SERVICE :: Service Tip #9

Senders vs. Switches and what they do

In our last Service Tip, we discussed what a Switch is. A common misunderstanding many people have is what a gauge sender and switch are and what job they do. We often have people walk in a request a gauge sender for their engine without any more information than it is a gauge and the type of engine they have. There is lot more that is necessary in order to get the right thing. Because electrical items are usually not returnable, it is important to get the right thing the first time.

Senders are a variable resistance device that alters the electrical signal that goes through them. That way they can move the needle on the gauge they are operating. The complicating thing is that there are many "Resistant Curves" on the Market. If your sender's resistance curve does not match your gauges, it will not work or will give erroneous readings.

There are 2 common Resistance Curves. We call them the US Standard and European Standard. The US Standard is 240-33 ohms and includes manufacturers such as Stewart Warner, Isspro, Teleflex, Medallion, Faria and many others. The European Standard is 10-180 ohms and includes VDO (just to confuse the situation, VDO also makes a line of senders and gauges that use the US Standard, but they are not as common). You don't need to know the numbers but it is important that you understand that they are backwards!

If you don't know the resistance curve of your sender, it is important to get the manufacturer and model of the gauge so that the resistance curve can be determined. Some manufacturers use multiple resistance curves!

The other thing that needs to be determined is thread size. Just as in Switches, most Pressure Senders have 1/8" NPT threads. But Temp Senders can use NPT, Metric, Straight Thread or even Whitworth. It is very important to determine this before you go looking for your sender. Isspro manufactures "kits" in both NPT and Metric. They supply a 1/8" NPT sender with various bushings in NPT and Metric. These are very handy when you don't know what you’re looking for. Usually the Manufacturer of the engine gives a clue as to whether it will be Metric or NPT.

To learn more about troubleshooting gauges and senders, see Tip #4 ENGINE GAUGES, SENDERS AND TROUBLE SHOOTING.

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Tel: +1(284)494-2830/440-2830

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