OK, there are lots of OBD questions popping up. how about we have a mega thread just for OBD discussion and to share readings etc. A "one stop shop" if you like.
Perhaps we can llis ones we ahve bought and how good they are and share abnormal readings etc. A lot of knowledge in tis area has the potential to save members money at the garage. I know its not T4 , but it can help identify simple problems
I will start with this rather good info I found tongiht telling you what all the sensor readings mean.
Perhaps we can llis ones we ahve bought and how good they are and share abnormal readings etc. A lot of knowledge in tis area has the potential to save members money at the garage. I know its not T4 , but it can help identify simple problems
I will start with this rather good info I found tongiht telling you what all the sensor readings mean.
OBD-II Sensor Description
This is a complete list of the sensors that OBD Gauge supports. Only those sensors that are also
supported by the connected vehicle are displayed.
Throttle Position
This sensor generates a voltage proportional to instantaneous throttle position (0-90°). It is
measured in degrees. Usually a closed throttle will read about 10° and a fully open throttle about
79.3° to 90°. This sensor is used to determine the pos ition of the throttle plate. It informs the
engine management computer if the throttle plate is closed, wide open or partially open. The
sensor is basically a potentiometer providing an output voltage. The closed throttle position may
be used to determine when the accelerator pedal is not being pressed.
Engine RPM
This sensor generates a pulse for every revolution of the engine. It is measured in revolutions per
minute. Usually the sensor is a pick up coil looking for a unique keyed tooth on an engine timing
rotor. Values range from 0 upwards.
Vehicle Speed
The vehicle speed sensor tells the vehicle’s engine management computer how fast the vehicle is
moving. Speed is measured in kilometers per hour (km/h) or miles per hour (MPH). The vehicle’s
engine management computer adjusts the injector pulse times to maintain the proper fuel mixture
at any speed and load. It also sends a signal to the speedometer and, in most cases, to the
cruise control computer.
Calculated Load Value
The vehicle’s engine management computer generates a value of between 0 to 100% to
represent current loading.
Ignition Timing Advance
This system is used on spark ignition and combustion ignition engines to start the ignition event
earlier by controlling the ignition or fuel system. As engine speed increases, start of ignition or
start of injection must occur earlier in order to result in the most effective downward thrust on the
piston during the power stroke. The vehicle’s ECU generates this value, measured in degrees
(−64° to 63.5°).
Intake Air Pressure
This sensor creates a signal that is proportional to the average pressure in the intake manifold.
The pressure should be low and fairly steady at idle and light loads and will go higher as the load
increases, up to atmospheric pressure.
Intake Air Flow Rate
This sensor provides a voltage signal, corresponding to the air mass entering the engine. The
electrical signal is transmitted to the engine control module and is used to determine the required
fuelling for petrol engines and for exhaust gas recirculation (EGR) control on diesel engines. It is
typically mounted inside the clean air duct (usually at the exit of the air cleaner). Modern vehicles
use a ‘hot wire anemometer” sensor for this measurement.
Short Term Fuel Trim
(STFT) – The neutral value for the short term fuel trim is 0%. Any deviation from 0% indicates the
short term fuel trim is changing the injector pulse width. The amount of pulse width change
depends on how far the short term fuel trim value is from 0%. The short term fuel trim is rich at -
99% and lean at +99%.
The short term fuel trim changes the pulse width by varying the Closed Loop factor of the base
pulse width equation. As the vehicle’s engine management computer monitors the oxygen
sensors input, it is constantly varying the short term fuel trim value. The value is updated very
quickly. The short term fuel trim only corrects for short term mixture trends. The correction of long
term mixture trends is the function of long term fuel trim.
Long Term Fuel Trim
(LTFT) – As the engine operating conditions change, the vehicle’s engine management computer
will determine what long term fuel trim factor to use in the base pulse width equation.
