Dispelling Electrical Test Myths Part VII By Dan Marinucci Alternative Alter- nator Tests. It’s a common electrical myth that diagnosing intermittent electrical problems requires outrageously sophisticated, expensive test gear. Not so! Using some digital multimeters, a competent technician can find many of the intermittent elec- trical problems he encounters. In previous installments of this se- ries, I have described digital multimeters equipped with a minimum/ maximum (MIN/MAX) memory ca- pability. Both Fluke and OTC offer professional-grade automotive meters with a MIN/MAX feature. With the touch ofa button, this feature cues the meter to display the minimum or maxi- mum reading that occurred during a test. This can be a powerful diagnostic tool when diagnosing intermittent elec- trical troubles. Open Output Circuit. For ex- ample, consider a symptom of re- peated diode trio failure. Diode trios are used on some Delco, Bosch, Mitsubishi, Hitachi, Nippondenso, Lucas and Motorola alternators. Many of us have seen cases where a loose connection in the alternator output circuit is the root cause of the diode trio failure. Un- doubtedly, the most common of these conditions is a failing fusible link ona General Motors product. Of course, the fusible link is usually located some- where out of sight, out of mind on that vehicle. Eventually, the failing fusible link remains open, causing a no-charge condition. But whether it’s a failing fusible link ona GM product or a loose connec- tion between the alternator and bat- tery on anon-GM product, it can still cause the same pattern of repeated diode trio failure. Here’s why—as well as how a MIN/MAX feature may help you pinpoint the problem. Suppose an intermittent open cir- cuit occurs between the alternator out- put terminal (BAT or B+) and the posi- tive battery terminal during a period of heavy electrical demand. That means the alter-nator’s pumping out 40 to 50 amps and suddenly there’s a momen- tary roadblock (open or nearly open circuit) between the alternator and bat- tery. The roadblock forces the current to seek an alternative path. Electrically speaking, the diode trio is a parallel output path, so the current tries to exit the alternator via the diode trio. The trio, which is supposed to handle less than about 10 amps, fails because it’s slugged with substantially higher cur- rent. Unless a technician corrects the root of the problem—the intermittent open in the output circuit—the new diode trio will suffer the same fate. Usually, performing the voltage drop test I described in Part VI, catches marginal connections. But sup- pose the intermittent connection in the output circuit isn’t obvious when you check the vehicle. Using long test leads, connect a digital voltmeter between the positive battery terminal and the alter- nator output terminal. That is, clip one lead to the positive battery terminal, the other lead to the BAT or B+ alter- nator terminal. Place the voltmeter on the dashboard or passenger seat. Turn on lots of electrical accessories and road test the vehicle so the engine com- partment gets good and hot. Some- times it helps to road test on bumpy MAY, 2001 roads too. A combination of real- world heat and vibration usually forces marginal connections to reveal them- selves. After the road test, recall the max1- mum voltage reading—which will be the maximum voltage loss that oc- curred between the alternator and bat- tery during the road test—by pressing the MIN/MAX button. Maximum al- lowable voltage loss is about 0.40 to 0.50 volt, but the MAX reading will probably be close to system voltage! (Remember the voltage loss across an open circuit equals source or system voltage.) A severe voltage loss con- firms the output circuit is failing. It’s time to carefully trace this circuit from the alternator to the battery, then locate and repair the problem. Quicker Than Your Eyes. The neat thing about the MIN/MAX feature is that you don’t have to watch the volt- meter during a road test. Even if you can watch the voltmeter during a road test, the MIN/MAX captures and holds readings your eyes may not. In this example, I “bridged” the output circuit with the meter and told it to watch for an excessive voltage loss across that circuit. Likewise, you can bridge the hot side or ground side of any circuit with the voltmeter and use the MIN/MAX to identify intermittent connections. Certainly, a professional-grade voltmeter with this feature can cost twice as much as meters offered by popular electronics retailers. But to me, the durability of the professional prod- uct and the usefulness of features such as MIN/MAX aren’t expensive— they’re priceless! Dan Marinucci, a former technician who is ASE certified, is a regular contributor to trade journals such as Motor and Tire Business. Page 1 of 1