Dispelling Electrical Test Myths Parts V and VI By Dan Marinucci Last month, I delved into the myth that alternator output must be within ten percent of specifica- ” tion. I pointed out that although output specs are impor- tant, specs alone won’t solve every charging system problem. Here, I’1l examine the myth that using a load tester is the only way to check an alternator on the vehicle. In fact, a well-equipped digital voltmeter can be an effective alternative test de- vice. What’s more, digital voltmeters increase shop efficiency by comple- menting the load tester. Real-World Scenarios. Many auto-electric shops install their own alternators and starters as well as per- form electrical system diagnosis and repair. Ideally, they would like to have a load tester available for each techni- cian, but things don’t always work out that way. However, diagnosis on other vehicles need not grind to a halt when all load testers are taken. Here’s why. Charging system complaints come in three varieties: no charge, under- charge, or overcharge conditions. No matter which one the technician pursues, his immediate concern is: Does he have to remove the alternator or alternator/ regulator assembly to fix the problem? If the unit does have to be removed, the rebuilding department will perform aroutine set of detailed tests on it and its components. Meter Recommendations. The first order of business is getting the right meter and options to do the job. The two leading names in digital automo- tive multimeters are Fluke and OTC. (Multimeter refers to a multipurpose meter that reads volts, amps, ohms and possibly other values such as fre- quency, rpm, etc.) Experience shows both companies produce durable, high-impedance digital voltmeters with lots of time-saving features. A high- impedance voltmeter is essential for testing low-current computer system circuits. Regardless of which meter you buy, I urge you to invest in three important features. First, buy a multimeter equipped with memory recall or “min/ max’ capability. The memory feature allows you to recall the maximum or minimum reading the meter detected with the push ofa button. Among other things, this capability is invaluable dur- ing battery and charging system tests. Second, always specify an auto- ranging multimeter. The autoranging feature speeds up diagnosis and re- duces mistakes because it always se- lects the correct scale for the reading you’re taking. In other words, an autoranging meter automatically distin- guishes between a 1.20-volt reading and a 12.00-volt reading without forcing you to switch from the 2-volt to the 20-volt scale. Automotive instructors and tech- nical hotline personnel agree that using the wrong voltage scale is one of the most common flubs technicians make during electrical tests! Third, invest in a meter that accepts an inductive ammeter pickup. An in- ductive ammeter option extends the utility of the meter by enabling it to measure relatively high current appli- cations such as alternator output and starter motor current draw. Not only does this feature extend the utility of APRIL, 2001 the meter, it saves the step of discon- necting wiring in order to measure cur- rent. Checking Alternator Output. A charging system check should include a battery test. Ifanother technician is using the shop’s load tester, do what I have recommended in other ERE ar- ticles. After the engine reaches oper- ating temperature, shut it off and safely disable the ignition system or electric fuel pump so the engine won’t start. Connect a digital voltmeter to the bat- tery terminals and crank the engine for 15 seconds. At the end of the 15-second pe- riod, note the voltmeter reading just before you release the ignition switch. Or, shut off the ignition and push the meter button that recalls the minimum reading. The minimum voltage reading on this test must never drop below 9.6 volts (at room temperature). Typically, ahealthy battery with adequate capac- ity for the application will maintain 10.0 to 11.0 volts during this test. Besides checking the battery, this 15-second crank also “conditions” the battery for an easy alternator output test. That is, it lowers battery voltage enough to prompt the alternator into producing a bunch of current quickly and safely. To do this output test, reactivate the ignition system or fuel pump. Con- nect the inductive ammeter adapter to the voltmeter and switch the meter to the correct position for measuring cur- rent. Then clip the inductive pickup around the alternator output wire. Now start the engine and immedi- ately floor the gas pedal. To see peak alternator output, push the meter but- ton that recalls the maximum reading. Experience shows that if the charging system’s in good condition, the maxi- mum reading the multimeter captures Page 1 of 2