CartThe days are long gone when automotive faults could be diagnosed using a multimeter. For work on modern cars many automotive technicians are turning to oscilloscopes.
Many automotive waveforms (ABS, injectors, crank sensors etc) can be measured using simple test leads (see the library of waveforms for examples). Some signals however can not be measured in this way, examples include secondary ignition where the voltages are too high to be connected directly to oscilloscopes or non electrical signals such as manifold pressures. Fortunately to measure such signals there are many commercially available probes. Some are designed for use with oscilloscopes, others for multimeters. The good news is that almost all can be used with the ADC-212 oscilloscope.
To simplify the use of automotive probes, we have produced a scaling file for PicoScope which adds support for:
- Secondary ignition pickups
- Current clamps
- Pressure/Vaccuum sensors
- Temperature probes
- Carbon monoxide sensors
To download this file simply click here and install it into the same directory as PicoScope (normally c:\pico). Example 1 below shows how to use the scaling provided by the scaling file.
If you have a probe that is not supported by the built in scaling, Example 2 below shows how to add your own scaling. Alternatively if you can provide us with the specifications of probes you have (or manufacture) by e-mail, we will do our best to support it in future releases of the software (so helping other users).
Example 1: Current Clamps
DC current clamps find uses in measuring: starter current, alternator charging and injector testing to mention just a few. Measuring starter current can also be used as a quick method of compression testing (see below). Some clamps are designed for use with oscilloscopes, some for multimeters, the main difference being the connector. Current clamps designed for use with oscilloscopes can be plugged straight in. If you have a current clamp with 4 mm 'banana' type connectors this can be used with the ADC-212 provided a 4 mm to BNC adaptor is used.
If you are considering purchasing a current clamp, a couple of points to be aware of, firstly make sure it can measure DC current as some can only measure AC, and secondly check it covers the range you wish to measure (at least 600 A for starter current).
Most current clamps have outputs of 1 mV per Amp (like the one pictured here from AES), some designed for measuring smaller currents have outputs of 10 mV or 100 mV per Amp. The automotive scaling file (see above) has support for all three types. If your current clamp has a different output, it can still be used with PicoScope, but the scaling will have to be setup manually (see the example on secondary ignition and also the PicoScope help file for more details on this).
To tell PicoScope you are using a current clamp, simply select the 'Settings' from the drop down menu and then choose 'Custom Ranges'. This brings up the 'Edit Custom Range' window (see below). From here select the correct type of current probe. (This assumes you have installed the automotive scaling file already).
The waveform below shows starter current from an Aston Martin DB7 Vantage (V12) measured using PicoScope scaling:
Another common use of current clamps is as a quick compression test. As each cylinder compresses, the current drawn by the starter motor increases. If all cylinders have equal compression then each peak is approximately the same amplitude, if however a cylinder has weak or no compression the peak is lower / missing (as with the waveform below). In order to capture the waveform, the engine has to turn over several times so its best to make sure the engine can not fire by removing the plug leads.
Example 2 : Using Secondary Ignition Pickups
Due to the high voltages involved in secondary ignition, it is not possible to directly (or safely) measure signals using a scope, instead non contact capacitive or inductive pickups are used. Typically these are just clipped around a plug wire.
This application note shows how to set up PicoScope to correctly measure from the popular AES#01-12 Secondary pickup from Automotive Electronic Services. Information on setting up the Pico PP178 secondary ignition probe, is covered in a seperate application note.
Displaying a waveform from the AES#01-12 Secondary pickup is fairly trivial, just plug it into the Pico automotive oscilloscope and set the timebase, voltage range and trigger. Your waveform should look like the one below.
There are however 2 problems with the above waveform, firstly as secondary ignition is inverted with respect to primary, many people consider the waveform to be upside down. The second problem is that the scaling is incorrect, the probe attenuates by a factor of 1000 so the peak amplitude is 9.6 kV not the -9.6 V displayed. Fortunately using PicoScopes custom range options it is easy to set a extra voltage range up for this probe. First select custom ranges from the settings menu:
From the custom range dialog, click add then fill in the form as shown below:
The above settings both scale the results and invert the result (20000 mV scales to 20 kV). Once you have entered in the information, click on 'OK' then select File | Save Settings. If you now click on the drop down menu that selects the voltage range, you will see a new 20 kV range for use with the probe. The graphic below shows the result – note the ruler is being used the measure the max amplitude.
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