Volt drop testing using scopes

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Volt drop testing using scopes

Postby Technician » Sat Nov 12, 2016 9:36 pm

From previous thread;

Earlier this year I was renewing my aTa certification, during conversations a Master Tech advised that vehicles electronics after year 2005 should not have the wiring load tested by volt drop testing, he advised that this could cause problems with ECM's, he recommended using an Ohmmeter and advised that "0 Ohms" should be achieved when testing wiring. I have no problem with the results of "0 Ohms" as a result but if modern scopes like the Pico can carryout volt drop testing, surely it must be safe to volt drop test wiring with tools like Pico and the HOOK made by Power Probe ?
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Re: Volt drop testing using scopes

Postby Technician » Sat Nov 12, 2016 9:42 pm

Dcunning35 wrote:Hook , captain hook ? Left hook? Meat hook? Coat hook? What are we discussing?


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Re: Volt drop testing using scopes

Postby andrewbishop66 » Sat Nov 12, 2016 10:39 pm

I think the problem here is the term Drop testing

1 is it used as slang to describe the voltage drop along a piece of wire though a bulb ect or maybe on a wiring loom to highlight a point of resistance to a given load which cant be found unless the circuit is under load

2 low resistance across a battery to test its capacity or health ?

3 or to add a artificial load on a circuit to test its ability to pass the the designed load with out failing WARNING TRYING TO PULL 1 AMP OUT OF A ECU SUPPLY CIRCUIT DESIGNED TO DELIVER 500 mA WILL KILL IT IF IT HAS NO OVERLOAD PROTECTION

1 is realy the Potential difference at points across any circuit and all you need is a volt meter to find on a steady circuit or scope on a intermittent I prefer scope

2 battery test

3 the reason i use the load that's part of the circuit and a scope to measure the potential difference

V I R
V x I = W



If you know ohms law and power equations all else is maths

Thanks Andrew
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Re: Volt drop testing using scopes

Postby Technician » Sat Nov 12, 2016 11:50 pm

Thanks Andrew,

Reading back through Steve's scope school bonus class I understand now what the scope(s) are doing, initially I thought because previous electricians on forums have always referred to a bulb and voltage supply for volt drop testing, that the scopes were actually doing the same, however the scopes are actually doing what you describe above and measuring voltage drop through a wire, which is a different test. It would appear then that unless absolutely certain that there is no possibility of a stray voltage occurring that load testing a wiring harness with either the Power Probe (HOOK) or a bulb and voltage supply, that is indeed possible to damage ECM's, if their not ALL disconnected with the last two methods.

It's a long time since I studied Ohms law, however as I remember Ohms Law it is;

Double the voltage and you halve the resistance, and if you double the resistance you halve the voltage. Power equations, somewhere in my old college folders I have the periodic table with them all in it, but I can't remember them all, its a very long time since I was at college. :)
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Re: Volt drop testing using scopes

Postby STC » Sat Nov 12, 2016 11:54 pm

Andrew
I think the problem here is the term Drop testing


I have to disagree. I quote the opening Post:
vehicles electronics after year 2005 should not have the wiring load tested by volt drop testing, he advised that this could cause problems with ECM's


It is very definitive on the point of "wiring load tested by volt drop testing" Which has no comparison to the old school Battery testers which used a large coil/ resistor to load the battery. That was "Drop Testing" not "Volt Drop"

As you say, adding an additional load to a power supply Whilst we "Volt Drop" becomes "Load Testing" Which is fine unless it is a supply switched from within an ECM transistor, then it becomes Risky to add an additional Load. So we Volt Drop to measure how much it has lost on its journey.

None of the above changes regardless of the year 2005, the principle remains the same on a car, van, central heating boiler or a washing machine. Pre or Post 2005 ??

I don't for a minute believe a Master Technician would have said that, or he has been misquoted.

I have been through MT assessment processes and loose speak (nonsense) of that nature would equal a fail !!!
The number of Diagnostic Master Technicians in the UK is just in 3 figures - We are a minority. I know many of them personally & have regular contact.
Resistance testing Actuator Wiring and accepting 0 Ohms end to end has the potential of a misdiagnosis. Not acceptable !!??

I have asked before and did not get an answer, so will ask again
Earlier this year I was renewing my aTa certification
In which discipline ?
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Re: Volt drop testing using scopes

Postby STC » Sun Nov 13, 2016 12:30 am

that is indeed possible to damage ECM's, if their not ALL disconnected with the last two methods.


