Mack ASET AC, Service Manual

Page: 55 / 536

Page 42
DESCRIPTION AND OPERATION
Electronic Unit Pumps (EUP)
The fuel system is pressurized by six Electronic
Unit Pumps (EUP), one per cylinder, driven by a
third lobe on the engine camshaft. Each one of
the single-plunger EUPs is actuated by a roller
follower (tappet) in contact with the engine cam
lobe.
The EUP is very similar to a unit injector. The
primary difference is that the EUP delivers fuel
through a fuel injection line to a
conventional-style nozzle-holder assembly;
whereas, a unit injector has a nozzle mounted
directly on it.
The EUP is capable of providing very high
fuel-injection pressures. The pump is controlled
by a high-speed solenoid valve (see Figure 38)
responding to electronic signals from the
V-MAC III engine control module. This electronic
control provides a greater timing range. The
combination of higher pressures and greater
timing control improves the combustion process
and optimizes engine performance.
38
Figure 38 — EUP Components
Although each EUP has three O-ring grooves on
the pump housing, only the top and bottom
grooves are fitted with O-rings. No O-ring is used
in the center groove.
For the occasional instances where the standard
lower O-ring fails to seal against fuel leakage into
the crankcase, a service parts kit with a
three-piece delta O-ring seal is available through
the MACK Parts System. The delta seal, a
triangular O-ring with two nylon support rings
(Figure 39), is used at the lower O-ring location
only.
39
Figure 39 — Delta O-Ring Seal Cross Section
5-111.bk Page 42 Monday, July 10, 2006 2:26 PM