How To Calibrate Your Tiger Instruments
By Cullen Bennett
B9472658
September 17, 1999
The battery voltage changes, the ambient temperature under the dash, the "exact" value of the resistance required to produce the correct current are all variables that will cause the needle to point to a different place on the dial. Ideally we would like for this to be the "correct" reading that it is supposed to be indicating.
In order for a gage to "read" correctly, for all possible conditions, requires that all of these input variables must be correct (or at least the total effect) needs to be what is needed for proper indication. In the mid 60's mass production environment of the automotive industry, the best that we could hope for is a "relative indication" on the meters.
One of the most common complaints about the Tiger is that it is running HOT. Sometimes the gage "indicates" an over heated condition, while none of the other symptoms are present. There is no gurgling in the radiator, no puking of the coolant all over the driveway, there is no steam leaking from anywhere or the tell-tale "ticking" of metal parts that are being over heated. Usually (but not always) this apparent overheating is bogus, and it is due to a temperature gage that is not "indicating" correctly.
Another major irritation is to have the engine quit only to find out that you are out of gas. After all, the gage still says that there are 3 gallons in the tank.
My design utilizes an LM317T, an inexpensive three terminal adjustable positive voltage regulator. For this application the TO-220 package was quite adequate to supply the power considerations. This particular device has an internal over current protection circuit which will turn itself off if the output current demand approached 2 Amps ( which would have to be caused by a fault somewhere in the wiring).. The LM-317T also has an internal temperature compensation circuit that will maintain a constant output voltage across a large range of ambient temperatures. My particular design simply utilizes two identical circuits in parallel. One for the Fuel gage and one for the temperature circuit. A single circuit example is shown in Fig. 1 . This circuit uses a multi-turn variable resistor or "pot" as it is commonly referenced to adjust the voltage that appears on the output of the circuit. The actual voltage that it will be set for is selected as part of the calibration procedure. It will normally be somewhere around 9.5 Volts +/- 0.5 Volt.
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