Figure 3-16. Diode bias block terminal identification

(4) Disconnect the lead to the A1 bus bar (27)

(4) If the voltage is incorrect, verify that the

by removing the nut, lock washer, flat washer, and

control unit is functioning by disconnecting the lead to the

screw. Connect the positive ohmmeter lead to the A1

bias block terminal 4 and measuring at the lead. It

bus bar and the negative lead to the B2+ bus bar (28).

should measure about 15 volts.

The ohmmeter should read about 70 ohms. Zero ohms

(5) Reconnect the lead. Measure for 1.5 volts

indicates a short circuit within the switch assembly. (one

10% at bias block terminal 2.

or more shorted transistors.)

(6) Measure for 3 volts 10% at bias block

To locate a shorted transistor:

(a) Remove the screw (30), lock washer

terminal 3.

(31), and flat washer (32) attaching the encapsulated

e. Removal. Remove the power switch assembly

resistor to the heat sink of the module (34). Do this for

according to paragraph 2-7.

each module, one on each side, one at a time, until the

f. Disassembly.

reading on the ohmmeter is normal. The last module

(1) Disassemble the power switch assembly

disconnected is shorted.

according to figure 3-15. A single or all power switch

(b) To check for additional shorted

modules can be removed as follows.

transistors, momentarily touch each disconnected

(a) Disconnect push-on leads (11, 12, and

resistor lead to the heat sink, one at a time. A zero ohm

13) from top and bottom of module.

reading indicates a shorted transistor in that module

(b) Remove screw (14), lock washer (15),

also.

and flat washer (16) connecting emitter lead of module to

d. Voltage Test. This test verifies that the diode bias

emitter bus bar (28).

block is functional.

(1) Connect battery and turn the key switch on.

and flat washer (191 connecting each encapsulated

(2) Connect voltmeter negative lead to the B2+

resistor to collector bus bar (27).

bus bar (28, (fig. 3-15) for this test. Measure voltage by

(d) Remove nylon screw (26) and

touching the positive lead to the point indicated.

associated parts connecting each end of collector bus

(3) Measure for 4.5 to 5.5 volts at diode bias

bar (27) to end plates (51).

block terminal 4. (fig. 3-16.)

(e) Slide collector bus bar out of assembly.

(f) Loosen nuts (421 at both ends of

insulated mounting rods sufficiently to allow removal of

desired module (34).

(2) Disassemble the individual power switch

modules according to figure 3-17.

Figure 3-16. Diode bias block terminal identification

3-27