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Unread 07-21-2009   #1 (permalink)
Ivy66GT is online now SHELBY GT 350 Member

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Default 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
65 E
39 1/4
24 1/2
15 3/4
10 F

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.
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Unread 07-21-2009   #2 (permalink)
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Nice work and good write up!
I wonder if adjusting the DC voltage of the regulator for the span adjustment and using the zero pot for Empty setting will give a better center point reading.
Here is a voltage reg that is adjustable ($15)
Adjustable step down switching voltage regulator
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Unread 07-21-2009   #3 (permalink)
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That might work if you don't move the zero very far. Certainly changing the CVR voltage will change the span of the gauges. Don't go too high though since the power consumption of the senders at the top end of the scale (10 Ohms) may get so large as to fry the senders. Also, as you turn up the voltage the needle positions for both E and F will move to the right and adjusting the needle back to E may screw up the calibration. I built several experimental circuits powering the gauge from 12V instead of the CVR and decided the best thing to do was go back to the stock CVR.

The original CVR is also adjustable. There is a little screw embedded under a blob of red something to keep it from turning. How well that might work I don't know. You also almost need an oscilloscope to adjust the CVR since what it will do is vary the on/off time of the psuedo turn signal flasher inside.

Speaking of CVRs: When the key is first turned on the CVR does not cycle but puts 12V directly on the gauges for a couple of seconds. That is why you will see the needles move up really fast (compared to otherwise) for the first couple of seconds and then slow down as the CVR starts to cycle and the gauges reach their final position with 12V only applied about half the time. This gives you around 2 seconds to make quick DC measurements before the CVR cycling kicks in.
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