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MG MGB Technical - Test a light switch

Hi, Just bought a new headlight switch (rocker type)and my sidelights will not work. They worked with the old swich but unfortunatly it broke when removing it from the dash. Can i check the switch is OK/faulty with a cucuit tester?. Thanks.
Chris82

yes-ish, you can click the switch into side and head position. It should be open on all pins when off and show a few milliohms to the circuits as it is moved to the postion where it energises them. The tester however only sends a few 10s of microamps ( up to miliamps depending on design) through the contacts so it is just possible to show continuity at low currents, but not be able to pass the heavy headlight current. If you test on the most sensitive (lowest resistance) range this avoids this problem as far as possible. If it does not change state , send it back.
Stan Best

What's the model year? Using a meter is the best way to test the switch because a meter will measure resistance and continuity. Continuity can be tested for using a test light or a continuity tester. If I know the model year for the switch and what type of tester you are using, I can tell you how to test the switch using the pin numbers on the switch.
Kimberly

As Stan implies the only real way to test a switch is to measure volt-drop when it is carrying its design load. With very small loads like an ohmmeter you may not break through an insulating film that can develop across the contacts so it shows a high resistance or open-circuit, OTOH the same small currents may not cause a bad connection to break down with the result that an ohmmeter shows very low resistance but it is still open-curcuit under load.

Have you got the wires on the correct spades? The common connects to no spades in the off position, one of the other spades in the middle position, and both of the others in the fully on position. As such it is possible to get the parking lights coming on with the headlights but not on their own if incorrectly connected. If however the parking lights don't work at all, and you confirm with a voltmeter or test-lamp that there isn't any voltage on their spade in *any* position of the switch, then the switch *is* faulty. But if the parking lights don't come on when the red or red/green spade is touched to the blue spade, and the headlights are on, then the problem is in the wiring or elsewhere.
Paul Hunt 2

This thread was discussed between 13/11/2006 and 14/11/2006

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