1
Introduction
1
Meritor WABCO
Maintenance Manual MM-1012 (Issued 02-10)
1 Introduction
System Overview
All vehicles equipped with a Detroit Diesel 2010 emissions engine
will have an air control unit (ACU) used to provide air pressure to the
urea dosing unit. The ACU is a combination valve integrating three
components: an electrical solenoid, a pressure reduction device,
and an outlet one way check valve.
The electric solenoid is a 12V continuous-duty solenoid. It is
controlled by an electronic control unit (ECU) dedicated to the
selective catalytic reduction (SCR) system. The pressure reduction
component of the ACU reduces the air brake system pressure to
80 psi (5.5 bar), providing pressure to the urea dosing unit and
injector. The outlet check valve provides protection to the air brake
system by preventing urea from entering the brake air system.
Identification
There are two ACU designs available; the most common will be a
normally closed design (part number 975 009 002 0) with one inlet
and one outlet. The second design is considered to be a normally
open design (part number 975 009 101 0) used on vehicles utilizing
a negative battery shut-off. This design will have two inlets and one
outlet. Figure 1.1.
Figure 1.1
How the ACU Works
The ACU is controlled by an ECU activated when the ECU sees an
ignition-controlled power supply. With the ignition key in the on or
run position, vehicle voltage is supplied to the ACU activating the
solenoid to the open position which allows air pressure to flow
through the ACU. The valve will remain open until the ignition key is
turned to the off position. When the key is turned off, the SCR ECU
will pulse a signal to the ACU to pulse air pressure through the SCR
lines pushing out any remaining urea fluid to keep the injector and
lines clean. This function is important because urea can crystallize
and clog the lines and the injector if the urea is subjected to too high
of a temperature.
If the ACU were to not operate correctly or stop operating altogether,
the urea in the SCR system may be incorrectly injected into the
engine exhaust causing higher emissions. When emissions exceed
the specification for any reason, the SCR ECU will turn on a warning
light on the dash panel and reduce the engine output and vehicle
speed. If the emission system is not repaired, the ECU will further
reduce the engine output allowing the vehicle to move at low
speeds, effectively a “limp home mode”, in order to have the
emission system repaired.
Figure 1.1
4007
99
3
a
NORMALLY OPEN
975 009 101 0
NORMALLY CLOSED
975 009 002 0
