revilla Posted June 27, 2013 Share Posted June 27, 2013 I recently noticed that my brake lights weren't working. I suspected the switch as the most likely candidate. Tonight I whipped off the lid of the pedal box, stuck my multimeter continuity tester across the switch (all switched off first), pressed the brake pedal and ... "beeeeeep" ... So switch looked fine. I then switched to voltage mode and checked for 12V on one side of the switch, then pressed the brake pedal expecting to register 12V and ... about 0.5V! A check in resistance mode showed about 150 ohms resistance through the switch in the "on" position, which as far as a 21W brake light bulb is concerned might as well be "off", but the continuity tester beeped as though all was fine. A replacement switch from spares fixed my problems. The moral of the story is that continuity testers beep when the resistance is below a certain threshold level, which may be surprisingly high. Just because a switch or earth connection passes the audible continuity test doesn't mean it's resistance is low enough to mean that it will perform well when asked to carry a more realistic current load. In my case 0.5V was appearing across the bulb, with the remainder of the 12V or so being dropped across the switch. Just wanted to mention this as a word of warning to anyone who is not experienced with electrics and may fall into the trap of thinking that a "beep" always means "good". Link to comment Share on other sites More sharing options...
tweeky Posted June 27, 2013 Share Posted June 27, 2013 hope this helps Link to comment Share on other sites More sharing options...
Leadership Team Garry7 Posted June 28, 2013 Leadership Team Share Posted June 28, 2013 Happy to confess to being somewhat less than a genius when it comes to car electrics, so thanks for pointing this out. I have exactly that kind of tester, which I guess has the potential to confuse me more than help!! Cheers Garry Link to comment Share on other sites More sharing options...
7TOM Posted June 28, 2013 Share Posted June 28, 2013 You should never trust a beep test always read the meter as it will beep way above a good low resistance of less than an ohm. Even this is not fool proof as a meter induces a voltage then measures it at the other end so if you have power connected your get a false reading. Also as the circuit is not loaded you have no current draw you can still get a good reading over a poor circuit . To sum up always load your circuit and check voltages across it this will quickly ascertain if a live,earth or consumer fault. Kind regards Tom Link to comment Share on other sites More sharing options...
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