MACS Service Reports - 2015 - APR3

General Motors

If there is only one resistance in a circuit (a load
such as a blower motor on high speed, for example)
that resistance drops all of the available voltage.
If there are other resistors in series with the load
(a blower resistor for example) the resistor drops
some of the voltage while the blower motor drops
the rest. That is, assuming there are no poor connections creating unauthorized voltage drops. That
is where we get the electrical rule "the sum of the
voltage drops in a series resistance circuit equals the
source voltage." In the case of the voltage divider
circuit as used in thermistor circuits, the fixed value resistors in the modules are always fairly high -
typically several thousand ohms. Thermistors, like
other sensor circuits, pass very little current. The
higher the resistance, the lower the current flow. ■

Figure 5: A typical pull down resistor internal to a module may be
around 10 kΩ (10,000 ohms). If the temperature being measured equated
to the thermistor also being 10 kΩ (around 120°F), half of the available 5
volts would be dropped at the pull down resistor in the module, while the
thermistor would drop the other 2.5 volts. This is called a voltage divider
circuit. The bigger the resistor, the more voltage it drops in comparison to
a smaller resistor in series with it.

ON BOARD DIAGNOSTICS
Any sensor, whether connected to the engine (OBD-II) or
HVAC controls have on-board diagnostics that follow two
general rules of sensor monitoring, regardless of the sensor
application or vehicle manufacturer. Those two rules are:
a. Electrical Malfunctions (Operating in a Dead Zone)
b. Rationality (Reading is Unbelievable)
Electrical continuity problems, such as shorts, opens and
out-of-range values, are checked by monitoring the input
signals at the analog/digital converter within the module.
To detect a severe continuity problem, all sensors have what
is called a Dead Band (Figure 6). The band is in an area not
normally able to reach certain voltage levels. Each manufacturer sets this level for each sensor. Using a temperature
sensor for our example: The manufacturer may determine
that readings from: 0.001V to 0.175V are out of range. At the
top, it may be that 4.75V to 4.99V is out of range. This is the
dead band, and is usually caused by an intermittent problem.
Anything that would reach 5.0V is open and anything at 0V
is shorted.

Delphi

kiatechinfo.com

Another input strategy consists of rationality checks. This
test is simply the use of other sensor readings and calculations to confirm or determine that a specific sensor input is
appropriate for the existing conditions. If the control module
sees a signal of 0.2V or less from a sensor, the control module
will identify this as an out-of-range signal, and will suspect
that the circuit is either shorted to ground or a loss of voltage
has occurred due to the low voltage input. Out of range does
not mean the signal is incorrect, it means it has seen voltage
reach an implausible level.
When the control module identifies a signal as out of range,
it is often termed as an implausible input, making valid signals plausible. This is another part of the rationality test. For
example, if an engine has been running for one hour and
the IAT (Intake Air Temp) sensor has increased from 45°F to
120°F, a reading of 50°F on the ECT is hardly believable. It is
not shorted or open (operating in either extreme of the dead
zone), but it isn't producing a rational (or plausible) reading either. This technique is similar to virtual sensors, where
other sensors on the vehicle work in concert with a module
to come up with a number that is close to what an actual
sensor would read. Implausible sensor readings and severe
continuity or voltage level problems, such as open and short

Figure 6: Thought automotive sensors operated between 0 and 5 volts? Not
exactly. The "dead band" is somewhere lower than 0.5 volts and above 4.5
volts. If the sensor is reading in the dead band, it truly is dead!

April 2015

Figure 7: P0117 would be the likely DTC with temperature this high /
resistance this low.

MACS Service Reports

http://www.kiatechinfo.com

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