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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