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Carman Scan Oscilloscope Use

In this issue we hope we can give you, our customers, some of the basic information on how to use the oscilloscope feature on most Carman diagnostic scan tools.


Most engine sensors can be diagnosed using basic oscilloscope functions; these sensors include throttle position, O2 sensors, cam and crank sensors, injector and some ignition signals just to mention just a few.

June-2012-Fig1

 

Before you begin using your oscilloscope, there are a few basic adjustments you should be familiar with. The first is adjusting the Y axis, or voltage scale. This adjusts the height of the displayed waveform and determines the voltage value you give to each of the vertical section of the display. This is adjusted using the up/down arrows located in each of the channels ID boxes.

The second adjustment is the X axis, or time base. This adjusts the width of the displayed waveform and determines the time value that you give to each of the horizontal sections of the display, and can be adjusted by using the left and right on screen arrow keys. The use of these controls allows the scope display to be adjusted for easy viewing and better diagnostics.  Below is the same scope pattern as in Figure 1, with the exception of the voltage and time base, rescaled. This can be used to “zoom in” as more events are collected on the screen.

June-2012-Fig2

 

Figure 2 shows the same scope event as Figure 1, with a lower value Y axis and a shorter time frame on the X axis.

Modern vehicles mainly use three types of sensors giving the following output:DC voltage switching used in cam and crank sensors: AC voltage, used for magnetic cam, crank, and some wheel speed sensors; as well as analogue voltage, which are commonly associated with throttle position sensors, O2 sensors and fuel pressure sensors.

The third set up parameter that requires attention is the use of either AC or DC. There is a rule of thumb that can be applied. If the sensor is a two wire sensor and used on engine cam or crank sensor and has a resistance of approximately 1KΩ then you have a good chance that the sensor will output an AC voltage and the voltage output will be proportional to speed. Therefore the X and Y set up will need to be changed as the engine speed increases or decreases, some examples of this are displayed below:

June-2012-Fig3

June-2012-Fig4

Scope signals can also be used to diagnose some common faults relating to the misalignment of cam and crank sensors.

June-2012-Fig5

 

The crank and cam signals shown here are indicative of an engine that has 4 teeth on the crank and one tooth on the cam (both cam and crank sensors are hall sensors). As can be seen, both the crank and cam waveforms shown here are quite clean.

June-2012-Fig6

However in the capture shown here, taken from the same vehicle, notice the positive-going spikes in the crank sensor waveform. This could be indicative of a faulty crank sensor or a bad sensor ground, and depending on the engine management system, the ECU will sometimes not log a fault code.

 

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