1999 BMW 735i Motor

FUNCTION
The DC Motor LDP ensures accurate fuel system leak detection for leaks as small as
0.5mm (.020”). The pump contains an integral DC motor which is activated directly by the
engine control module. The ECM monitors the pump motor operating current as the mea-
surement for detecting leaks.
The pump also contains an ECM controlled change over valve that is energized closed dur-
ing a Leak Diagnosis test. The change over valve is open during all other periods of oper-
ation allowing the fuel system to “breath” through the inlet filter (similar to the full down
stroke of the current vacuum operated LDP).
DC MOTOR LDP INACTIVE -- NORMAL PURGE VALVE OPERATION
In it’s inactive state the pump motor and the change over valve of the DC Motor LDP are
not energized. When purge valve operation occurs filtered air enters the fuel system com-
pensating for engine vacuum drawing on the hydrocarbon vapors stored in the charcoal
canister.
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LEAK DIAGNOSIS TEST
PHASE 1 - REFERENCE MEASUREMENT
The ECM activates the pump motor. The pump pulls air from the filtered air inlet and pass-
es it through a precise 0.5mm reference orifice in the pump assembly.
The ECM simultaneously monitors the pump motor current flow . The motor current raises
quickly and levels off (stabilizes) due to the orifice restriction. The ECM stores the stabilized
amperage value in memory. The stored amperage value is the electrical equivalent of a 0.5
mm (0.020”) leak.
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PHASE 2 - LEAK DETECTION

The ECM energizes the Change Over Valve allowing the pressurized air to enter the fuel sys-

tem through the Charcoal Canister, The ECM monitors the current flow and compares it

with the stored reference measurement over a duration of time.

Once the test is concluded, the ECM stops the pump motor and immediately de-energizes

the change over valve. This allows the stored pressure to vent thorough the charcoal can-

ister trapping hydrocarbon vapor and venting air to atmosphere through the filter.
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