Temperature - Fuel - Oil Pressure gauges

The temperature gauge I examined from my 1970 Europa S2 is designed as in the figure below. Its construction is the same as the fuel and oil pressure gauge. The TC uses a mechanical oil pressure gauge, otherwise its gauges are the same and they were all made by Smiths. The Amp gauge is of a different design.

Operation

Electrical power from the "voltage stabilizer" is applied to one lead of a coil wound around a metal strip. The coil is made of about 30 turns of insulated resistance wire with a resistance of about 60 ohms. The other lead of the coil connects to the sender which completes the circuit path to ground. The sender is simply a variable resister whos resistance changes with change of temperature. As temperature goes up, its resistance goes down. This allows more current to flow through the meter coil which causes the metal strip to get hotter. The hotter the metal strip gets, the greater the deflection of the gauge pointer. According to Ohm's law, power is equal to voltage times current or P=EI. The "voltage stabilizer" is supposed to keep the voltage at a constant 10v so the sender varies current (I) by changing its resistance. This happens because current is equal to voltage divided by resistance or I = E/R. The coil wound around the metal strip heats from power applied to it. It may not be obvious but unlike many DC electric meters, this meter is not polarized. Because of this, it doesn't matter how the wires are connected to the meter. All that is needed to make the meter deflect is current flow through the coil. It doesn't matter which way it flows.

There is substantial thermal hysteresis (lagging effect) in the gauge. It takes time to heat or cool the metal strip. This is a good thing because the "voltage stabilizer" is not a very accurate device. It is more correctly a power averager. It's job is to try to make the meter behave as if it sees a constant 10 volts. In reality, it's simply a thermally activated switch that turns on and off at a rate determined by its own thermal properties. It doesn't actually lower the cars voltage to 10 volts. The slower it switches, the higher the average voltage to the meter because it is on longer. If the meter was a fast acting device, you would see the pointer alternating between its lowest reading and a reading at whatever the electrical system voltage happened to be at that moment at the switching rate of the stabilizer . There is also considerable mechanical hysteresis in the meter. After getting your engine up to a stable operating temperature, lightly tap the rim of the meter once or twice. You may notice a slight change in the reading.

BTW - the "voltage stabilizer" can be a source of interference to your radio. The on an off action of the stabilizer can make enough of a spark to generate RF energy. It will sound like clicking in your speakers, especially on AM. See the link to plans for an electronic regulator on the previous page for the ultimate solution.

The following diagram shows the layout of the internal components of the temperature gauge.

The following picture shows the actual insides of the temperature gauge from my old S2. You can see the coil and other parts. Notice the Calibration Adjuster. I have no documentation on how the gauges were originally calibrated but if you insert a suitable size flat blade screwdriver in the opening at the back of the gauge, you should be able to move the needle resting position. Notice that I installed a yellow LED (and resistor). I did this so that I could have a visual indication of when the radiator fan was on. Between the radio and the engine it is sometimes hard to hear the fan run. I think "idiot" lights have a place in a car as long as the car has a full set of gauges.

The following picture shows the radiator fan LED indicator.

How I Installed the LED in the Temperature Gauge

In theory it's simple. In practice, it's a bit of a challenge mainly because you have to disassemble the gauge and provide a method for connecting the LED to the world outside of the gauge housing. All it really consists of is a very small LED and a 1,000 ohm resistor. The resistor limits the current through the LED. Otherwise the 12 volts would burn it out. The resistor value is not too critical. Just use the formula R=E/I where R= value in ohms, E= 12v and I=max rated LED current in amps. For example, if the LED max current is 20ma, then the resistor should be at least 600 ohms (assume the LED to be nearly zero ohms). I didn't want the LED to be so bright so I used a higher resistance value. This is something you can experiment with. You will probably be happy with something between 800 and 1,200 ohms. Just use a 1/4 watt resistor.

I carefully drilled a small hole (5/64" - 1.9mm) in the meter face and mounted the LED bedind it with a small dab of J-B Weld Kwik. You can use any suitable glue but the J-B Weld is fast, strong and opaque. This prevents the instrument panel lamp from shining through the LED case. Before mounting the LED, I bent its leads back a bit so that I could solder wires to them. Don't bend them too far or they might interfere with the meter pointer. You will have to be careful and a bit creative in how you route the wires to the LED inside the gauge. If you are not careful, the wires will interfere with the operation of the pointer. You can see in the picture above how I looped the wires so that I could install the face and keep the wires away from the pointer.

I first did this to my old 1970 S2 (which I bought new). In the original factory design, +12v was always connected to the fan motor and the thermostat switched the negative (ground) side. My TC is the same way. Therefore, I connected the anode of the LED to one of the internal (left in picture above) meter leads and brought the LED cathode lead out through the meter body with a little jack I scrounged up somewhere. I ran that lead through the cabin to the thermostat switch or motor ground connection. I don't remember which I did but they are equivalent so do whichever is most convenient (neither - haha). Wire size is not critical. You can use 22ga stranded wire. When the thermostat switches on, the LED should light. If you can't find a jack to make a connection like I did, you could just run a pigtail lead out the back of the gauge. Just install a small grommet to protect the wire insulation.

I'm rewiring my Europa with relays and circuit breakers so I'm going to do things differently. I'm going to ground the negative side of the fan motor and switch the +12v side so I'm going to ground the LED inside the meter case and bring out the anode lead to the fan relay. I'm also going to use relays for the heater fan, horn and headlights. I was surprised to learn how few of those 12 volts make it to the headlights and fans. I expect to have much brighter headlights after I'm done. I'll document here the new wiring scheme.

The LED I used was like the one that Mouser sells for 24 cents. I think Radio Shack has something similar. The LED color is a matter of personal choice. I used yellow. I subscribe to the red is "alarm", yellow/amber is "alert/attention" and green is "working ok" scheme.

If you want to try this but are still confused after reading the above (it could happen), send me an email.

Last update: 27 Nov. 2003