MACS Service Reports - 2015 - APR5

Testing OAT Sensors

A grounded signal wire (low resistance) pulls the reference
voltage down. If a ground fault cannot be found in the signal
wire, check for reference voltage at the PCM. If the scan tool
or meter indicates the value that is equivalent to a high voltage reading, the ground circuit is checked next. A next logical step would be to use a jumper wire to short both leads of
the thermistor's harness together simulating a shorted sensor condition. Your scan tool / meter reading should display
a low or zero voltage and a DTC for a thermistor circuit short
/ low voltage should also set. You can also set a lab scope for
a long time period and watch a complete warm up cycle to
test the ECT and thermostat. (Figure 11). ■

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Testing an OAT is exactly like testing an in-car temp sensor as far as the use of an ohmmeter or voltmeter. It's when
observing the sensor with a scan tool or watching it on the
DIC (Driver Information Center) when you have to remember about the software filtering of the sensor's output. If
you cool the sensor down with some circuit chiller (while
the ignition is turned on), you'll see an immediate update on
most models. But when you heat the sensor up with a heat
gun or hair dryer, don't expect to see a change in the PID
(Parameter Identification) / DIC on most vehicles without
performing a road test, or at least lifting the vehicle and getting the drive wheels to spin at about 30 MPH or so. One
notable exception to this filtering of OAT signals is within
GM's line-up. For reasons that are a mystery to me, the OAT
on 2001-2005 full size GM trucks with the Duramax engine
doesn't get its update delayed based on VSS. Therefore you
will see a jacked up temperature reading on the DIC when
the engine sits and idles (thereby roasting the sensor with
engine heat). The solution is to remount the sensor to a radiator support screw near the left headlight. The wiring will
be too long now, so be sure to secure it with a zip tie or two.

Diagnostics Technician Methods - ECTs
Disconnecting a thermistor in order to determine if the
expected result occurs is one simple testing method. With
the key-on engine-off, the reading on your scan tool or meter will indicate a value that is equivalent to a high voltage
reading. This indicates reference voltage is getting from the
module to the thermistor. A DTC for sensor voltage high /
sensor open should also set. If not, this portion of the circuit
is at fault. If not, look for a ground fault in the signal wire.

Figure 11: When the going gets tough (multiple comebacks), the tough
(smart) techs get out their lab scopes! This scope's time base was slowed
down to capture several minutes of the ECT's warm up performance.

VOLTAGE DROP TESTING ON SENSOR CIRCUITS? YES!
You might think that because sensors are such low
current devices that your ohmmeter is sufficient for
circuit testing. While it is true that high current items
such as blower motors and fuel pumps are the more
common components to suffer from voltage drops in
a circuit, you can and should check sensor circuits as
well. Simply disconnect the sensor 's wiring from
the module then substitute an electrical load such
as a halogen headlamp in place of a sensor. At
the disconnected wires on the module end apply
power and ground into the sensor V-Ref and Return (ground) in order to get the light to burn at the
other end of the circuit. Now compare your applied
voltage to the voltage available across the substitute load to determine if you are dropping voltage along the way. If more than 0.5 volts has been
dropped, connect the meter between a known good
ground and the ground side of the substitute load.
Follow this by the same test, only this time between
a known good power source and the power feed to
the substitute load to determine if the voltage drop

April 2015

is on the power or ground side of the circuit. IF
you exercise caution in removing any connections
from the module prior to installing and powering
the substitute load and adhere to common sense regarding matching the substitute load current draw
to the wiring size, you'll be fine. What I mean by that
is if the sensor circuit is comprised of 18 gauge wiring,
you wouldn't want to grab a ½ ohm substitute load
that would draw 24 amps. That sensor wiring will be
bad then! Why go the extra step of voltage drop testing
low current sensor circuits? Under normal diagnostic
routines, you would read DTCs and follow OE service
diagnostics. Under unusual circumstances, you want
to go the extra mile.
Coolant sensors are one of the most important sensors on any engine. They must be dead on accurate in
order to get proper drivability, cooling fan operation,
TCC lock up, emissions, etc. The extent of engineering many OEMs are now utilizing to ensure this little
unsung hero is dead on will become apparent as you
read to the very end of this report!

MACS Service Reports

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