Ford gauge calibration
A couple of weeks ago Jonsee mentioned in a thread that the original Ford gauges could be calibrated. I, nor few others, knew anything about this. I recently bought a new gas gauge sender and investigated the 'calibration' of these gauges.
The gauge head is electrically an 18 Ohm resistor which heats a bimetallic strip that very slowly responds to changes. The senders must be wound with a very non-linear response if the gauge is to be anything close to accurate. What surprised me was to learn that the gauge responds linearly to the VOLTAGE across its thermals, not the current or the electrical power used to heat the bimetallic strip. At E the gauge will have about 1.3 V across it, at F the voltage is around 3.7 V and at 1/2 it will be halfway between at 2.5V. Although the deflection is linear with voltage the resistance needed to general those linear voltages are very highly nonlinear.
The photo below shows the back of one of the original gauges. The adjustments appear to be mechanical and simply change the geometry of the parts inside. Since I couldn't really see all those parts how the adjustments should work is somewhat of a mystery. What I do know is you can really screw up one of these gauges by moving those toothed adjusters.
Question for anyone: Do repro gauges have these adjuster holes?
The small adjuster hole in the lower left mostly adjusts the zero position of the needle and one in the upper left mostly adjusts span or how far the needle swings when the sender resistance changes from 78 to 10 Ohms. However, the adjustments are not independent. Changing either one will change the zero of the meter. Also, if you only use the max/min resistances to set the end points where you might like you can screw up the response in between such that the gauge becomes useless as a fuel gauge.
I tweaked the two adjustments to perfectly position the max/min points about a needle's width above and below E and F using the actual resistance of my new gas tank sender. When I later checked the gauge with a half full tank of gas the gauge read about 1/4. Some calibration. Luckily, I had taken measurements a few months ago as to what resistance caused what deflections BEFORE I moved either of these adjusters. As you can see in the picture I had also marked the original location of some of the teeth for each adjuster but failed to mark which teeth should point to those locations. That made it more difficult to figure out which tooth to align with which mark. I had to tear it all down again and use several resistors over the full range of the gauge while tweaking these two back to where they were originally with a correct meter deflection range.
Although these are related to calibration, unless you know a whole lot more about how they are supposed to work I wouldn't try to use them to make any major changes to how your gauge works. If you do, you may have the end points correct but the rest of the gauge range may not be what you expected.
There are a couple of indentions that look like dots on either side of the E and F of the gas gauge. The oil and temp gauges have similar dots but the descripton of where they are located is more difficult. It seems that the proper position of the needle for any of these gauges when the key is OFF is to be centered over the left most of these dots, for the gas gauge that is the one to the left of E.
For the gas gauge my revised chart of the correct deflecton for various resistances (in Ohms) for a properly operating '66 Mustang gauge is:
78 lower side of E
When you turn the key ON with a completely dry tank the needle will move from covering that lower dot to just below the E mark as long as the sender is hooked up. Don't confuse that movement with meaning there is anything in the tank. As gas is added to the tank the needle moves higher although there will likely be a gallon or so at either end of the scale that causes no needle movements.