With them "all disconnected" would ensure no current flow ??
A Complete Circuit, hence Current Flowing being the fundamental Prerequisites to valid Volt Drop Testing.

Periodic Tables ???
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Re: Volt drop testing using scopes

Postby andrewbishop66 » Sun Nov 13, 2016 2:29 am

Got to agree STC there that's my dyslexia it does say voltage drop testing (potential difference ) so in that case he was wrong

the components must be in circuit for voltage drop testing as STC says
High resistance or broken wire would show near battery voltage (little or no current flow ) a large voltage drop

low resistance or corrosion in wire very little voltage drop (would expect near normal current draw depending on load )

But a modern volt meter or scope has a input impedance of at least 10 mega ohm so without any other resistance in circuit

current draw would be 0.0012 mA for the meter connected to a circuit no pcm or fraggly wiggit is going to know
its there

Ohms law is a formula I afraid double this double that doesn't work

V x I = P
V/ I = R
I X R = V
V/ R = I


P = WATTS

Know any 2 of the above and all the others can be worked out I remember it like this


_____V_____ volts always divided by one of the lower 2
I x R Amps x Resistance = volts

Hope that helps

Andrew
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Re: Volt drop testing using scopes

Postby STC » Sun Nov 13, 2016 3:56 pm

Ohms law is a formula I afraid double this double that doesn't work


Indeed, it is a kindergarten / nursery rhyme approach to the subject, making a mockery of the complex nature of automotive diagnosis.

Power equations, somewhere in my old college folders I have the periodic table with them all in it, but I can't remember them all, its a very long time since I was at college.


I am guessing it has not been more than 50 years since you went to college, these theories certainly haven't changed in that time. I can only guess that you cannot remember them because you are not utilising them on a regular basis.

but if modern scopes like the Pico can carryout volt drop testing,

Not a case of "Modern" or "If" - Any instrument that can measure voltage accurately will be capable of measuring "Volt Drop" Whilst Current is Flowing.

surely it must be safe to volt drop test wiring with tools like Pico and the HOOK made by Power Probe ?

Volt Drop testing with the Hook would be "Safe" but only if in the right hands. Before you press the "HOT SHOT" button you really ought to know what it is doing internally and to the circuit under test.
Anyone with an obvious lack of knowledge (Ohms, Watts, Kirchhoff's Law) should not be using the HOOK. It will be counter productive.

Whilst all 3 tools you mention have the potential to measure "Volt Drop" Some will be of more use than others dependant on what activity is taking place within the circuit. For example, Volt Dropping the Permanent Power Supply on a fast ground side switched actuator (anything above 0.5 hz) and you are limited to your tool of choice right there.

For those reasons your "Alleged" statement from the Master Tech is just plain and simple Wrong !
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Re: Volt drop testing using scopes

Postby Liteace » Sun Nov 13, 2016 10:57 pm

There's no need to keep jumping in with all your:

Volt Dropping the Permanent Power Supply on a fast ground side switched actuator (anything above 0.5 hz) and you are limited to your tool of choice right there.

looks like your trying to tell ALL of us you are No1 top tech and no one else knows what they are talking about, its getting pretty darn boring
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Re: Volt drop testing using scopes

Postby Technician » Mon Nov 14, 2016 12:36 am

It is some 28 years since I studied auto electrics at college and although a wire is still and wire, electronics on vehicles have significantly changed in that time, we never had CAN, FLEXRAY, PWM signals etc back then, in fact the most up to date electronics back then besides the electronic ignition distributors were the Montego with the Programmed Ignition Control. Anything that included fuel mapping etc was there but not in the direct public domain at that time, the population of vehicles being points, condensors and Bosch among other electronic ignition systems. I have made my auto electrical experience clear before.

Andrew, it seems I remember Ohms law like probably at lot of other things to date, maybe I am getting too damn old for this job now. Maybe trying to remember an examination question via aTa some 3 years ago I am not remembering that correctly either, dare I now say;

12V / 6A = 2 Ohms, and if the voltage is doubled and the current held constant then the resistance will double like this;

24V / 6A = 4 Ohms.

Its very clear when your young your full of energy and more than eager to push it forwards, but we older people make mistakes and don't push it back into peoples faces
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