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How to test an analogue resistance gauge

What is the resistance of a gauge?

Analogue gauges work on variable resistance.  As the input resistance changes, so does the needle on the gauge.  Senders (sensors) and gauges must both have the same resistance range.  Some gauges will work on a rising resistance – with the needle rising as the resistance increases – and some gauges will work on a decreasing resistance – with the needle falling as the resistance increases.  There is no advantage to either type, the only important thing is that the resistance range of the sender matches the resistance range of the gauge.

If your gauge is working backwards – the needle rising as the level of the tank is falling – then you have a mismatched sender and gauge. 

If an analogue gauge has failed, the needle will not travel from completely full to completely empty.  Generally, if you have the full range of movement on the gauge needle then the gauge is working correctly.

To test an analogue gauge, just disconnect the sender wire (usually black) and touch it to battery negative.  By connecting and then disconnecting this wire you should see the needle travel from full to empty.  If it does this successfully, it is most like the sender that needs replacing.

 The most common two resistance ranges are know as the European specification which works on a range of 0–190ohms or 10–180ohms [there is no practical difference between the two] or the American resistance specification which works in the range of 240-30ohms.  

There are other resistance ranges but these tend to be on very old products or for very specific applications. 99% of gauges will be working on Euro or American specifications.

 

To check to see what resistance the gauge is, follow these simple steps...

 

If you have access to the wires on the back of the gauge;

  • Power the gauge off
  • Disconnect the black wire (sender) 
  • Power the gauge on

  • If it's an American gauge the needle will show EMPTY
    • Touch the black wire to earth, the needle will go to FULL

  • If it's a European gauge the needle will show FULL
    • Touch the black wire to earth, the needle will go to EMPTY

 

If you don't have access to the back of the gauge;

  • Disconnect the two wires in the loom that attach to the sender.
  • Touch the two wires together (on gauge side of loom!)
    • If it's an American gauge the needle will show EMPTY
      • Touch the black wire to earth, the needle will go to FULL

    • If it's a European gauge the needle will show FULL
      • Touch the black wire to earth, the needle will go to EMPTY

 

To test the sender, click here