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Electrical troubleshooting


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#1 Alc ONLINE  

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Posted March 07, 2011 - 09:00 PM

Hope this might help someone with an electrical problem some day. Sometime you need to check for voltage under load . Here's an example.
I work as a fleet mechanic , had a complaint of a spot light on a truck not working .First thing I checked ,the light didn't work on high nor low beam , 2nd check fuse , it was an old style glass 20 amp ( same one as in the picture ) looked OK so check the voltage at the light switch , had 12 volts there , until I tried using the spot light , ???? Went back to the fuse and looked closer at it and there was a slight darkness at one end , replaced the fuse and everything worked fine. I set this up so maybe it will help.
DSC07677.jpg This picture I have 12volts at the fuse going to the switch.

DSC07678.jpg This is what happened with the switch closed , light not working and voltage went to .006mv which is really nothing.
DSC07679.jpg This picture shows the power taken before the fuse.
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#2 Bolens 1000 OFFLINE  

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Posted March 07, 2011 - 09:02 PM

Thanks for the informative post. Alc

#3 wvbuzzmaster OFFLINE  

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Posted March 07, 2011 - 09:07 PM

good troubleshooting.

#4 caseguy OFFLINE  

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Posted March 07, 2011 - 09:16 PM

Alc Thanks for the tip. That's a good indicator that you have a bad fuse masquerading as a good one. An easier method (for future reference) is to read the voltage drop across the fuse with the switch open and closed. (For those of you who aren't as familiar with these types of tests) With your meter in the "volts" position, put one of the leads on each end of the fuse. If the fuse is good, there will be 0V across it. If it is bad, you will see all or most of the voltage developed across the fuse, in this case, it would've read 12 volts when the switch was closed or "on". Just a little trick that we use in the mill to find a bad fuse without having to disconnect the power first.
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#5 WQDL753 OFFLINE  

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Posted March 08, 2011 - 04:27 AM

I had a voltage drop issue once myself, they can be frustrating to isolate.
For me it was the foglights on my '72 bug. They worked before I went on deployment, but when I got back they didn't.
checking continuity, and the bulbs everything looked good.
But checking voltage drop at various points, I found I had a drop between the bumper and frame. So unbolting the bumper cleaning up a contact area and putting it back on with some dielectric grease at that spot gave me trouble free use from then on...
On a 12 volt system, 2 or 3 volts is a big big differance, and it's not always easy to see just checking ohms.
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#6 KennyP ONLINE  

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Posted March 29, 2011 - 04:52 PM

Good info. Can someone correct the spelling on the title? Bugs the daylights out of me.
KennyP

Edited by KennyP, March 29, 2011 - 04:53 PM.
spelling


#7 Bolens 1000 OFFLINE  

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Posted March 29, 2011 - 08:35 PM

Good info. Can someone correct the spelling on the title? Bugs the daylights out of me.
KennyP

I fixed it :thumbs:

Dont know how I missed that the first time :wallbanging:
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#8 cubcrazy OFFLINE  

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Posted April 05, 2011 - 10:30 AM

Thanks! That is good information to know for any future problems!

#9 rwhiteley OFFLINE  

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Posted May 01, 2011 - 07:59 AM

Thanks for the information.

#10 gimmodog OFFLINE  

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Posted September 07, 2011 - 07:46 PM

I would like to add that everyone should have & learn to use a multimeter, a test light just doesn't find all problems, a meter will find all of themif you know how to use it.

#11 KennyP ONLINE  

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Posted September 07, 2011 - 08:06 PM

Besides the HF cheapy , I have this one:
Knight.jpg

Now, if I knew what to do with it, I'd be fine. Don't have a manual.

#12 JDBrian OFFLINE  

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Posted September 07, 2011 - 08:17 PM

I've worked in the medical electronics field for almost 30 yrs and I use a multimeter daily for troubleshooting.
I was helping a factory tech to get a major piece of gear going and I wanted to check the fuse. He says "no! the operator already checked it and it's fine". I insist and sure enough the fuse is NG. You can't tell from looking at them. To be sure you have to test them. My preference for checking fuses is to power down the circuit and check the resistance of the fuse. If you can troubleshoot a circuit with it powered down that is the safest way to do it. It also prevents shorting something out with a test probe and causing more damage.
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#13 gimmodog OFFLINE  

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Posted September 07, 2011 - 09:10 PM

on a tractor it is much easier and faster to just turn the key on and start your testing

#14 gimmodog OFFLINE  

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Posted September 07, 2011 - 09:10 PM

especially with safety circuit problems

#15 rammer OFFLINE  

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Posted October 28, 2011 - 11:21 AM

gentlemen:
any troubleshooting tips re: electrical stuff is greatly appreciated.
I don't know what it is with me (cuz it's almost the same as water thru pipes/valves etc.) BUT i just don't get it.
Learning how to use the multimeter ... PRICELESS.
thanks & please keep those tips coming,
Tom (rammer)




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