Walneck's Classic Cycle Trader - March 2008 - (Page 43)

SPECIAL FEATURE STORIES TIP OF THE MONTH inexpensive 1-ohm resistors. I thought the resistors would be used, but the one I received (see photo) was brand new in its original box. It was also rated at 300 watts, well above the one detailed in the shop books. Now armed with both a high and low watt resistor, the question of results and comparative figures could be answered. First up was a high rate discharge test of a 12-volt battery as given in the shop manual. The specs call for using a battery charged to at least 70 percent of its capacity. Minimum voltage for a good battery is given at 9.4 volts after 15 seconds under load. Test one was made with the 10-watt rig and a known weak battery that I replaced on another service job. After charging, it tested out at slightly more than 12 volts DC Test rig is shown with resistor in parallel, the way it should be for many tests. without resistance. After 15 seconds with the 10-watt resistor, the gauge read just a touch over 8 volts, meaning it flunked the test. The battery was recharged back to the same voltage level and tested again at 15 seconds with the 300-watt resistor and gave an identical reading of just a bit above 8 volts. A second test was made of a known good 2-wire Lucas alternator residing in my ‘69 BSA. The shop manual test calls for running the bike at 3,000 RPMs and getting a reading of at least 9 volts AC with the one ohm resistor in parallel, just like the battery test, except for using an AC voltmeter instead of DC for the battery test. I’ve always done alternator tests with a multi-tester set at 10-volts AC, because if it’s over that we’re OK, and it’s hard to read a meter accurately when it’s resting on the ground while you’re sitting on the bike, but it is easy to see the While a multi-tester is good for lots of tests, amps must also be measured to test devices like a zener diode. needle swing to the far side. Testing with both the 10-watt and 300-watt resistor pegged the meter, meaning the alternator passed the test, and again, it should have as it charges perfectly. Conclusion? While there may be some slight variation that one could get with more accurate digital meters, useful figures can be obtained from inexpensive resistors commonly available. If we were working on something like a new computer controlled fuel injection system where tiny variations in resistance, volts and amps can produce enormous performance changes, it wouldn’t be prudent to vary any testing procedure. But with most of our old bikes and for many tests, all we need to know is that the system keeps the lights on bright without discharging the No matter what sort of testing is to be done, most figures battery (or over charging it and blowing bulbs) and that obtained are based on tests with a good battery. Consequently, the spark is sufficient to properly fire our old motors. it’s a good idea to charge and test the battery first before proT’aint rocket science, McGee, and anyone with a ceeding to other checks. common background in basic mechanics can accurately diagnose and fix electrical gremlins. If you do use a light resistor, however, watch the fingers and be advised if you keep it on too long you can smoke it, and this would most definitely invalidate test results. Oh, by the way. I have 20 feet of nichrome wire I’ll sell cheap. WALNECKS.COM I WALNECK’S CLASSIC CYCLE TRADER® I MARCH 2008 43 http://WALNECKS.COM

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Walneck's Classic Cycle Trader - March 2008

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