Troubleshooting guide

determine the specific component. Electrical isolation is accomplished by unbolting the pigtail of

the diode being tested.

(b) If one or more fuse links (F3/F4) are blown, the diodes must be isolated and checked individually.

NOTE

Some analog meters and most digital meters will read similarly high both before

and after reversing the meter leads when testing good diodes and SCRs.

1 Place one test probe on NEG heatsink (diode stud) and the other probe to each diode pigtail's

SCR heatsink. Note the resistance readings obtained. All three readings should be similar

(high or low).

2 Reverse the test probes and recheck the diodes. Note each resistance reading. All three

readings should be similar; but, opposite of those noted in the first reading.

3 Diodes are good if they test high in one direction and low in the opposite direction. Diodes

are bad if the resistance readings are low in both directions. High readings in both directions

when using a digital ohmmeter indicates a good diode.

4 Diodes are bad when they show no, or very low, resistance in both directions indicating they

are shorted. Although it is possible for power rectifier diodes to fail "open", such failures are

rare, particularly for the larger diodes. In the vast majority of cases, a high resistance reading

in either or both directions indicate a good diode.

2. Checking Freewheeling Diode D7.

This diode must be checked in the circuit using the same procedures outlined in steps 1.(a), 1.(b), and

1.(c). If the diode tests "good", nothing else need be done; however, if the diode tests "bad", you must

disconnect its pigtail and retest it for verification.

3. Checking SCR Group SCR 4, 5 and 6.

NOTE

Before proceeding with the following SCR tests, all diodes D4, 5, 6 and 7 should

have been checked, and defective (shorted) diodes electrically isolated.

(a) If fuse links F3 and F4 are not blown and are still connected to the bridge heatsink, use an ohmmeter

set to its lowest resistance scale and check the SCR group as described in step 3.(c). Checking

any one SCR within the group will determine if the group is good or if a shorted condition exists.

If there is a short, you must electrically isolate to determine the specific faulty component, see step

3.(d).

(b) If one or more fuse links (F3/F4) are blown, the SCRs must be isolated and checked individually.

Refer to step 3.(d).

MAINTENANCESECTION 3