Thermo King SB-190 30, Manual

Page: 61 / 119

Engine Maintenance (Rev. 12/03)
51
POSITIVE CRANKCASE VENTILATION
The Positive Crankcase Ventilation (PCV) system ducts
gases formed in the crankcase directly to the intake. Harm-
ful vapors that would otherwise collect in the crankcase and
contaminate the oil, or escape to the outside, are now drawn
back into the engine and burned.
The PCV valve is located in the valve cover. It uses a spring
and diaphragm to maintain a constant flow of crankcase gas
regardless of the intake manifold pressure. The crankcase
pressure is consistently regulated, even in the presence of
ring wear and air cleaner clogging.
The following schematics illustrate the Positive Crankcase
Ventilation (PCV) operation. In the unrestricted position,
Figure 1, gas flow exits the crankcase via the push rod pas-
sages and past the orifice and diaphragm. In the past, as air
cleaner restriction increased, vacuum and flow would
increase in the crossover line—lowering the crankcase pres-
sure. To prevent this, the PCV diaphragm, Figures 2 & 3,
expands into the passageway—restricting the flow to main-
tain a constant, slightly positive crankcase pressure.
Figure 1: Figure 2: Partially Figure 3: Fully
Unrestricted Position Restricted Position Restricted Position
1. Diaphragm 5. Push Rod Passages
2. PVC Valve 6. Baffle Plate
3. Spring 7. To Crossover Tube
4. Valve Cover
Positive Crankcase Ventilation System Operation
AGA152
Checking Crankcase Pressure
(See Figure 2-63, Measuring Crankcase Pressure, from
Overhaul Manual - di 2.2 and se 2.2, TK 8009-3; pg. 33.)
Use a 127 to 254 mm (5 to 10 in.) Magnehelic Gauge to
measure crankcase pressure. At High and Low Speed the
crankcase pressure should be between 51 to 76 mm (2 to
3 in.) H
2
O.
If crankcase pressure is more than 76 mm (3 in.) H
2
O:
• Restriction in hose (dirt or ice). -or-
• PCV diaphragm is frozen to seat in valve. -or-
• Plugged atmospheric vent hole in PCV cover.
If crankcase pressure is below 0 mm (0 in.) H
2
O (in
vacuum):
• PCV diaphragm has torn and is no longer controlling
gas flow.