Innovate Motorsports LogWorks 3 - Manual (Page 35)

LogWorks3_Manual_1.01.doc - 35 -
Of the three measurement points, the easiest to get is the middle point. Just leave the sensor in a
warm room (Room Temperature at or above 25 degC or 77 degF) for at least an hour. Measure
the resistance of the sensor with a digital multimeter and write it down. Also record the
temperature with a good thermometer.
Two other easy temperature standards available to everybody are the freezing point of water (0
degC or -32 degF) and the boiling point of water (100 degC or 212degF). Water does not usually
hurt NTC resistor sensors. Just make sure they are thoroughly dried out after the measurement.
To get the 0 degC resistance, fill a small plastic container with a mixture of ice-cubes and water.
Connect a multimeter to measure the sensor resistance and put the sensor in the ice water. Wait
until the resistance has stabilized, indicating the sensor is at the same temperature (0 degC) as
the water.
Next put the sensor in a pot of boiling water and repeat the measurement, again waiting for the
temperature and resistance to stabilize.
4.2.1 Thermistor circuit
Most ECU’s and also the Innovate MTS products do not measure resistance directly, but
measure a voltage. To convert the resistance of the sensor into a voltage requires a 5V power
supply and a load resistor.
The typical circuit for this looks like one of the following schematics:
A. Sensor Grounded
B. Sensor at 5V
In case A, the sensor is grounded on one side. This is the most common case where the sensor
body is in physical contact to the engine, and is grounded through its body. Very often these
sensors have only one pin.
To find out, which circuit your ECU uses, measure the voltage between each of the two sensor
pins (if it has two) and ground. If one voltage is zero and the other is some voltage between 2 and
5V when the sensor is cold, you have circuit A. If one voltage is 5V while the other is lower, you
have circuit B.
To get the voltage curve, you also need to know the value of the load resistor. You can’t measure
it directly with the multimeter though, because it would interfere with the ECU and can possibly
damage it. The method to use goes as follows: