Eye On Electronics

Motor, Sep 2004 by Dale, Mike

Like any transformer, there's a low-frequency cutoff point where the AC probe is no longer able to accurately measure current. This includes DC, hut also some frequencies just above DC that are not moving fast enough for the transformer action to really pick them up. AC/DC current probes include an added Hau effect sensor type of device that can measure the amplitude of DC current pulses and low-frequency AC pulses, These probes are not passive in that they must have power to run the Hall effect sensor. In addition, it may be necessary to have a current probe amplifier built into the probe to combine and scale the AC and DC levels so you see them as a single trace.

Its important to keep in mind that the current probe operates as a closely coupled transformer to the wire whose current is being measured. The typical probe tip is designed to open the core of the transformer so the wire being measured is completely surrounded by the probe and that the probe is at a 90° angle to the wire. For automotive test purposes, the probe end is usually one of the larger, more rugged "clamp" hand grippers, but the principle is the same.

In addition to the multitude of probes offered by the traditional manufacturers of industrial scopes, there are specialty probes being offered specifically for the automotive test market. Its not possible to mention all of them, but a few are worth mentioning because they show how the basic oscilloscope lab instrument can truly be transformed into an automotive diagnostic and repair tool.

Pico Technology (wiow.picotech.com) makes a tool that allows an oscilloscope to be connected to the high-voltage side of the ignition without the need for a highvoltage probe. This is done with a springloaded clip that attaches to the high-voltage spark plug wire. A capacitor is formed between the clip, the insulation of the wire and the conductive core. This capacitor can pass the AC signal through the clip and on to the scope. Electronics in the cable attenuate and calibrate the signal so you can view amplitude information as well as waveform shape.

I had a chance recently to see a demo of a transducer made by SenX Technology. This device, called "First Look," uses a piezoelectric sensor mounted between two tuned cavities. One end of the device has a hose that goes into the exhaust pipe, while the output end goes to the oscilloscope. This transducer takes advantage of the pressure pulses in the exhaust gas that are direcdy related to the engines ability to act as an air pump. Gases, as in the case of exhaust gases, act as an incompressible fluid. This means when a valve opens or closes, or when a piston moves, this can be seen as a pulse in the flow rate of the exhaust. With the ignition disabled, you can see the whole pumping cycle of each cylinder. If you sync the scope up with the ignition system, you can match up each pumping cycle with the hole responsible for creating it.

While SenX's brochure talks about a five-minute setup, I don't think it took me even two minutes. With the ignition disabled, it was easy to see that cylinder 4 was not behaving the same mechanically as the other five. Pulling an injector lead or, for diat matter, a spark lead was easy to spot and identify from the scope as to which one it was. When you're trying to diagnose a misfire, one of the first things you'd like to know is the basic mechanical condition of the engine. Using this transducer and a scope is faster and easier tiian performing a conventional compression check. SenX's vvebsite (mvio.senxtech.com) goes on to talk about added tests that can be done with the same device.