Oscilloscope Patterns and Waveforms
Engine Coolant Temperature Sensor:
BACKGROUND AND APPLICATION INFORMATION:
Most Engine Coolant Temperature (ECT) sensors and Intake Air Temperature (IAT) sensors on 1980-and-newer vehicles operate in the same way, so the same Digital Storage Oscilloscope (DSO) setup and test procedure will apply. Most ECTs and IATs are Negative Temperature Coefficient (NTC) type thermistors. This means they are primarily two wire sensors that change resistance when their temperature changes. They are supplied with a 5-volt VRef power signal and return a voltage signal to the Powertrain Control Module (PCM) proportional to the engine coolant or intake air temperature. The rule for NTC resistors is: their resistance goes down as their temperature goes up. When the DSO or Digital Multi Meter (DMM) is connected to the signal from an ECT or IAT, what is being read is the voltage drop across the sensor's NTC resistor. Remember: when the sensor is cold the resistance (and voltage) is high, and when the sensor is hot it's resistance (and voltage) is low. The DSO setup (and what to look for in a bad sensor) is the same.
CONNECT AND SETUP THE DIGITAL STORAGE OSCILLOSCOPE (DSO):
1. Connect the "COM" probe to the ECT or IAT ground circuit, engine block, or negative battery post.
2. Connect the "COM" probe to the ECT or IAT signal wire to the PCM. Use a wiring diagram to obtain the PCM pin number or the color of the wire.
3. Set the voltage scaling to 1 V/div.
4. Set the time base to 20 s/div (20 seconds per division).
5. Select �GND" coupling and position the trace on the second division line from the bottom of the DSO display.
6. After ground position is set, select �DC" input coupling.
7. Select �NORMAL" acquire mode.
8. �TRIGGER" mode is usually set automatically (Roll Mode).
EXERCISE THE SENSOR
It's best to start the testing procedure with a stone cold engine unless the problem being dealt with is temperature dependent and it's already known (from customer input, etc.) what temperature to start with. Start the engine and hold the throttle at 2500 RPM until the trace goes from the left side of the screen to the right side of the screen. At 20 seconds per division this will seem like forever, but it is only 3 minutes and 20 seconds. Press the "RUN/HOLD" button on the DSO to freeze the waveform on the display for closer inspection. The sensor has now been put through its entire operating range, from stone cold to operating temperature.
Good Engine Coolant Temperature Sensor Pattern (Typical):
EXPECT THIS WAVEFORM RESULT
Check the manufacturer's specifications for exact voltage range specifications, but generally the sensor's voltage should range from between three volts to just under five volts when stone cold, dropping to around one volt at operating temperature. The Critical Dimension of this DC signal is amplitude. The sensor must generate a signal with a certain amplitude at any given temperature for it to be good. Opens in the ECT or IAT circuit will appear as upward spikes to VRef. Shorts to ground in the ECT or IAT circuit will appear as downward spikes to ground level.
IMPORTANT NOTE #1:
It is sometimes advantageous to perform this test procedure in "SPIKE DETECT", or "MIN/MAX" mode to find quirky opens or intermittent glitches, provided that the waveform on the display doesn't get too "noisy" in this very sensitive mode. A lot of systems are too noisy to use "SPIKE DETECT" mode when the engine is running or accessories like blower motors are on.
IMPORTANT NOTE #2:
It may be advantageous to shorten the time base to something on the order of 200 ms/div (2/10ths of a second per division) or even less to hunt for fast, intermittent glitches in "NORMAL" acquire mode.
IMPORTANT NOTE #3:
Beware of 1985-and-newer Chrysler and GM vehicles that switch a 1000 OHM resistor into the circuit at about 125° F (at about 1.25 volt). This makes the waveform have an upward step in it. The waveform jumps up to about 3.7 volts and then continues to drop until it reaches its fully warmed up voltage of about 2.0 volts. The step up from 1.25 volts to 3.7 volts is normal on the 1985-and-newer Chryslers and GM's and some other vehicles. When this is witnessed for the first time, it can be surprising. If the engine is running okay when this happens, check the vehicle manufacturer's specs to see if the system being serviced switches a resistor into the circuit.