how do you test a gauge to find out what sending unit to use? I have a small gauge cluster with gas, water , oil pressure, i'm insure how to find compatible sending units. i.m sure it takes more than a multimeter. thanks for any advice.
If the gauges are SW then you need SW units. I have no experience with other brands but a multimeter will work for a gas gauge.
use a variable resistor, and connect it in place of the sender. Turn the resistor to get the gauge to read at the low end of the scale, disconnect it, measure the resistance. Re connect it, turn to read full scale, disconnect, measure the resistanc.e Then look in a sender spec book (such as a Standard Motor PRoducts buyers guide) to find a sender that has the appropriate resistance at both ends of the scale. yes, it's kind of tricky! and probably requires stuff you don't have.
You can test them with potentiometers and a meter. Make sure you're using the correct voltage when testing or that will skew the results. Most automotive-type gauges operate in a range, which could be as little as 5 to 30 ohms or as much as 70 to 250, so you'll probably need more than one size potentiometer. Connect power to the + terminal, then connect the potentiometer in series with the - terminal. Adjust the potentiometer so the gauge reads minimum value, then disconnect the pot and read the pot ohm value with the meter. Repeat to find maximum gauge value. This will give you the range, then find a sending unit that matches. If you can get within +/- 10%, the gauge will still be reasonably accurate. Also be aware that not all gauges work the same; some will have 'reversed' ranges. So a 5 to 30 range won't be the same as a 30 to 5.
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