Actron AutoTroubleShooter CP7677 - Manual (Page 27)

27
Testing Engine Sensors
In the early 1980’s, computer controls were installed in vehicles to meet Federal
Government regulations for lower emissions and better fuel economy. To do its job, a
computer-controlled engine uses electronic sensors to find out what is happening in
the engine. The job of the sensor is to take something the computer needs to know,
such as engine temperature, and convert it to an electrical signal which the computer
can understand. The digital multimeter is a useful tool for checking sensor operation.
The Oxygen Sensor produces a voltage
or resistance based on the amount of
oxygen in the exhaust stream. A low
voltage (high resistance) indicates a lean
exhaust (too much oxygen), while a high
voltage (low resistance) indicates a rich
exhaust (not enough oxygen). The com-
puter uses this voltage to adjust the air/
fuel ratio. The two types of O
2
Sensors
commonly in use are Zirconia and Tita-
nia. Refer to illustration for appearance
differences of the two sensor types.
Test Procedure (see Fig. 31):
1. If engine is HOT, let it COOL down
before proceeding.
2. Remove Oxygen Sensor from ve-
hicle.
3. Insert BLACK test lead into COM
test lead jack.
4. I n s e r t R E D t e s t l e a d i n t o
test lead jack.
Titania-Type
Oxygen Sensor
Zirconia-Type
Oxygen Sensor
Exposed
flat element
Flutes
Rich Lean
Red
5. Test heater circuit.
• If sensor contains 3 or more wires,
then your vehicle uses a heated O
2
sensor.
• Refer to vehicle service manual for
location of heater pins.
• Connect RED test lead to either
heater pin.
1-wire or 3-wire: Ground is sensor housing
2-wire or 4-wire: Ground is in sensor wiring
harness
Ground
Black
Fig. 31
Oxygen (O
2
) Type Sensors