How to connect the oscilloscope, check the signal of the car’s sensor or the actuator? I will be categorical – there is only ONE way, how to do it!
a. GND conection of oscilloscope probe is connected to the car’s Ground (body; chine No.31);
b. oscilloscope’s probe connected to the measured signal.
Are any other connection types acceptable? No, not “in parallel to the sensor,” “in line with the valve,” are not acceptable! Why are such “alternative” connections not acceptable? In case of an unsuccessful connection, you will permanently damage the input or output of the exact electronics module; you can damage the oscilloscope itself too.
Several significant nuances when connecting the oscilloscope:
a. if you are looking (in the amplitude) at “large” signals (for example, actuators of 12V; signals of ignition coils 0r injectors), GND for the oscilloscope can be taken from the closest available location (even the metal part of the engine, body screws will be fine);
b. if you are looking at “smaller” signals (for example sensors; data buses), to reduce the interferences – create the extension of the probe GND cable and connect the GND at the place where the GND of the exact module (DME or other exact/corresponding electronic module) is taken;
c. connect the probe GND first, only after connect the signal input. When disconnecting, the first one should be the signal connection, GND should be disconnected as last. So you will avoid surprises, damaging input of the electronic module with static electricity;
d. if you are planning to measure high voltage sources (for example, primary chains of the ignition coils), use 1:10 probes or switch the probe in position 1:10 (if the probe has such an option). If you have a 1:1 probe, definitely check the Umax, which is allowed to the current oscilloscope. The voltage in these chains can reach 400 .. 500V (peak value); I do not know any oscilloscope, which would allow (with a 1:1 probe) input of such a large voltage! Measuring such signals with a 1:1 probe, you will damage the oscilloscope!
e. by no means do not close the probe to the secondary chain of the ignition coils! Voltage in this chain reaches several tens of kV – such a voltage will permanently damage the oscilloscope! In addition – it will be enough to just bring the probe close by several cm from the high-voltage wires. Be very careful!
How to measure the signal “directly on” the actuator mechanism or the sensor?
Why such a question? There are cases when there are doubts both for the supply voltage of the actuator of the sensor (maybe it’s jittering?), or the actuator is managed via both outputs (for example motors, which turning direction can be changed). Also, CAN data buses have two active signals. In sush cases, both outputs/signals should be seen!
a. check the signals successively. I usually start by checking the supply voltage. If everything is fine with it, I move to the signal/management;
b. checking both signals at the same time. In this case, the oscilloscope with 2 channels will be necessary – connecting both probes and output both signals on the screen;
c. if both outputs of the actuator (or sensor) are active, in the case of using 2 channels, the math function of the oscilloscope can be turned on – in this case; the oscilloscope will show the third curve, which displays the data according to the chosen formula (A+B or A-B). The oscilloscope will calculate the sum or the difference of both signals in your place.
The more popular problems:
1. incorrect choice of the probe GND connection place when measuring small signals. In this situation, interferences are possible, which by amplitude can be compared to the signal of the sensor itself. For example – when checking the ABS sensor data of the older cars (their voltage does not exceed 1V peak-to-peak) and connecting probe GND at the closest point (for example, the bottom part of the car’s body), significant interferences are possible.
at first – make sure that these are the interferences of the GND connection. Connect the signal input of the oscilloscope probe to the GND of the sensor. If, at this point (GND of the sensor), the “signal” appears – these are interferences.
Correct solution – connect probe’s GND connect (as mentioned in this entry before) to the appropriate module (in this case: ABS/DSC) GND place, creating the extension of the probe GND wire. I assume that you can be tempted to connect the probe GND not far from the sensor signal measuring place. But, take into account:
a. in case of such connection, the oscilloscope itself can become a source of the interference;
b. in case of unsuccessful connection (for example, confusing the wires of the sensor) of damaged GND wire of the sensor, you can damage the input of the block!
Such connection “parallel to the sensor” I don’t recommend categorically! Several minutes saved can cost a lot!
You can use both (if there is such a possibility) inputs of the oscilloscope:
a. connect one channel to the input of the sensor signal;
b. connect the second channel to the output of the sensor GND;
c. in the oscilloscope, choose the math function A-B. The oscilloscope will show a “clean” signal, automatically calculation off all interference in the GND connections.
2. in the input of the oscilloscope, only some interference, not a signal, are visible.
Such a situation is possible if:
a: lost contact of probe GND connection;
b. lost contact of probe signal connection;
c. the sensor or the actuator itself is damaged (lost connection in it).
If the first two cases are self-evident, the third case is more cunning. If the connection in the sensor or the actuator itself is lost – this connection chain has become high-Ohm (with high resistance). Interferences caused by other hubs/wires/units to the high-Ohm chain could be significant. Not taking it into account, the diagnostics specialist can waste a lot of time trying to find the problems of the wire connection. Everything could be fine with the wires, but the interferences can induce in them (wires).
In the end – several suggestions.
1. if possible, use a battery-powered oscilloscope and disconnect it from the main supply while performing the measurements;
2. if possible, DO NOT use additional car chargers while performing the measurements.
If you, however, use the car charger and the oscilloscope, powered from the mains supply, be very careful! More about ground loops and safety – at the next entry.