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Voltage Drop Tests

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rms827 View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote rms827 Quote  Post ReplyReply Direct Link To This Post Topic: Voltage Drop Tests
    Posted: Jul/09/2018 at 6:56am
I see alot of folk here struggling with electrical issues.  It can be a pain for sure, even on older cars with "simple" systems (compared to the latest stuff).  Voltage Drop testing was something I was taught ages ago when it was largely confined to the electronics industry.  Now it's fairly well known among the new breed of auto technician.

It's similar to ohm testing but is more accurate and won't damage modern computer components like ohm tests can.  It's particularly good for picking up loose and bad connections.

So here's how it works:

Set your voltmeter on low setting (assuming it's not auto-ranging).

Check any sort of connection, etc... as if you were doing an ohm test.  The readings will be how much voltage is lost across the tested area.  Here's the acceptable ranges I was given back in the day:

.0 Volts on any sort of connection.
.1 volt on a switch
.2 volts on a cable (ie the length of a battery cable or spark plug wire)
.3 volts across a solenoid
.5 volts on a complete circuit.


Where I've personally found this test excels is finding bad grounds.  Paint, rust, only slightly loose connections that seem tight...  There's all manner of stuff that can interfere with a good ground connection.  This test does a bang up job of finding bad grounds an ohm test will miss.





1964 AMC Rambler Classic 660

"You can think I'm an idiot, just don't talk to me like I'm one." - Batman
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billd View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2018 at 1:16pm
Originally posted by rms827 rms827 wrote:

I see alot of folk here struggling with electrical issues.  It can be a pain for sure, even on older cars with "simple" systems (compared to the latest stuff).  Voltage Drop testing was something I was taught ages ago when it was largely confined to the electronics industry.  Now it's fairly well known among the new breed of auto technician.



I beg to differ on the last sentence I snipped from your quote, above.
We taught this in HIGH SCHOOL auto in the 70s. 
I say "we" because of the fact I was already running a business by the time I was 14 and doing as much as was taught in the first two auto courses so I assisted the teacher. 

Voltage drop was a big thing in the electronics classes I took in HS, but it was also very telling in automotive, too. 
Then when I went on to college, it was taught in our auto electric courses, and of course, it's in the early Motorola, Prestolite and Leece-Neville literature and tech documents of the early 1960s, too.
the problem is that too many in the business when I started out were, well, won't go there.........

It's one of the first things I do for troubleshooting certain issues.
Of course checking for the presence of voltage at all where there should be some is a form of checking for voltage drop - if you expect 12 volts and see nothing, wow somewhere the system has dropped 12 volts! LOL
But seriously, the latest generations do it as a matter of necessity and because it's harped on, where when I was young and getting into it, the smart ones, the TRAINED ones, those who paid attention, were doing it waaaay back.
In fact, one of my early bosses was likely in his 60s close to 70 when I worked for him and he knew what and how to check.............

I have for a good ten years talked of checking connections by using a voltage drop test, talked of how much drop was in the wiring to my Eagle headlights when I ended up putting in relays and making a new custom harness, I have told how to do it for checking alternators, and more...........
Maybe if you get involved with step-by-step from a different viewpoint or personality, the word will spread??

A volt meter is your friend - and it doesn't have to be a FLUKE!! (although for my purposes, yeah it does, but my ten dollar RadioShack pocket meter it still used a lot, too.)
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rms827 View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote rms827 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2018 at 2:35pm
Really, Bill?!?  One statement, and you want to go on the attack for 4 paragraphs?  Completely hostile and uncalled for.  You could have disputed that in two polite sentences.

FACT of the matter is I talked to numerous technicians over my time as one.  They all knew their way around a multi-meter fairly well, but none of them had heard that you could see if a cable was good via looking for a .2 volt drop across it vs a "normal" ohm test.  Not until late in my career anyway.  I went by personal experience and what my instructors told me.  Sue me.

You learned earlier.  Great for you.  Your neck of the woods was obviously ahead of the curve.  I didn't see voltage drops mentioned here?  Blame your site's complete crap search feature.  I looked.

Yeah, I'm sick of people using me as a punching bag (here and elsewhere).  Particularly when they apparently agree with 95% of what I say, but have to raise this much heck over one detail.  Your job as an administrator is to keep the site functional and friendly.  Not go off on people for no reason.  Half the problem I suppose is I'm not in the cool kids club.  Precisely why I haven't really posted any diagnostic help

Last time I try to be helpful here.  Last time I post here period actually.
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"You can think I'm an idiot, just don't talk to me like I'm one." - Batman
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billd View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2018 at 2:58pm
Ouch - Sorry, it was not intended to be hostile or any attack at all - just saying that it's not just new techs. We did that also. 
I have no idea of your age, I don't know if you worked in the field or work in the field.
My apologies, there was no attack or dig or whatever, intended!

