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Is there any way to test a flasher it see if it's good or not?
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For a '72 or later model, probably the best way to test it is to replace it and see if it works. Go to your local auto parts store and buy a type 552 flasher, for about 2 bucks or so. Pull the old flasher and reconnect the wiring to the new one. The terminal designations on the Lucas flasher are "B" and "L". On the one you buy at the parts store will have terminal designations "X" and "L". The "X" correspondes to the "B" and the "Ls" are the same.
If you have a '69-'71 model, you will need to buy a Tridon model 550 in the short can (any other make or the tridon in a tall can won't work properly). In this case, the Lucas terminals are "B", "L", and "P". On the aftermarket, they are "X", "P", and "L". "X" = "B", "L" and "P" = "L" and "P".
The only other way to really test one is to hook up a load and see if it works. I keep a pair of bulbs handy that I've soldered together with short pieces of wire for testing purposes. The flasher won't work unless it has a reasonable load on it, so a simple voltmeter won't help.