Understanding the system’s wiring pattern is very important
for correct diagnosis. A quick look at the wiring diagram will reveal the
particulars to the system being worked on. On certain systems, it is common
to see one of the APP potentiometer signal increase (low to high) when
pressing on the accelerator, while at the same time the other potentiometer
signal decreases (high to low).
The APP signal characteristics is dependent on how the APP
sensor is wired. If the potentiometer’s wiper rests (throttle closed) at
ground, then the signal output increases as the throttle is pressed. And if
the wiper rests at the 5 volt reference, then the signal decreases as the
throttle is pressed. The important thing to remember is that regardless of
how many potentiometers are within the APP, they all have separate reference
voltage and grounds lines.
A throttle control actuator is also employed in
drive-by-wire systems to do the actual throttle opening. The ECM responds to
the APP signal changes by opening the throttle plate. Some systems also use
a separate throttle control computer or throttle control module.
The throttle control module (if separate) will always work
in conjunction with the engine control module in order to control the
vehicle’s acceleration. Both modules communicate with each other via a
serial data line. In older systems there were multiple sensor connections
from and to each of these modules. But newer and faster ECMs have made it
possible to rely on data line communications and not hard-wired systems. As
the APP signal changes, the ECM commands the throttle actuator motor to open
the throttle plates a certain amount. The throttle actuator assembly TPS
sensors then relay the throttle opening back to the module. The APP signal
is constantly being compared to the throttle actuator assembly
NOTE: It is important to know that under no circumstances
should the throttle plates be forced open with a screwdriver. Some techs
have tried this using screwdrivers or pliers to accelerate the engine, as
with older systems (non drive-by-wire). This action will cause a discrepancy
between the APP signal and the throttle actuator TPS sensors causing the
vehicle to go into limp-in-mode (loss of performance). In most cases, the
use of a dedicated OEM scan tool is needed to reset this condition and
re-learn a new throttle actuator position adaptive memory.
position sensors. Any discrepancy between the APP and the
TPS sensors will set a code, with the system also going into limp-in-mode.
The ECM flags a discrepancy when the deviation or difference between the APP
and the TPS signal goes above a maximum pre-programmed amount. All
drive-by-wire systems use dual TPS sensors. Again this is all done for
In drive-by-wire systems, idle speed is achieved in two
ways. Some systems use a regular IAC valve to control idle speed. There is
no basic difference between the old and this new IAC valve. The only
exception is in the internal ECM programming, which takes into account the
differences in the throttle actuator.
The second way to control idle speed is by the ECM using the
actual throttle actuator to slightly open the throttle plates. This action,
in conjunction with the engine speed input, and the TPS signal determines
the idle speed. As a side note, it should be pointed out that in older (non
drive-by-wire) systems the throttle plate and bore carbon deposits were the
cause of a good deal of idle air/fuel mixture problems. However, in
drive-by-wire systems, dirt and carbon deposits could render the engine
inoperative by making it unable to idle properly. It all goes back to the
fact that any discrepancy between the APP and the TPS will cause a variety
of different problems. In this example of an engine with a dirty throttle
body, the carbon and dirt creates an air restriction causing the ECM to
increase or change the throttle plate opening. The ECM has pre-programmed in
its internal memory the maximum throttle opening at idle possible. If this
limit is reached, (because of the carbonized throttle body) a trouble code
is set in memory and the system goes into limp-in-mode. A throttle body
cleaning will correct any of these problems.
Conditions that Affect Operation
As stated before, dirt and carbon deposits in the throttle
plates have a negative effect on throttle control. The ECM will always try
to adjust or compensate for the air restriction in the throttle body. This
excessive throttle opening creates the APP to TPS signal discrepancy,
triggering the ECM into setting a code.
The APP dual or triple potentiometer signal must also be
within proper specifications. An improperly adjusted (if adjustment is
available) APP sensor at the rest position will be picked up by the ECM, as
is a bad or erroneous signal. Also, the two or three APP signals must be
without discrepancies within each other. If the main signal fails the ECM
then looks at the other signals for proper accelerator pedal position. This
will set a code and performance will be reduced.
The throttle actuator is an actual electric motor (stepper
or DC motor) that is energized by the ECM or throttle module in response to
the APP signal. The actuator has a set of gears and springs which enables it
to work the throttle plates. Any binding of the gears or breakage of the
springs will create an immediate discrepancy between the APP and the
throttle TPS signals. The same applies to a defective throttle actuator
motor itself. Again, a discrepancy beyond the preprogrammed acceptable limit
will set a code and the ECM will go into reduced engine performance. On
certain occasions, if the TPS dual signal is completely lost the engine will
shut down. Hence the use of two TPS signals with independent voltage
reference and ground. Again, all done for redundancy.
An invaluable tool in
diagnosing APP and drive-by-wire system problems in general is the scanner.
An after market or preferably an OEM scan tool analysis, with all the
parameters (PIDs) necessary for proper signal monitoring makes for a quick
preliminary testing of the system. A quick analysis of the APP PIDs will
indicate if the potentiometer signals are out of specifications and any
possible signal discrepancy can be picked up by analyzing the different
PIDs. A careful observation of the APP signals, while slowly pressing on the
accelerator cable, will reveal a faulty APP sensor (provided that the scan
tool is fast enough, as with some OEM scanners) . Otherwise, a multi-channel
VOM or scope should be used. It is also worth knowing that most systems will
put out throttle position ERROR PID. Watch carefully for this parameter,
since an error flag will reveal an APP to TPS discrepancy.
NOTE: If an OEM scan tool is available and the
manufacturer has bi-directional control of the throttle actuator, perform an
increasing throttle opening command and observe the TPS output on both the
scope and the scanner PIDs. Always be on the alert for any APP to TPS
discrepancy that might send the system into limp-in-mode.
Despite today’s faster and better scan tools, the second
step to this procedure should always be followed by a manual electrical
• The first step is to prove the APP voltage reference and
ground circuits. These circuit are provided by the ECM independently of each
other, for redundancy.
• A couple of other scan tool data PIDs are also helpful in
diagnosing this system. If the system uses the throttle actuator for idle
control, look for an out of balance air/fuel ratio mixture. The long term
and short term fuel trims (LTFT & STFT) are helpful PIDs when it comes to
diagnosing air/fuel ratio problems. Provided that the vehicle has no vacuum
leaks or fuel restrictions, an out of adjustment or faulty TPS will send the
wrong signal reading to the ECM. A maladjusted TPS could make the ECM react
as if the throttle plates were in a different position than they really are.
The net effects will be the ECM increasing throttle plate
opening (this will create a lean condition), or reducing throttle opening
(this action will create a rich condition or a possible engine stall).
By studying the system PIDs carefully before jumping into a
long diagnostic routine, it is possible to pin point the fault faster and
easier. Knowing the system is the first rule to remember in modern