Theory of Operation
Since the introduction of OBD II in 1996, it became
mandatory for the EVAP system to be checked for leaks. The EVAP system is
comprised of the fuel tank, hoses, charcoal canister and related
solenoids/sensors. A program subroutine inside the ECM’s memory called the
EVAP monitor is responsible, with the help of all related sensors and
solenoids, for the leak detection ability of the EVAP system. Early EVAP
systems were mandated by law to detect a .040” leak or EVAP aperture. Most
systems rely on a fuel tank pressure sensor and engine vacuum for leak
detection. In recent years, however, more stringent OBD II regulations
mandate a leak detection of .020” and even tighter restrictions (around
.010”) are projected for the future. For this tighter leak detection
standard, some manufacturers require the use of a device called the leak
detection pump (LDP).
The LDP is a totally
integrated unit. It is comprised of the pump itself, LDP solenoid, reed
switch and related vacuum hoses. The LDP is an electromechanical device. It
has the duty of pressurizing the EVAP system so as to make it possible for
the ECM to check for leaks. It is capable of pressurizing or inflating the
fuel tank and canister to about ¼ psi, which is enough for leak detection
The LDP is seeing wider use due to its more sensitive leak
detection capability. It is absolutely important to understand how the LDP
works so as be able to properly diagnose the system. This phrase is repeated
throughout this book and takes us to one of the main rules of diagnostics,
“Know the System First”.
The LDP pump itself is composed of two chambers separated by
a diaphragm. The lower chamber is connected to the canister, air filter, and
atmospheric pressure line. The upper chamber is connected to the vacuum port
and the other side of the atmospheric pressure line. The two chambers are
separated by a rubber diaphragm, which moves up-and-down creating the
pumping action. The diaphragm itself is connected internally to a small
shaft so that as it moves up-and down it interrupts a reed switch magnetic
field. The reed switch, as it opens and closes, sends a square wave signal
to the ECM for feedback. This reed signal is used by the ECM for diagnostic
The following is a detailed analysis of the LDP’s operation.
1. The LDP’s natural position is with the diaphragm all the
way down. Therefore, the bottom of the diaphragm rests on the vent valve,
keeping it open. Normal canister purge can take place at this time.
2. As soon as the ECM initiates the EVAP monitor, the
canister purge solenoid closes (no purge) and the LDP solenoid is activated.
The ECM uses a duty cycle signal to activate the LDP vacuum solenoid.
3. When the LDP solenoid is activated, engine vacuum pulls
the diaphragm all the way up. This closes the vent valve and opens the inlet
one-way valve so that fresh air can enter the lower chamber through the
small filter attached to the LDP. This action also pushes the upper part of
the diaphragm and shaft through the reed switch breaking the small magnetic
field and deactivating the reed switch.
4. As soon as the ECM senses a break in LDP reed switch
signal, it deactivates the LDP solenoid. This cuts the engine vacuum and
opens the atmospheric line to the upper chamber. With air filling up the
upper chamber, the spring pushes the diaphragm down. This closes the inlet
one-way valve and opens the outlet one-way valve causing fresh air to enter
the canister and the entire EVAP system. The LDP actually works like a
regular diaphragm pump, but vacuum is what moves the diaphragm up-and-down.
5. When the diaphragm moves further down the reed switch
closes telling the ECM to activate the LDP solenoid once again. And the
cycle repeats itself. (It is important to understand that while the EVAP
monitor is running (LDP pumping air to the EVAP system) the diaphragm never
touches the vent valve. So the vent valve remains closed at all times. This
is necessary to pressurize the EVAP system otherwise the pumped air would
escape through the vent valve. Then, after the EVAP monitor has completed,
the vent valve stays open at all times once again while the ECM is in
canister purge mode.)
As soon as the EVAP system is pressurized, the diaphragm can
not move up-and-down any more because the pressure in the lower chamber is
too great to overcome the tension of the upper spring. The ECM actually
looks at the reed switch frequency to determine when the EVAP system has
pressurized. If a leak exists, then the diaphragm will keep moving
up-and-down with a high reed signal frequency. Please note that when the
EVAP monitor first runs, the ECM disregards reed signal and fast pump the
LDP at 150 to 200 Hz. This pressurizes the EVAP system very quickly.
Conditions that Affect
Basically a leak in the EVAP system will make the LDP keep
pumping, since there is an escape for the air pressure. It is precisely the
job of the LDP to detect such leaks. A vacuum problem to the upper chamber
will render the LDP inoperative. The atmospheric line can actually be broken
without affecting the system. In this case, the LDP will suck air through it
instead of the LDP air filter.
Last but not least, any electronic or electrical problems
with the LDP solenoid will be detected by the ECM and set a trouble code.
The ECM first checks for electrical problems before running any tests. In
the event that a solenoid or reed switch fault exists, the ECM will abort
the EVAP monitor.
• The first step in testing the system is to make sure that
no electrical problems exist. Use a scanner or a voltage injection probe to
actuate the LDP solenoid. First, feel for the solenoid clicking and then
apply vacuum with a hand vacuum pump to the vacuum port. Make sure the
vacuum drops when the LDP solenoid is activated. If it doesn’t, then it is
clogged. If it does not click, then the internal coil has an open circuit.
Verify with an ohm meter.
• If the solenoid test has passed, then connect a VOM or a
scope to the reed switch output while activating the solenoid. Every time
the LDP’s diaphragm moves up-and-down the reed should switch on-off. This
should be readily seen on the scope or VOM display. On some vehicles, the
scanner can read the reed signal as a data-stream PID . Look for it. Failure
of the reed switch will render the EVAP monitor inoperative, since the ECM
uses the reed switch as an indicator of EVAP pressurization.
• With the EVAP LDP monitor commanded to run (using the scan
tool or equivalent), probe the reed switch output. As the fuel tank and
canister begin to pressurize the switching frequency should begin to drop.
This is an indication of EVAP pressure buildup.
• With the LDP solenoid switching (EVAP monitor running),
feel for suction at the LDP inlet (filter side) and pumping action at the
LDP outlet (canister side). If no suction or pumping is felt, internal
damage might be the cause.
• Check the atmospheric line for obstruction. An obstruction
will not let the diaphragm move up-and-down preventing the LDP from working
• If the LDP keeps working continuously, suction is felt,
air is being pumped, and the reed switch cycling on-off then there is an
EVAP leak. In this case, the unit is doing exactly what it is supposed to
do. Remember that continuous LDP operation (with reed switch signal
cycling) is telling the ECM that there is an EVAP leak. And with no EVAP
leak, as the system builds up pressure the reed signal output frequency will
begin to drop.
• Manual actuation of the LDP solenoid with the power probe
is sometimes necessary in the absence of a good scanner with bi-directional
control. In this case, connect a VOM or GMM/scope to the reed switch and
manually energize the LDP solenoid. As soon as the signal cycles,
de-energize the LDP solenoid. Keep doing that until the reed signal stops
switching, which is an indication that the EVAP system is pressurized. If
not, there is a leak in the EVAP system. Start crimping and isolating the
different sections of the EVAP system so as to find the faulty section (fuel
tank, canister, hoses, purge valve, etc).
This blog describes some of
the techniques that could be used with all LDP EVAP systems. Some
differences exist among makes and models. It’s always a good idea to study
the wiring and vacuum diagram of the particular vehicle you are working on
so as to get a deeper understanding of that particular system.