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#16 JD DANNELS OFFLINE  

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Posted April 03, 2012 - 03:58 PM

Being's we are on the subject of multimeter and electrical testing. I'll ask this here.
I have a Troy Built 21" push mower, I wanted to use to mow my property thats on my way home from work.
I can throw that in the back of my buick and not need an extra trip with the truck & trailer.
I was getting no spark at all, but eventually traced it back to a draggy flywheel brake(loosened that up). And it's running fine.

But in the meantime I thought it was a bad coil. I went to the mower shop and the tech gave a used coil and told me to take it home and try it. It was about a screw width too wide and would not work. But while I had it I took an ohm test and it showed a resistance exactly the same as the coil I had. So I sumrized my coils was good. and nothing was open or shorted out.
When I took the coil back to the mower shop and told him what I had done, He told me you can't check a coil with an ohm meter.. Did I just get lucky or is he mistaken?

Edited by JD DANNELS, April 03, 2012 - 04:00 PM.


#17 HowardsMF155 ONLINE  

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Posted April 03, 2012 - 04:12 PM

He may have meant that while your ohm test showed no broken wires, it can't show if the insulation has broken down and is allowing high voltage electricity to run to ground inside the coil.
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#18 coldone OFFLINE  

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Posted April 03, 2012 - 09:00 PM

Yes you can test a coil that but it is not a comprehensive test. A coil is nothing more than a step up transformer. The primary side is nothing more than straight piece of wire wrapped into a coil. The secondary is the same only it has more turns (wraps/coils) longer piece of wire. There are no moving parts, only the coils of wire. The two sides (primary and secondary) have no connection to each other. (they may physically wrapped around each other but not electrically connected) The way it works is pretty simple. You need three things to generate electricity
1: Conductor (in this case the wire)
2: magnetic field
3: Motion of the conductor through a magnetic field

In the primary side, when the points are closed, DC current is passed through the primary windings. This creates a magnetic field around the windings. (thing electro magnet). When the points open, the current flow stops which allows the magnetic field to collapse. The magnetic field surrounded the secondary windings (conductor). When the field (magnetic) collapsed (physical motion) current flow was created in the secondary winding. The condenser (capacitor) controls the rate of collapse. Since we start with 12VDC and 2amps and end up with roughly 24000VDC and 0.001amps then we have a 2000:1 step up transformer. So we can estimate that for every 1 turn of the primary there is 2000 turns in the secondary.

Now back to the testing. You can ohm out the primary (+ pole to - pole) and the secondary (Plug wire connection to case). Then you can ohm out the primary TO the secondary to see if they are shorted together. They shouldnt be, but because the most voltage that your meter can produce is what every its battery is (exp. 9VDC) then you cannot see if the internal insulation is breaking down under high voltage.


Hope that helps.
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#19 Amigatec OFFLINE  

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Posted April 04, 2012 - 06:25 AM

A couple of more tips to rememeber, just because it's direct current doesns't mean it only flows one way. It only flows one at a time, it can flow either way over the wire. A good example is the wire between the battery and the charging system, when starting the power flows from the battery into the wiring harness, and after starting the power flows from the harness back into the battery. If you have 2 wires that are touching, you can have power from one wire flowing into the other circuit, and providing power from the wrong end of the wire. I have seen taillight bulbs with 2 filiments do this. One filiment will wrap around the other so when when you turn on the turn signal both tail light blink because the taillight bulb is suppling power to the other bulb on the wire for the tailght.

I hope that makes sense.

The other thing to remember is that diodes pass electricity both ways, they pass a lot one way and a very small amount to other way. I know the older GM cars would flow about 2 volts from the battery with the key off because of the diodes in the altenator. After you disconnect the wire from the altenator the voltage should drop to zero. It doesn't mean the altenator is bad, it just means the diodes are leaking just a bit of voltage.

Edited by Amigatec, April 04, 2012 - 06:28 AM.

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#20 ducky ONLINE  

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Posted April 04, 2012 - 08:30 AM

DVOM Training.

http://www.cdxetextb...Test/title.html


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#21 caseguy OFFLINE  

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Posted April 04, 2012 - 09:01 PM

...while I had it I took an ohm test and it showed a resistance exactly the same as the coil I had. So I surmised my coil was good. and nothing was open or shorted out.


You can test a coil for shorts between the primary and secondary as well as the primary to ground and the secondary to ground. If you read a dead short (0 ohms) to ground from anywhere, the coil is bad. That's only the first step in the troubleshooting process though. A reading of "X" number of ohms above a short does not necessarily mean that the coil is good.

