Section IX. HYDRAULIC CONTROL VALVE

TM 10-3930-621-34

i. Check pump pressures as follows:

Caution: Do not force pump into position as this

(1) Install a 2000 psi pressure gage in the

will damage the bearings, resulting in poor operation

pressure side of the power steering circuit.

and short pump life.

(2) Start engine and run pump under no load

conditions.

f. Secure pump plate to pump bracket and timing

(3) With engine running at 1400 rpm, turn

gear cover with screws and lock washers.

steering wheels to their limit and observe pressure gage.

g. Connect hydraulic tubes to power steering,

Opening pressure should be between 1000 and 1200

control valve, and suction ports.

psi.

h. Run the pump at no load as follows: (1) Check

(4) If opening pressure is above 1200 psi,

for required reservoir oil level.

remove shim(s) (7, fig. 10-11) from relief valve, and if

(2) Run pump for one minute at no load to allow

pressure is below 1000, add shim(s).

system to fill. Check for pressure or air leaks in the

Caution: Pressure should never exceed

system during run.

2000 psi, as higher pressures severely reduce pump

(3) Shut off engine and recheck reservoir oil level. If

life.

h)w, fill to proper level. If severe foaming is noted, it

indicates a suction leak or improper oil and must be

corrected.

Section IX. HYDRAULIC CONTROL VALVE

f. Remove screws (1, fig. 10-13), flat washers

10-43. Description

(15), and nuts (16) attaching valve to truck and remove

The hydraulic control valve serves to direct

valve.

pressurized hydraulic oil from the hydraulic pump to tilt or

g. Clean valve exterior with cleaning compound,

lift cylinders. It contains two operating plungers, two

solvent (Spec. P-S-661) and dry with compressed air.

check valves, and an adjustable relief valve. The single

acting plunger operates the lift cylinders and the double

10-45. Disassembly

acting plunger the tilt cylinders. When over 1200 psi oil

pressure is applied, the check valves allow oil flow to

a. Refer to figure 10-12 and disassemble as

either lift or tilt cylinders when levers are activated. The

follows. Only components requiring service need be

adjustable relief valve is a protective device that prevents

removed, unless complete overhaul is necessary.

damage to the hydraulic system due to pressures over

Caution: It is extremely important to keep all

2000 psi. Such pressures are normally caused when lift

parts clean at all times. Machined surfaces must be

or tilt cylinders reach full stroke and continued pump

protected from nicks, scoring, gouging and other

operation builds up pressure. If pressures rise above the

damage.

relief valve pressure adjustment (1850 to 1950 psi), the

b. Remove end plugs (6) and packings (7 and 22).

valve unseats and allows oil to by-pass the plungers and

Discard packing.

return, through a filter, to the reservoir.

c. Disassemble tilt plunger as follows: (1) Place

valve in vise in horizontal position.

(2) Remove socket head screws (32) and

10-44. Removal

remove plunger cap (33).

Make sure lift cylinders are collapsed

(3) Place a drift pin through eye of tilt plunger

and mast is tilted all the way forward.

(10) and install drive head socket adapter on plunger

Remove right side cowl from truck (TM

sleeve screw (34).

1030)30-621-12).

(4) While holding adapter, strike drift pin with

Remove cotter pins (13, fig. 10-13), flat

hammer. Blow should loosen sleeve screw (34) from

washers (14) and, pins (10) attaching lift lever links (11)

plunger (10). If sleeve screw does not loosen, remove

and tilt lever links (12) to lift plunger (38, fig. 10-12) and

adapter and insert brass pin through either valve port into

tilt plunger (10).

plunger holes. Strike brass pin with hammer.

Tag all hydraulic lines so they will be

Caution: To prevent damage to plunger holes,

reconnected to proper ports when the valve is installed.

use brass pin only.

Disconnect all hydraulic lines and

(5) Remove screw (34), seats (35), and spring

remove fittings from valve housing (37) connections.

(36) from tilt plunger (10).

Remove poppets from valve.

(6) Remove upper seal plate screws (12) and

upper seal plate (11).

10-24