If the short term fuel trim is far enough from 0%, the vehicle’s engine management computer will
change the long term fuel trim value. Once the LTFT value is changed, it should force the STFT
back toward 0%. If the mixture is still not correct, the STFT will continue to have a large deviation
from the ideal 0%. In this case, the LTFT will continue to change until the STFT becomes
balanced. Both the STFT and LTFT have limits which vary by calibration. If the mixture is off
enough so that LTFT reaches the limit of its control and still cannot correct the condition, the
STFT would also go to its limit of control in the same direction. If the mixture is still not corrected
by both STFT and LTFT at their extreme values, a Fuel Trim DTC will likely result.
Air Temperature
The intake air temperature sensor tells the computer the temperature of the incoming air. Since
cold air is denser than warm air, it needs more fuel to achieve the ideal air to fuel ratio. To do this,
the computer will open the injectors for a longer period of time.
Coolant Temperature
This sensor measures the temperature of the engine. When the engine is cold, it needs more fuel
to operate correctly. In very cold conditions, a large quantity of fuel is needed to start the car. This
is called “Cold Start Enrichment” and replaces the function of a choke.
Coolant Temperature
This sensor measures the temperature of the engines coolant. Normally about 82 to 104°C (180
to 220°F) on most cars once the engine has warmed up .
Fuel Pressure (not avialble on F/TF?)
This sensor measures the regulated pressure on the fuel rail. This is a very important value; the
fuel/air ratio is determined by length of time the fuel injector is pulsed on. If the fuel pressure is
not constant, then the ECU cannot maintain the correct fuel/air ratio, causing emissions or
performance problems.
Most regulators have a static pressure of between 38 and 44 PSI.
Oxygen Sensors
All modern vehicles have multiple oxygen sensors. The number of sensors is always even. The
sensors are part of the emissions control system and feed data to the engine management
computer. The goal of the sensor is to help the engine run as efficiently as possible and also to
produce as few emissions as possible.
A gasoline engine burns fuel in the presence of oxygen. An oxygen/fuel ratio of approximately
14.7:1 is ideal (stoichiometry). The ratio depends on the amount of hydrogen and carbon found in
a given amount of fuel. If there is less air than this ideal ratio, then there will be fuel left over after
combustion. This is called a rich mixture. Rich mixtures are bad because the unburned fuel
creates pollution.
If there is more air than this perfect ratio, then there is excess oxygen. This is called a lean
mixture. A lean mixture tends to produce more nitrogen-oxide pollutants, and in some cases, it
can cause poor performance and even engine damage.
An oxygen sensor is positioned in the exhaust pipe and can detect rich and lean mixtures. The
mechanism in most sensors involves a chemical reaction that generates a voltage. The engine's
computer looks at the voltage to determine if the mixture is rich or lean, and adjusts the amount of
fuel entering the engine accordingly.
When the oxygen sensor fails, the computer can no longer sense the air/fuel ratio. This sensor is
much more complicated than the other sensors and deserves a more detailed explanation.
Basically, the oxygen sensor indicates the presence or absence of oxygen in the exhaust stream.
The ECU, in order to keep emissions low, adjusts the fuel/air ratio at stoichiometry, the exact
mixture which will completely burn all fuel with the oxygen present in the cylinder, leaving no fuel
or oxygen in the exhaust.
As the fuel/air ratio deviates from stoichiometry, then either oxygen or hydrocarbons (unburned
fuel) will be present in the exhaust. The oxygen sensor produces a low voltage (< 0.1V) when
oxygen is present in the exhaust, and produces a high voltage (> 0.8V) when there is no oxygen
in the exhaust. The ECU uses the voltage output from the oxygen sensor to make slight
adjustments to the amount of fuel injected to keep the fuel/air ratio near stoichiometry. It does this
by increasing the fuel to get a high reading, meaning that there is no oxygen in the exhaust, the
slightly decreasing the amount of fuel until the oxygen sensor reading drops low. This process
goes on continuously to keep the average fuel/air ratio as close to stoichiometry as possible.