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Power and sensor grounds with DMM and Scope?

I just want to confirm (or dispel!) that I'm using good basic technique

To ck the integrity of the sensor ground for a 3-wire (alternator/batt current supply, ground, and 'square' output signal) Hall-Effect sensor, would the positive scope or DMM probe be placed on the ground sensor wire (back-probed at computer w/engine cranking or running), and the COM or ground clip routed to a known good ground? And, if all is in top form, the scope waveform should be flat-lined at 0 V DC, and DMM should read 0.0 V DC? Or is there a more definitive method to test a sensor ground?

Even simpler - if I want to V-drop test the battery's ground cable - postive probe on batt's neg terminal, COM/neg probe on ground termination and/or grounding area (recently stated to me by a Tech)? Optimally 0.0 VDC? I was previously placed the DMM's negative probe on batt's neg terminal, positive probe on batt's positive terminal - taking a baseline reading (say 12.6 V DC) - then moving the DMM's negative probe to the ground termination and/or block/chassis, looking for anything less than the baseline reading. It doesn't seem that either of these techniques loads the battery's ground cable...

I think this is just an extension of the para above - if I want to test singular wires (within bundled wiring harnesses), running between two computers (one fuel, the other ignition), and I want to disconnect both computers to isolate the wires (carrying various signals, some with known waveform samples, but several without, hence no reference wf for comparison | max voltage or wires is alternator voltage; amperage unmeasured, but assumed (!) to be low)...do I use Ohm's Law, add a resistor of known value into what will ultimately be a closed circuit, including either the scope or DMM, to simulate a load in order to perform a V-drop test (preferred over continuity, and/or running a known voltage thru the wire w/o any load?)? Will I ultimately need a ammeter w/an inductive clamp (vs 'regular' piercing probes)? Or, again, is there a better/more definitive method to test the wire's integrity?

I _thought_ I understood how to V-drop a circuit w/a load, such as headlights on, cooling fans, etc, but am somewhat fuzzy on applying that to testing wires w/o a load. The grounds are shiny clean, but I'd like to electrically test them

Thank you, Lance

Reply to
LanceM
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The ground circuit includes all the way back to battery negative, so it's a good habit (with exceptions) to connect your meter - lead directly to the battery negative terminal.

The meter's negative lead goes to what would be "most" negative in the circuit, so in the case of V-dropping the battery ground cable, the meter's negative lead goes to battery negative, the meter's positive lead goes to where the cable is connected to the block of chassis. Once the meter is properly connected, the circuit needs to be made to function, if one is tracking down a cranking problem and suspects it to be the cable, one would then engage the starter motor. If one were tracking down a dim headlamp, one would turn the headlamp circuit on. In effect, you are paralleling the circuit in question with your VOM, if there is resistance in the circuit, it will take the easier path thru the VOM with a resulting voltage reading on the VOM.

I've been known from time to time to substitute a heavier load when tracking down a circuit problem, i.e., disconnect the ECM and use a sealed beam headlamp jumpered onto the power feed to the ECM. When a splice is corroded down to one strand of wire and your DVOM is fibbing to you (voltage available), believe me, this method works.

You'll never see a perfect 0.00 voltage drop across any circuit on an automobile. For the normal non computer circuits, .2 volt

  • .1 volt for each switch in the circuit is a rule of thumb. For computer circuits, 100mv is the industry norm. (IIRC)
Reply to
Neil Nelson

Thanks again for coaching a rook along (and reading my wordy/stumbly explanations). Good trick with the headlamp - and it clarifies things.

I've made some headway with the scope, t-shooting and finding some abnormalities on my Saab, and now am trying to make sense of the data

Why are power grnd circuits sometimes/usually(?) distinct from sensor grnd circuits, when they are sometimes/usually(?) terminated at the same exact point (MAF power and sensor grnds, for example). If one gets spiked, they would both get spiked...?

Reply to
LanceM

Neil Nelson wrote: snip

Huh??? mike

Reply to
mike

Approximately 11/12/03 13:46, mike uttered for posterity:

This is an issue mostly on high impedance circuits, such as checking old tube cathode circuits. Fairly easy to calculate for any given VOM. The meter will have a resistance of N ohms/volt full scale. Meter resistance is N x [Ohms per Volt] x [Full Scale Value] Dunno if anything in an auto is high enough impedance that this matters.

Reply to
Lon Stowell

By all means, if you have a better way of explaining it in laymans terms, have at it.

Reply to
Neil Nelson

OK, The current through the resistance of the circuit path creates a voltage drop that can be measured with a voltmeter. Lower voltage measured is better. By subdividing the path, you can zero in on the defective wire/connection/component.

How's that?

The basic problem with your explanation is the word "it". The only parameter that I can find that "it" could refer to is "resistance". I claim the resistance does not TAKE any path. In fact, (most of)the "current" takes the easier path and it's NOT through the VOM if you're measuring voltage.

mike

Reply to
mike

My usage of the term VOM was strictly in the generic sense given the subject line...

Reply to
Neil Nelson

Ahh, yes, I see and agee 100%. Bad phrasing on my part.

I should have posted "it (voltage) will take."

Brain and fingers got out of synch... ...better now.

Reply to
Neil Nelson

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