When I took the coil back to the mower shop and told him what I had done, He told me you can't check a coil with an ohm meter.. Did I just get lucky or is he mistaken?


The resistance of a certain coil should be within a specified range if it is good (i.e. 25 to 30 ohms, or 50 to 75 ohms etc...). The problem with this is that the manufacturers do not provide the information on what that range should be.

Yet another aspect is what coldone was talking about. Electrical insulation is ALWAYS rated for up to a certain voltage. The insulation on a coil winding keeps it from shorting from one wind of wire to the next. because the insulation of the individual wires in the primary and the secondary is physically touching inside the coil, the insulation must be able to withstand the highest voltage that the secondary will produce. as it ages, the insulation can loose some of its insulating properties and start to "short" between windings in an erratic way. It may read fine at the 9 volts that your meter puts out, but be intermittently shorting at 20,000 volts. There is a high voltage device that is used to test the integrity of motor and coil winding insulation called a Megger (Megohmeter), but they're expensive and not practical for the hobbyist to own. I hope that this answers some of your questions.
There's a lot of great information in this thread for someone who wants to learn.
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#22 CASENUT OFFLINE  

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Posted April 11, 2012 - 01:49 AM

Power Probe...

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#23 Michiganmobileman OFFLINE  

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Posted April 11, 2012 - 07:23 PM

That would be a nifty tool to add to the tool box, thanks!!
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#24 fishmow OFFLINE  

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Posted July 04, 2012 - 10:46 AM

Yeah, I agree, voltage drop is the most accurate. It can help find poor connections and anything. There also is nothing better than a really good Multimeter. I still have my Fluke 88 from my dealership days. They are pricey, but work forevere. They can do just about anything, including point dwell/duty cycle. Just throwing out there.

Edited by fishmow, July 04, 2012 - 10:46 AM.

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#25 larrybl ONLINE  

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Posted August 25, 2012 - 06:30 PM

I made about 15 or so youtube vids on how to on electrical. Mine are GT specific. Here are a couple Enjoy.

Intro to the multimeter


Using a diagram


The ignition switch
http://www.youtube.com/watch?v=17AfF11-nU0&feature=BFa&list=UUVVLQg6L-tLIZkrLxjJEtbA
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#26 mrmd OFFLINE  

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Posted November 07, 2014 - 04:38 PM

Maybe someone can help me with this issue. Allis B 112 w16hp Briggs. When I disconnect the neg bat cable and put my meter between it and the post I,m getting about 1'5 volts. If I don,t disconnect bat it will slowly lose juice. Replaced VR, had s/g checked, checked a mmeter and everything else I could. This is older.model with push button start. Has me totally stumped(doesn't,take much).
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#27 larrybl ONLINE  

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Posted November 08, 2014 - 09:29 AM

Disconnect the battery and place the meter as you previously did to see the 1.5V, then disconnect the connections (one at a time) till you see the 1.5 disappear. I could see a crudded ignition switch leaking current. The VR shouldn't be an issue as the ignition SW should disconnect it when in the OFF position. I know you said this has a push button start, and I suspect that the ignition switch is being left on, That is why I am focusing on it. 


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#28 Kmac1 ONLINE  

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Posted December 27, 2014 - 05:22 PM

Diagnosing electrical problems is the reason I coined the phrase "I have to work twice as hard to be half as good as everyone else".  I am getting better at it, but it still takes me along time to be sure I have found the issue.  Willie



#29 lrhredjb OFFLINE  

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Posted June 23, 2015 - 06:39 AM

I drove around in my 1950 Ford with the overdrive not working for a long time. It didn't work when I got the car, used of course, and I checked the fuse first while troubleshooting. It was good as anyone could plainly see so I took it to a mechanic who diagnosed it as internal problems that would cost about $300 to fix. Since I didn't have the money I just lived without OD. One day I was playing around with a battery powered test light I had built and just for fun I put it across the OD fuse. Oddly the light was not on so I tested some other stuff and came back to that fuse again and still no light. I replaced the fuse and I had a working OD. The bad fuse had no dark ends and you could plainly see the wire was intact. I'm glad I didn't give it to the mechanic to fix.

Another lesson I learned was to make sure you have good grounds. All sorts of weirdness happens if there is a high resistance ground path.



#30 Unoindio OFFLINE  

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Posted August 21, 2015 - 08:54 PM

How do test my rectifier on lgt 145 hydrostatic
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