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1. Reference leak measurement phase
For the reference current measurement, the motor-
pump is switched on. In this mode fresh air is pum-
ped through a 0.02-inch reference orifice, situated
internally in the module, and the pump motor current
is measured. At some unusual operating conditions
the pump current may not stabilize. In this case the
leak check is aborted and a new leak check will be
performed in the next after run. To prevent a perma-
nent disablement of the leak check due to a DM-TL
module problem, the number of subsequent irregular
current measurements is counted and a module error
is set as soon as the counter exceeds a calibrated value.
2. Rough leak test phase
In this monitoring mode the changeover valve is
switched over (the purge control valve remains closed).
The motor current drops to a zero load level. Fresh
air is now pumped through the canister into the tank.
This creates a small overpressure at a tight evaporative
system, which leads to a current increase.
The rough leak check (≥ 0.04-inch) is performed by
monitoring the pump motor current gradient. Relative
pump motor current is created by using minimum
pump motor current and reference pump motor cur-
rent. Area ratio is created by dividing integrated relati-
ve current with ideal area, which is the linear integrated
area from minimum pump current to current sample
of the current. If the relative current has increased
above an upper limit but not exceeded a calibrated
area, within a calibrated time, the rough leak check has
passed without a fault. If the calibrated area ratio is
reached before the relative pump current limit, within
the calibrated time, a rough leak fault code is set.
The integrated relative pump current area
Aint
is
defined by;
Aint = A1 + A2
and the ideal area
Aideal ,
Aideal = A2 .
See figure below.
