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Power and sensor grounds with DMM and Scope?

L

LanceM

Jan 1, 1970
0
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
 
N

Neil Nelson

Jan 1, 1970
0
LanceM said:
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?

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.
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...

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 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'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.
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

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)
 
L

LanceM

Jan 1, 1970
0
Neil said:
technique


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.

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...?
 
M

mike

Jan 1, 1970
0
Neil Nelson wrote:
snip
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.

Huh???
mike


--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
Color LCD overhead projector
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
 
L

Lon Stowell

Jan 1, 1970
0
Approximately 11/12/03 13:46, mike uttered for posterity:
Neil Nelson wrote:
snip

Huh???
mike

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.
 
D

Dick C

Jan 1, 1970
0
Neil Nelson wrote in rec.autos.tech
By all means, if you have a better way of explaining it in
laymans terms, have at it.

Electricity follows the path of least resistance. Thus, if you have a low
impedance vom you will be getting current flow through the meter that will
give a false reading. However, this is usually not an issue with automobile
electric systems. It is a factor with electronics. Myself, I always use a
digital multimeter, high impedance, and usually quite accurate.
I prefer to read the numbers rather than watching the swing of the needle,
easier for me to remember what is going on. But with a car the vom will
work just fine, so long as you have a good one. The little $5.98 specials
usually aren't worth much, and you cannot rely on the voltage readings.

--
Dick #1349
"Believe those who are seeking the truth; doubt those who find it."
Andre Gide, French author and critic (1869-1951).
Home Page: dickcr.iwarp.com
email: [email protected]
 
M

mike

Jan 1, 1970
0
Neil said:
By all means, if you have a better way of explaining it in
laymans terms, have at it.

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

--
Bunch of stuff For Sale and Wanted at the link below.
laptops and parts Test Equipment
Honda CB-125S
Color LCD overhead projector
Tek 2465 $800, ham radio, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/
 
N

Neil Nelson

Jan 1, 1970
0
Dick C said:
Electricity follows the path of least resistance. Thus, if you have a low
impedance vom you will be getting current flow through the meter that will
give a false reading. However, this is usually not an issue with automobile
electric systems. It is a factor with electronics. Myself, I always use a
digital multimeter, high impedance, and usually quite accurate.
I prefer to read the numbers rather than watching the swing of the needle,
easier for me to remember what is going on. But with a car the vom will
work just fine, so long as you have a good one. The little $5.98 specials
usually aren't worth much, and you cannot rely on the voltage readings.

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

Neil Nelson

Jan 1, 1970
0
mike <[email protected]> said:
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.

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...
<thwack> ...better now.
 
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