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Scope grounds
Posted to Technical Tips Forum on 11/18/2015 70 Replies

Yesterday in TDF there was some discussion about "noise" in some of the CKP/CMP waveforms in that thread. While this tip may or may not apply in that particular case, it's worth bringing up -probably again ;)

There can be several causes for noise or hash that appear in a DSO waveform. The "source" is typically something on the car (ignition system) or in the shop environment (fluorescent lights, power lines, etc) that emits electromagnetic inference. The scope leads can act as an antenna for this, or it can affect the scope itself. So lead quality and condition are factor; as is scope design , its location, battery power vs AC mains, etc.

Generally you always should have the scope connected to vehicle ground. However, WHERE you connect your ground can have a noticeable affect on your waveform. Here I have a generic 0-1V O2 (tomato) sensor signal. First is the wavform captured when the scope ground was connected to battery negative terminal. [scope ground at signal return] There's lots of hash present. Often on a perfectly normal car, the battery can be a very (electrically) "dirty" place.

Next is the same car, but with the scope ground connected to the signal return at the sensor connector. [Scope ground at signal return] Note the waveform is much cleaner. So if you're having problems with a signal, remember that the pcm often has filters on its sensor grounds- try connecting to one & see if it helps.

Chris from New Jersey

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