MACS Service Reports - 2015 - APR6

Fail-soft - Completely Made Up Scanner Readings

puts. To determine if the value is substituted, disconnect the suspect sensor, jumper its two wires together
and recheck the scan tool value. If it is the same, it is
likely a substitute value. In these instances, you must
rely on your meter. Most of the time, your scan tool's
OBD-II generic section will display the true reading
instead of the substituted 'fail-soft' value. ■

Some scan tools may display a "substituted value"
during this test. This is due to the PCM recognizing an
out of range value, and substituting another made up
value based on what the sensor should be according to
the operating conditions indicated by other sensor in-

CRAZY KIA CASE STUDY
2006 Kia Sedona 63,000 Miles
Complaint: Intermittent MIL (Malfunction
Indicator Lamp)
P0118 History Code

wired in series. When the voltage being dropped
across the ECT gets low enough, the larger of the
two fixed ECM internal resistors is bypassed via a
transistor inside the ECM (Figure 12). The lower re-

History
1. Shop could only get code to set once.
2. Scan tool PIDs all look normal.
3. Saw some 'crazy stuff ' on meter once.
4. Replaced ECT and PCM (2X each).
5. Still having problems with MIL.
General Description from Kia Service Manual

Dave Hobbs

The Engine Coolant Temperature Sensor
(ECTS) is located in the engine coolant
passage of the cylinder head for detecting
the engine coolant temp. The ECTS uses a
thermistor whose resistance changes with the
temperature. The electrical resistance of the
ECTS decreases as the temperature increases, and increases as the temperature decreases. The reference 5 V in the PCM is supplied
to the ECTS via a resistor in the PCM. That
is, the resistor in the PCM and the thermistor
in the ECTS are connected in series. When
the resistance value of the thermistor in the
ECTS changes according to the engine coolant temp., the output voltage also changes.
During cold engine operation the PCM increases the fuel injection duration and controls the ignition timing using the information of engine coolant temperature to avoid
engine stalling and improve drivability.

Figure 12: Two resistors in series: R1 which is 3.65 kΩ and R2 which
is 348 Ω make up an effective resistance of 3,998 Ω (let's just call it
4 kΩ). When the microprocessor internal to the PCM turns on the
transistor for the stepped coolant sensor circuit, the larger of the two
pull down resistors (R1 / 3.65 kΩ) is bypassed, allowing for a much
better match between the lower value "hot" ECT under the hood and
the single (R2 / 348 Ω) pull down resistor in the PCM.

sistance, warm ECT's resistor then has a more suitable mate in its voltage divider network to allow
for more resolution. You won't see this shift from
a two resistor arrangement to the single pull up resistor on a scan tool; the ECM knows it's doing this
little trick and responds with a smooth movement
of temperature readings, gradually increasing with
engine run time. You will, however, see the transition if you are measuring voltage of the two step
ECT circuit live with a meter or scope (Figure 13). It
is very abrupt (not something you usually observe
when watching a coolant sensor) because a transistor is making the change from a 2 resistor match
within the PCM to a one resistor match within the
PCM. Why use a scope meter when you have a scan

Exceptions to 'Simple' Circuits - GM 2 Step ECT
That Kia service manual description sounds pretty much like normal thermistor strategy. Sometimes
there is more to the story though. Take GM for example. For many years, they (and other OEMs) have
refined the voltage divider circuits to basically be
flexible depending on engine temperature. Instead
of matching the thermistor 's huge resistance span
with a fixed resistor in the ECM, GM matches the
ECT with two fixed resistors internal to the ECM

April 2015

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2015