I was just pointing out that the GOOD techs knew this as it was taught in school - but whether or not they paid any attention (we started with 30 and only 18 graduated)
Apparently there weren't that many good ones and the others didn't pay attention in class?

Honestly, sorry - there was no intent of any attack. I was differing on the age - because it was taught, but judging by those I worked with over the years, not everyone paid attention in class back then???

I was agreeing with you on how to do it and why to do it (and people should do more such testing) - but pointing out that one shouldn't dismiss some of the old farts here - 
because there are plenty who do that sort of testing.
It's the YOUNG people IMO who have no idea how or what to look for or even why or how it works.
(I generally seek a shop with older folks working there, similar for parts stores)

Again apologies - there was no intent of attack.
Just pointing out to lurkers that you had a good idea - if people would follow it. In my personal experience, older folks know how, young are oblivious..... 

Cool kids club? Never even been invited. Don't know what it looks like inside.

Uh, Maybe this isn't a good time to mention a search for "exact phrase" voltage drop for the past year only, not all years, yielded 6 pages?
Well  because the topic was already brought up - I will show for those lurking and non-members who are having electrical system troubles and are looking for clues.

.......just saying......... for those who have electrical trouble and are looking in - do advanced search, type in voltage drop, change the drop-down to exact phrase or whatever it's called, choose any date........ I chose 'past year' (the default) and got 6 pages...............

For those with charging system or headlight troubles - here's a clue, places to look and how to look since it seems to be a problem finding such things. 


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Post Options Post Options   Thanks (0) Thanks(0)   Quote vinny Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2018 at 6:34pm
Hey rootmeansquare827, how's that 64 Classic running? I had lots of good comments about mine on Saturday at a little car show less than an hours drive away. 

Speaking of voltage drop, mine's been starting in the spring time after sitting over winter for a month or two or three at a time with a 20 year old battery and cables still connected.

Edit: Sorry, I guess the rms is your initials.


Edited by vinny - Jul/09/2018 at 6:45pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Jul/09/2018 at 11:48pm
Shush. Don't give away all of our industry secrets. Good electrical repair men who know these little magic tricks are rare and hard to find. Their skills demand a premium salary. If you give away our secrets and everybody knows how to properly fix their electrical systems, nobody will hire and pay those of us who know these tricks to fix their electrical systems for them. Or they will pay us less than they already do. 

It's like back in the olden days when they had craft guilds to protect the knowledge of an industry and keep the knowledge in the hands of a select few people so those people would remain in high demand. That way their business is in high demand and is reflected in their pay. The industry would collapse if everybody knew the skills to do it themselves. 

When you have a skill and can do something that few other people can but everybody needs your service, you are in high demand and can charge accordingly. Don't share your skill with just anybody because you are undermining yourself. 




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Lucas660 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lucas660 Quote  Post ReplyReply Direct Link To This Post Posted: Jul/10/2018 at 7:18am
Don't worry Matt, not everyone can retain as much information as people like you. But I agree, I usually wouldn't share all my knowledge, but on a American Motors forum it's ok. I live in rural Australia. I don't think I will lose work sharing information here. By the way I understand voltage drop, but I make a living from working with electricity.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jul/10/2018 at 7:20am
Dang Vinny, you have me beat at 13 years..........
Junkie - not all secrets are given away. There's a few protected.
But here's one I'll share that's related to the topic - 
To do a voltage drop test the area being checked must be under load.
For example, checking the drop across the field diode (internal in Motorola, GM calls it the diode trio as that's what it is.) or checking across the isolation diode, the engine must be running, charging system putting out, etc so best tested with a load like headlights on, etc.
To check the voltage drop on a battery cable, you must be cranking the engine with the starter or there's no load.

I was once asked to teach the other IT techs (actually those a level ABOVE ME) how I knew or did certain things - and asked to pass along all I knew. Dude, that would takes a lifetime!
I operate or think mostly using perceptual reasoning and that is based on lifetime experiences. Others use it but to a far lesser extent. Besides - they could never keep up with my ADHD brain - I was always thinking 200 mph and they were just looking for the accelerator by the time I was finished. HA. 

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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jul/10/2018 at 8:16am
Originally posted by Lucas660 Lucas660 wrote:

Don't worry Matt, not everyone can retain as much information as people like you. But I agree, I usually wouldn't share all my knowledge, but on a American Motors forum it's ok. I live in rural Australia. I don't think I will lose work sharing information here. By the way I understand voltage drop, but I make a living from working with electricity.


And I suspect it is checked from time to time for residential and business wiring.(I held a maintenance electrician license for a few years) I checked a water heater issue and found a large drop at tbe breaker connection.

Edited by billd - Jul/10/2018 at 8:22am
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