Re: generator test?

Posted by Gerald J. on October 13, 2004 at 11:08:34 from (67.0.96.53):

In Reply to: generator posted by mike langran on October 13, 2004 at 09:20:46:

Its not much of a test. First off a generator often is disconnected by the reverse current cutout at engine idle. You didn't say the engine speed. Then at low engine speeds the reverse current (generator discharging the battery) often happens at engine speeds just above idle, so the perfect generator AND regulator won't support engine ignition under each of these normal operating conditions. When the generator is turning faster then it should supply ignition power. But because of these slow speed conditions your "test" isn't much of a test.

The generator can be perfect but if the regulator is out of adjustment, has rusty parts or burnt contact points, or is just missing the case ground, the pair won't charge. The best test is an ammeter in the battery charge and load line. Everything but the starter should pass through the ammeter. Then the generator shows charge or discharge on the ammeter and proves the quality of the battery, the generator, AND the regulator.

Working more slowly you can check the specific gravity of the battery electrolyte over an hour of running (well above idle speed) to see it if rises. If the battery is pretty good you can also check the battery terminal voltage with a digital volt meter to see if the voltage rises. It should get up to 7.1 volts for a perfect 6 volt system, 14.2 volts for a perfect 12 volt system.

If the voltage setting on the regulator is perfect, the ammeter should show maximum charge right after starting, and should taper down to zero charging current in time. A double check of full charge is finding the gravity of the electrolyte is 1260 in most common batteries. And seeing that there is a bit of bubbling of the electrolyte as the battery charging current has gone to practically zero.

Old generators tend to need the commutator cleaned, the brushes replaced or at least sanded to fit the commutator, an often they need bearings and pulley. When the pulley sides aren't V shaped any more the generator may create almost enough voltage to charge but won't supply the current to charge in any reasonable time. A glazed generator belt does the same. It will show voltage to a voltmeter but not put any energy into the battery. A glazed belt cost me a car once.

Gerald J.

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