Quieting noisy grounds?

A voltage is, by definition, between two points. When you say that you have 0.15 volts of noise, it raises the question "referenced to what?" In other words, when you attached the tip of your scope probe to your ground, what did you attach the ground of your scope to?

I adjust the channel's initial vertical DC trace position, to coincide with the Ground ref line (switching back and forth between DC and ground)

The probe head is contacted to the test point(s). The probe's ground clip is, via various short routes (some require extension), attached to the battery negative post. I've also grounded the probe to various chassis points.

The scope itself is grounded thru the AC three prong outlet, grounded to earth. I have not made use of the front panel's chassis ground connection (should I?)

I've tried to follow the techniques described at

I've tried various connection paths, and probably don't have enuf experience to recognize extraneous noise vs true signal noise, and/or ringing or abberation.

I understand. Here is something that you can rule out with a simple test; the loop formed by the scope's ground clip to the battery post back to the ground point you are measuring is an antenna that may pick up extraneous noise. The way to rule this out is to keep the scope's ground clip as it is and touch the scope tip to the battery negative post. Your actual noise will disappear but any noise that is picked up by the loop will remain.

The scope being grounded through the AC three prong outlet is fine. To insure a solid measurement, make sure that this is the only connection between the vehicle and earth. You can test this with an ohmmeter. Vehicle tires are surprisingly conductive for something that looks like rubber (lots of carbon it them) but garage floors are often insulators as long as they are dry. If needed, one can always put cardboard between tire and floor.

You just did me a big favor by accident; I have been looking for the "ECB toprobe tip adaptor" shown in figure 6-2, page 37, and nobody in sci.electronics.design knew where to buy one from my description. Now I have a picture! :) But I digress...

Page 36.

Ringing isn't an issue for you. As you can see from figure 6-1, ringing only happens when there is a fast step change. You are measuring a ground trace and don't care if any step changes ring a bit.

Now that I understand the situation a bit better, let's revisit your original post and see if I can be of assistance...

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Lance Morgan says...

This gives us a chance to partially isolate grounding issues. When you go to higher RPMs and the AMM voltage stabilizes/normalizes, is the 0.15 V noise you are seeing in the power and sensor ECU grounds still there? If so, I would suspect that it's not the cause of the AMM voltage fluctuations at lower RPMs. Check it with the scope, though; it could be that the AMM voltage is still fluctuating but the engine isn't sensitive to the noise at higher RPMs.

If it weren't for you reported misbreathing/pulsing problem, I wouldn't worry about this at all. Try this; do the same test on any sensor ground on any other vehicle you have laying about; I suspect that they will have much the same measurement.

That tends to make me think it's not a ground noise problem, but I am not 100% sure.

I doubt if it will help, but it's really easy to try, so you might as well give it a shot. Get a 1N4001 (or 1N4002 to 1N4007, they will all work the same in your application) and connect the anode (bottom of the triangle on the schematic) to ground and the cathode (bar on the schematic) to 12V. Do this right at the sensor, keeping the leads fairly short. Vehicles tend to have really nasty negative spikes that are hard to see on a scope, and he diode will suppress them. The reason I think that this won't help is because the sensor was designed to ignore the kind of spikes often seen in automobile electrical systems. It's cheap and easy to try, though.

Also see:

http://68.18.136.52/electrical_system_folder/lh24text.htm Let's try this:

Put your scope ground lead on pin #1 of 205 and measure all of the pins (#1 through #5) with the probe tip. Tell me what you see on each pin.

Given the skill level of the person asking the question (skilled in automotive repair, unskilled in electronics) and the high probability that the "scope" we are talking about is an automotive scope as opposed to an electronics engineering scope, (think Sun/Snapon rather than Tektronix) I wouldn't advise differential measurements. Connecting the scope ground to the vehicle ground at the ground wire of the sensor will, in my opinion, work just fine.