Throttle Potentiometer TTT2.0
What it does
It measures the throttle valve angle and sends a
proportionate voltage signal to the ECU. The ECU uses
this information as a load signal and calculates the
engine’s ignition and fuel system requirements.
It is also used to recognise specific operating conditions,
such as idle and full throttle, that need to be controlled.
How it works:
Physically attached to the throttle body, it is an
electromechanical device that converts the rotary
motion/angle of the throttle shaft into an electrical
signal that the ECU can understand.
It is generally a three or four terminal device,
one terminal is the electrical feed (generally five
volts) and the other two terminals, being the output
are connected to internal variable resistors. Two
internal resistors are used to ensure that whatever
the throttle position the ECU will have an exact measurement.
The fourth terminal, which is not fitted on all throttle
potentiometers, is an earth connection.
The format of the two internal resistors varies,
some manufacturers utilise opposite variance resistors,
so as the throttle angle increases one resistor increases
and the other decreases. Other manufacturers use two
different value resistors, the first one has a very
“steep” resistance “curve”
and is used to measure low throttle openings, the
other resistor has a very “shallow” resistance
“curve” and is used to measure mid to
full throttle openings, both these resistances cases
will increase with throttle opening.
Note:
Before the introduction of full engine management,
throttle potentiometers consisted of just two switches.
One for full throttle and one for an idle position,
they appear similar but they are operationally different.
Ensure that you are fitting/testing the correct type.
Reasons for failure:
Constant throttle movement wears through the resistive
wiper-tracks, the first 30 degrees of throttle movement
is the area in which most wear failures take place.
This usually results in flat spots or holes in acceleration
or tick-over problems such as stalling.
Other failures are due to the environment in which
the potentiometer is located; it suffers from dirt,
water and vibration damage as well as over-voltage
damage due to wiring faults.
Testing:
A simple multimeter will highlight most faults. Check
the voltage supply to the unit, it is generally five
volts but a short to 12 volts is common fault on some
vehicles. Check the voltage feed with the sensor connected
so as to “load” the power supply; this
should show up any poor connections on the feed side.
Check the earth for a zero voltage reading.
Check the voltage output at the idle position, compare
this with OE specifications. Check the output of both
resistors with both an opening and closing throttle,
it is often best to do this from within the car and
operate the throttle with your foot as this tends
to be easier to control.
Check the resistance throughout the range, paying
particular attention to the first 30 degrees of movement
as above. If the potentiometer is faulty, it usually
shows up as an open circuit at one particular throttle
opening, this can be difficult to spot. The ECU diagnostic
function may or may not, as is often the case, record
the fault. It is also good policy to “tap”
the potentiometer with a screwdriver, or the like,
when testing, to simulate vibration; this can show
up track connection faults.
Caution:
The idle position of the sensor is critical, there
is often a specific voltage setting that should be
checked during any engine management system check.
Any wear in the vehicle’s throttle linkage,
mechanism or body will cause a misreading at the ECU
which will result in a tick-over problem.
For further technical information
concerning Throttle Potentiometers sensors please
phone our technical help line 01527 839307
