more importantly, the bulkhead was at operating temp, I confirmed the coolant temperature with a candy making thermometer. It was a steady 180 degrees. The bulkhead temperature and ambient temperature behind the dash also affect the bi-metallic strip inside the voltage stabilizer. Luckily, it was an unseasonably warm 78 degrees in Louisiana's winter. I decided that this was an adequate "normal" temperature. I took a look at the gauge, and saw that it was reading a bit on the high side, just below the red. Yet I knew that my engine was in ideal conditions, and not running hot at all. This is a common complaint with the late IIA and Series III Rovers.
Time to test theory. I loosened the gauge panel, and reached in with forceps to turn the tiny thumbscrew. As I turned it outward, I increased the contact gap in the stabilizer. This lowered the gauge reading. I turned the thumbscrew until I had the gauge on the N for normal. Because the voltage stabilizer also powers the fuel gauge, I saw it's needle move down as well. I must assume that the bi-metallic strips in both the fuel gauge and temperature gauge are identical, and therefore the fuel gauge is now reading more accurately. At any rate, with a lower reading on the fuel gauge, I am less likely to run out of gas. I reinstalled the gauge panel, this time with all the screws.
After doing this procedure, I searched the net and found that my tinkering had a firm basis, and others had been there before me. Here are links to a Sunbeam Tiger site that gives information on how to precisely set your gauges by this method. Because the Rover gauge is not calibrated, I felt this was not necessary, as an accurate "normal" reading would suffice.
Link 2
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