Kind of a noob question here, but I wanted to use my scope to test for the ground control signal (originating from PCM) at the ignition coil connector, with it unplugged and I'm a little unsure how to connect my leads up. If probe positive goes on the ground control wire and probe negative goes on battery ground will my scope show anything as, I imagine, there won't be any potential between the 2 leads?
Good question I have not tried with coil disconnected as I haven't felt like I needed to just see an unloaded command but I don't see why it wouldn't work as long as you have a floating ground scope or are only using 1 channel of a shared ground scope. Typically we leave the coil connected and attach ground lead to ground and positive lead to ground side of ignition coil than you can see the grounding command and the primary ignition signal.
With the suggested hook up while not pulling the coil control wire (#1) to ground you'll be dealing with an open circuit and measuring "ghost voltage" that turns into a direct short once pulled to ground, showing 0V.
From a diagnostic standpoint that way you're not loading the circuit adequately and may miss high resistance issues on the control wire.
Hooking up a test light to B+ and the control wire or installing a noid light in place of the coil will allow for checking coil control quickly. The test light check will work even with the coil in place.
Some pcm may cancel coil activation when seeing an open circuit with the coil disconnected.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
Thanks for the replies. I was watching a DiagnoseDan vid on the youtube and at one point he got a trace of the PCM signal and just was curious how he did that. He was working on a Beemer, so maybe it used a +12V signal instead of ground control?
I'll post a link to the vid below, if anyone's curious to see it. Scroll to ~20:25 mark.
The signal shown in the video is actually a control signal which the pcm sends out to the power transistor which controls the specific coil. The scope in this case is NOT connected to the negative side of coil primary, only to coil control. This is a regular square wave and there is no inductive kick as you would see when connected to coil negative as around 7.00 of the same video.
It's easy to see this kind of control signals on 3 and 4 wire coils. They internally feature an ignition control module which is told by the pcm when to switch on and off primary current. This order is sent from the pcm to the coil's integrated ICM as a discrete voltage signal and you would connect the signal probe of one scope channel to this control wire and the ground lead to chasis ground or bat negative.HTH.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
Okay, thanks very much for clearing that up. Probably should have watched it a few more times before posting... Just so I'm clear, this system is using it +5 or so volts to activate the transistor there, is that right?
