Mechanical Switching Tests

TM 10-3930-631-34

typical waveform and its relationship to the zero

9-12.

Speed Potentiometer Adjustment

reference line.

The shape of the waveform is

Note

unimportant. To check peak current set the vertical

Make certain battery is disconnected and

input at .01 V/cm. Connect oscilloscope signal lead to

capacitors

are

discharged.

Isolate

terminal on shunt towards power

fuse.

Connect

potentiometer leads by disconnecting speed

oscilloscope common or ground lead to terminal on

control wiring harness connector P4 (fig. 9-3).

shunt towards battery negative cable.

a. Remove nuts and lockwashers that secure

c. If the peak current is less than the minimum,

cover to accelerator control box (fig. 9-3) which is

disconnect the white wire from the S1 terminal and the

located on the frame beneath the instrument panel, and

S1 bus bar and apply a jumper to the white wire and

remove the cover.

connect to each drive regulator individually. The

b. Set VOM on RX1 scale and connect meter

minimum peak current of each power switch should be

leads to the two potentiometer terminals Where the

200 amperes. Replace any defective power switch

white/black and the blue leads are soldered or to P4-4

modules (para 9-28).

and P4-5 pins of the harness connector (P4).

9-11. Mechanical Switching Tests

c. Meter reading should be less than 30 ohms with

a. The following tests are conducted with the

accelerator pedal in the released position.

battery connected and the normally open contacts on

d. Depress accelerator pedal fully to the lowest

the forward and reverse directional contactor with thin

position: meter reading should be approximately 7K to

cardboard between contacts.

10K ohms.

b. Turn key switch to the ON position and depress

e. If speed potentiometer requires adjustment,

the seat switch, the emergency cutout contactor (fig. 2-

loosen locknut on setscrew (fig. 9-3) in actuator and

8) should close when the switch is activated. The lift

loosen setscrew. Turn potentiometer shaft until less

truck should show no other evidence of being turned on.

than 30 ohms resistance value is obtained with the

c. Check for operation of the horn by depressing

accelerator pedal in the released position. Tighten

horn button. Move valve control lever and check pump

setscrew in actuator and tighten locknut on setscrew.

contactor. Both should operate. Make certain that

Note

definite pressure is applied to the control valve levers

To aid in the speed potentiometer adjustment,

and noticeable motion occurs before the pump contactor

remove button plug from side of speed control

contact closes. When the contact closes, the pump

box so potentiometer shaft can be turned with a

motor should start operating under no hydraulic load and

screwdriver.

with no movement of any of the hydraulic cylinders.

f. When accelerator pedal is depressed fully to the

d. With the key switch in the ON position and the

lowest position, meter reading should be approximately

seat switch closed, move the forward and reverse lever

7K to 10K ohms.

to select a forward direction. Depress the accelerator

g. Remove meter leads and connect wiring

pedal slightly; the for- ward contactor should energize

harness connectors.

and its normally open contacts should close.

h. Check operation of speed control shaft (fig. 9-3)

e. Check directional interlocking by manually

and accelerator pedal. Make certain shaft bearing

operating (bringing in) the reverse contactor.

The

bracket is securely mounted to frame.

forward contactor should deenergize (drop out).

i. Install cover on speed control box with lock-

f. Repeat steps d and e with the forward and

washers and nuts; tighten nuts securely.

reverse lever in the reverse position. The reverse

Note

contactor should energize when the accelerator pedal is

After speed potentiometer is adjusted, it may be

depressed and the reverse contactor should drop out

necessary to adjust the inching potentiometer

when the forward contactor is manually operated.

(para. 9-13).

g. Turn the key switch OFF. Try to operate the

9-13. Inching Potentiometer Adjustment

pump contactor and directional contactors by use of the

a. Make certain drive wheels are blocked up and

control levers. Also depress the ac- celerator pedal.

are clear of blocks and floor. Check to make certain

Nothing should operate electrically.

directional contactors are not isolated.

h. Remove the cardboard from all of the contactors.

b. Sit on the operator's seat and turn the key

switch to the ON position. Move the forward and

reverse lever to forward. Depress the accelerator pedal

slightly until the forward contactor closes.

c. Slowly continue to depress the accelerator

pedal. Wheels should begin to turn and pick up speed

slowly and smoothly.

9-14