Troubleshooting guide

1. Place one test probe on the POS heatsink (same as the shunt) and touch the other test probe

to each of the SCR's anode heatsinks. The resistance reading across each SCR should be 1000

(1K) ohms or higher.

2. Reverse the test probes and repeat the check. Again, the resistance reading across each SCR

should be 1000 (1K) ohms or higher.

3. SCRs are bad if they show no or very low resistance reading in either direction.

(d) If the diagnosis indicates a defective SCR within the group, you must disconnect fuses F3 and F4 to

electrically isolate the SCRs (assuming all diodes D4, 5, 6, 7 have checked good, or defective diodes

previously isolated), and repeat the ohmmeter tests in steps 1, 2, and 3 above for each SCR.

C. Testing Boost Bridge Rectifier (BBR) Assembly.

1. Remove the 30 ampere fuses, F1 and F2, to electrically isolate the diodes of the BBR assembly from each

other and the SCRs of the BBR assembly from each other. (Its always good practice to check the fuses for

continuity to make sure whether they are good or bad. Use ohmmeter set to RX1 scale.)

2. Check diodes (D1, 2 and 3) with an ohmmeter set to the RX1 scale, as follows:

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.

(a) Place one test probe on the NEG heatsink (diode stud) and the other probe to each diode pigtails SCR

heatsink. Note the resistance reading obtained. All three readings should be similar (high or low).

(b) 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.

the resistance readings are low in both directions. High readings in both directions when using a digital

ohmmeter indicates a good diode.

(d) 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.

3. Check the SCRs (SCR1, 2 and 3) with an ohmmeter set to RX1 or RX100 scale, as follows:

(a) Place one test probe on the POS heatsink (SCR pigtails) and the other probe to each SCRs stud

heatsink. All three readings should show a high resistance of 1000 (1K) ohms or higher.

(b) Reverse the test probes and recheck the SCRs. Again, all three readings should show a high

resistance similar to the first test.

SECTION 3

MAINTENANCE