474 Auto Electricity and Electronics
The fuel sending unit is normally held in the fuel tank
by a large locking ring. Rotating the lock ring will free the
sender from the tank.
Warning: Make sure the fuel tank is almost
empty before removing the sending unit, or
gallons of highly fl ammable gasoline or
diesel fuel can pour out of the tank.
Water-In-Fuel Sensor Service
A bad water-in-fuel sensor can improperly illuminate
the dash light, or it may not warn when water is in the die-
sel fuel. Many diesel fuel systems use this sensor to protect
from internal corrosion caused by water contamination.
Figure 24-25 shows how one automaker recommends test-
ing the water-in-fuel sensor.
Knock Sensor Service
A bad knock sensor can upset ignition timing and affect
turbocharger boost pressure. It is used to detect abnormal
combustion or ping. When it “hears” pinging or knocking,
it will retard ignition timing or lower turbo boost with the
turbo wastegate. Many computers will store a trouble code
if there is a potential problem with the knock sensor.
To check a knock sensor, tap on the engine right next
to the knock sensor. This will simulate pinging or knocking
and should make the computer retard the ignition timing.
The light taps should make the engine speed drop slightly.
You might need to prop open the throttle to increase engine
speed slightly so timing is advanced and will retard.
If tapping next to the sensor has no effect on timing and
engine speed, you can check it with a VOM. Refer to the manual
for recommendations. Remember to check the wiring leading
to the knock sensor before removing or replacing the sensor!
Diesel Metering Valve Sensor Service
A bad diesel metering valve sensor can affect how
much fuel the diesel injection pump injects into the cyl-
inders. This can upset the diesel’s air-fuel ratio and reduce
performance and fuel economy. This sensor is only used on
late-model diesels with computer control.
A digital voltmeter or an ohmmeter can be used to
check the metering valve sensor. As in Figure 24-26, sensor
voltage output or resistance must be within specs. If not,
replace the sensor.
Actuator Service
Actuator service involves testing solenoids, servo
motors, displays, and other output devices for possible
Figure 24-24. Fuel sending unit or sensor should have a spe-
cifi c resistance value for specifi c fl oat arm positions. (Toyota)
Ohmmeter Should
Read Specs in
Each Position
Spec
Heights
70mm
168mm
235mm
F
1/2
E
Figure 24-25. Note how you can test this water-in-fuel detector.
(Ford)
Drain Fuel
Filter Module
Sensor Lead
12 Volts
12-Volt 2-Candle
Power Test Lightbulb
Bench Test Set-Up for Testing Drain Fuel Filter Module
Jumper
Water Must
Be Grounded
by Jumper
Water-in-Fuel
Sensor
Water
Ground
Ground
Battery
Figure 24-26. Digital voltmeter is needed to check the conditions
of this metering valve sensor in the diesel injection pump. (Ford)
Digital Voltmeter
Should Show Specs
Test Leads
Adapter Harness
Diesel
Injection Pump
Engine
Harness to
Computer
Jumper Wire to
Specified Pins
Fuel Metering
Valve Sensor
Sensor Harness
Sensor
Harness