OPERATION
5-32 / Engine Exhaust After Treatment Systems
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5-14. Selective Catalytic Reduction (SCR) and DEF Operation
The operation of the SCR device requires Diesel Exhaust Fluid (DEF). The driver is responsible for filling the DEF
reservoir to maintain the proper amounts of DEF fluid required by the engine. The DEF reservoir on Pierce custom
fire apparatus is 4 1/2 gallons. The fluid level sensor transmits the fluid level directly to the Dosing Control Unit
(DCU). The DCU is programmed with specific operational parameters which will trigger the DEF indicator as well as
induce vehicle performance derates*.
Table 5-7: Cummins and Detroit DEF Fluid Indicator Lamps and Warnings
* Vehicle performance derates and speed limits do not apply to 2013 and later EPA engines.
The Diesel Exhaust Fluid (DEF) Level Low light (2010 EPA and later) indicates the fluid
level has reached a low level and a refill is needed. The operator should refill the reservoir
when refilling the vehicle with diesel fuel or at the end of their shift. Refilling the DEF tank
under any of these conditions should extinguish the light.
• DEF level between full and 1/4 full: DEF Fluid Low light will remain OFF.
• DEF level between 1/4 full and 1/8 full: DEF Fluid Low light will illuminate solid.
• DEF level below 1/8 full: DEF Fluid Low light will flash.
• DEF level below 1/4 gallon:
— Cummins: DEF Fluid Low light flashing and amber Check Engine lit solid. The
engine will induce a 25% engine torque derate*.
— Detroit: DEF Fluid Low light flashing and Check Engine illuminated. The engine will
induce a 55 mph speed limit* and 25% engine torque derate*.
• DEF tank empty and system can no longer maintain DEF pressure:
— Cummins: DEF Fluid Low light flashing, amber Check Engine lit solid, and MIL light
lit solid. The engine will induce an engine torque derate* that begins a 25% and
ramps to 40% at a rate of 1% per minute of operation.
— Detroit: DEF Low light flashing, Check Engine light and Malfunction Indicator Lamp
(MIL) illuminated. The engine will induce a 55 mph speed limit* and 25% engine
torque derate*.
• DEF Tank Empty and ignored:
— Cummins: DEF Tank empty and ignored, engine intentionally shut down or in
extended idle: DEF Fluid Low light flashing, amber Check Engine lit solid, red Stop
Engine lit solid, and MIL light lit solid. The engine will induce an engine rpm limit of
1000 rpms*, a 5 MPH vehicle speed limit* and a 40% engine torque derate*.
— Detroit: DEF Tank Empty and ignored (fuel tank refilled by more than 30% or engine
start after the consumption of more than 350 gallons of diesel fuel): DEF Low light
flashing; Check Engine light, Stop Engine light, and MIL illuminated. The engine will
induce a 5 mph speed limit* and a 25% engine torque derate*.
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