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DEF dosing system. DEF is supplied from a chassis mount tank to a Supply Module
which is controlled by the ECM. The Supply Module is located inboard the frame rail.
The Supply Module uses a very accurate metering and pumping system to supply
DEF to the Dosing Module located at the Decomposition Reactor in the exhaust sys-
tem. The amount of the DEF injected into the exhaust is controlled by the ECM. Water
evaporates quickly and the DEF turns into amonia when in contact with hot exhaust
gases.
Diesel Exhaust Fluid. DEF is carried on board the bus as an aqueous urea solution in
a 15 gallon storage tank clearly identified as DEF. The storage tank is sized to mini-
mize operator filling, but within packaging and weight constraints of the vehicle.
Diesel Exhaust Fluid is urea mixed with water to a 32.5% solution. DEF solution is a
clear liquid with a weak amonia odor.
The SCR system consumes DEF at a rate of approximately 2% of the amount of fuel
used, depending on bus operation. Therefore a 15 gal tank of DEF will treat approxi-
mately 750 gallons of diesel fuel.
The DEF tank level indicator can be found in quadrant 3 of the message display
center located on the instrument cluster. This display provides a constant reading
indicating the percent of DEF in the tank. If a problem is identified with the amount
of fluid or the quality of the DEF, a priority message will appear in quadrant 1 of the
message display center indicating the fault.
Keeping adequate levels of DEF in the tank is crucial to the operation of the SCR
system. When low levels in the DEF tank are indicated, refill the tank using only ap-
proved DEF solution. The DEF has its own tank and fill port with a cap. You will notice
the fill port is smaller than the diesel fuel port. This is to prevent putting diesel fuel
into the DEF tank. Conversely, care should be given not to put DEF in the diesel fuel
tank. Read all decals carefully before refilling. When filling the DEF tank ensure the
vehicle is on level ground and insert the fill station nozzle as far as possible into the
fill port adapter to ensure proper auto-shutoff levels.
[CAUTION] DO NOT overfill the DEF reservoir. DEF will freeze and expand at
temperatures below 12° F (-11°C). If the reservoir is overfilled and freezes, the
expansion of the frozen DEF will cause catastrophic damage to reservoir and/
or the vehicle SCR system.
Refer to Blue Bird and Cummins Service Manuals for detailed description of the af-
tertreatment system.
DEF Fill located just forward of the rear axle, right side.
DEF Fill Door
Diesel Fill
[CAUTION] DO NOT put diesel
fuel in the DEF tank. DO NOT put
DEF in the fuel tank of the bus.
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