Chapter 24 Sensor, Actuator, Computer Service 469

Tan sensor tip—could be lead contamination. It can
sometimes be cleaned away by briefl y running the
engine lean on unleaded fuel.

Black sensor tip—normally indicates a rich mixture
and carbon contamination. It can sometimes be
cleaned after correcting the cause.
Note that some manufacturers recommend oxygen
sensor replacement after only 12,000 miles (19,308 km)
when the sensor is removed. Therefore, the O2 sensor is
normally replaced with a new one when unscrewed from
the exhaust system.
Manifold Absolute Pressure Sensor Service
A bad manifold absolute pressure sensor can affect the
air-fuel ratio when the engine accelerates and decelerates. It
serves the same basic function as a power valve in a carburetor.
It senses engine vacuum to signal when more fuel is needed
under a load or when gaining speed. It might also have some
effect on ignition timing and a few other computer outputs.
To test a manifold absolute pressure sensor, measure sen-
sor circuit voltage or sensor resistance while applying vacuum
to the unit. As shown in Figure 24-11, a vacuum pump is
attached to the sensor’s vacuum fi tting. Apply specifi ed vac-
uum levels while measuring the output of the MAP sensor.
Some manuals instruct you to measure sensor voltage
at a specifi ed test terminal. Others might have you discon-
nect the wires from the sensor and compare ohmmeter
readings to specs.
In any case, sensor values must be within limits at the vari-
ous vacuum levels. If testing at the test terminal, check the wir-
ing harness before condemning the sensor. A poor connection
or short could upset a reference voltage fl owing to the sensor.
If problems are indicated, measure the voltage going to the
sensor. Make sure any reference voltage is up to specs (usually
about 5 volts). If needed, check harness resistance. Replace the
sensor or repair the wiring as needed, Figure 24-12.
Jumper wires are often used when testing a MAP sen-
sor. Connect the jumpers between the sensor’s terminals and
the wiring harness. This allows you to measure actual sensor
circuit voltage with the unit operating. See Figure 24-13.
Throttle Position Sensor Service
A bad throttle position sensor (TPS) can also affect fuel
metering, ignition timing, and other computer outputs. It
can also trip several trouble codes on some systems and
can be a frequent cause of problems. Many throttle position
switches use contact points or variable resistors that can
wear and fail after prolonged operation.
A throttle position sensor is comparable to a carbure-
tor accelerator pump and metering rod. It signals the com-
puter when the gas pedal is depressed to different positions
for acceleration, deceleration, idle, cruise, and full power.
It can cause a wide range of performance problems. If
shorted, it might make the fuel mixture too rich. If open, it
can make the mixture too lean.
Figure 24-11. Manifold absolute pressure sensor is tested by
applying vacuum to the unit while measuring voltage or resis-
tance. A—Test connection. B—One example of specs. (Honda
Motor Co.)
Pump
Handle
Check Pressure
Sensor Output
with Voltmeter
or Ohmmeter
Vacuum
Pump/Gauge
B
A
Voltage Drop Change
with Different
Pressures Applied
4.5
V
1.41
0.5
100
0.14
(660)
350
0.47
(410mmHg)
1200mmHg
1.6 kg/cm2
( ) : Gauge reading
Vacuum
Hose
Figure 24-12. Always closely check the vacuum line running to
the MAP sensor. Any leaks can upset its operation.
Map Sensor
Wires
to ECM
Vacuum Line
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