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Gauges.. How to...
How to determine which sender is needed for a gauge
If you have a gauge, and you don't know the resistance needed for a matching sender, you can determine that resistance by doing the following:
First - Go to Radio Shack and buy an inexpensive potentiometer (variable resistor).
Get a 500 ohm or 1000 ohm (1 Kohm).
1. Remove the sender wire from the sender and attach it to the middle terminal on the potentiometer. Make sure the other end of this wire is connected to the "sender" terminal on the back of the gauge. Run another wire from one of the outer terminals on the potentiometer (either one) to ground.
2. Make sure the +12 volts and a ground wire are also connected to the gauge whose sender resistance (ohms) you’re trying to determine.
3. With the key on (power turned on to the gauge), rotate the potentiometer until the gauge reads "empty" (for a fuel gauge) or "low" (for a pressure gauge).
4. Disconnect both of the wires from the potentiometer. Take an ohmmeter (set to the lowest range possible). Measure the resistance between the center terminal on the potentiometer and the outside terminal that was connected to ground. Then write the value down.
5. Reconnect the wires to the potentiometer and rotate the pot until the gauge reads "full" (for a fuel gauge) or "high" (for a pressure gauge).
6. Again, disconnect both of the wires from the potentiometer. Take an ohmmeter (set to the lowest range possible). Measure the resistance between the center terminal on the potentiometer and the outside terminal that was connected to ground. Then write the value down.
You now have determined the resistance range of the sender you need for the gauge in question.
For example, if the value for "Empty" was 3.4 ohms and the value for "Full" was 90.8 ohms, you would need a 0-90 ohm sender.
If the value you wrote down for "Empty" was 17 ohms and the value you wrote down for "Full" was 188 ohms, you would need a 10-180 ohm sender.
The values listed for each sender are nominal, and vary plus or minus 20 ohms or so at either end. If you perform this test as described, it is a fail-safe method for determining the value of the sender you need.
Fuel Senders
10 ohms to 180 ohms VDO Ohm Range
0 ohms to 30 ohms GM Range (before 1965)
0 ohms to 90 ohms GM Range (after 1965)
73 ohms to 10 ohms Ford Range (before 1987)
16 ohms to 158 ohms Ford Range (after 1987)
73 ohms to 10 ohms VW Beetle Range
240 ohms to 33 ohms U.S. Ohm Range
Pressure Senders
10 ohms to 180 ohms VDO Ohm Range
240 ohms to 33 ohms U.S. Ohm Range
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