CHARGING CIRCUIT DIODES

When an alternator fully produces, each of its diodes conducts an equal share of the current. This condition is indicated by a ripple pattern that appears on the screen of the engine analyzer. (See fig. 4-16.) But a single nonconducting diode places a strain on the charging circuit which causes a decrease in the output of the alternator. Whereas an ammeter or voltmeter may not detect this strain, the analyzer can do so easily. The strain brought on by an open field condition, for example, will stop the alternator output ripple entirely. See the screen display of figure 4-17.

A likely result of decreased alternator output is an undercharged battery, and without a fully charged battery, there may not be enough current available to start the engine or meet the demands of the electrical circuits. When a good battery cannot be fully charged, the fault is usually in the alternator or voltage regulator. The engine analyzer can help you determine which is at fault. However, the regulator has to be bypassed altogether and battery voltage applied to the field terminal of the alternator. Not all alternators can be full fielded. Refer to the manufacturer's fieldtest procedure.

Open file stops the ripple

Figure 4-17.-Open file stops the ripple.

Bypass adapter

Figure 4-18. - Bypass adapter.

BYPASS PROCEDURE

The first step in the procedure for bypassing the voltage regulator is for you to turn OFF the engine. Next, disconnect the regulator and place a jumper wire between the positive (+) battery terminal and the field terminal of the alternator. You can also use the bypass adapter hooked up as shown in figure 4-18. Again start the engine and slowly increase its speed until the rated alternator output is reached. DO NOT RUN THE ENGINE FOR MORE THAN 20 SECONDS.

If the ripple pattern now appears on the screen of the engine analyzer, the regulator is faulty. No change in the screen pattern means the alternator or output wiring is at fault. Stop the engine, disconnect the jumper wire or bypass adapter, and reconnect the voltage regulator.

OPEN AND SHORTED DIODES

A shorted diode or shorted winding will usually burn itself open. The pattern on the screen will show a shorted diode (fig. 4-19) or open diode (fig. 4-20). Notice the similarity in the patterns. At any rate, the alternator will require service or replacement even

Shorted diode pattern

Figure 4-19.-Shorted diode pattern.