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Using the "Poor man's 4-wire milliohmmeter"

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kjashton(at)vnet.net
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Posted: Tue Aug 02, 2016 11:02 am Post subject: Using the "Poor man's 4-wire milliohmmeter"

Hi Bob,



	I made one of these to troubleshoot a sluggish starter on a Long-EZ from your excellent discussion here  http://www.aeroelectric.com/articles/grnding.pdf    I suspect I have some bad solder joints in connectors on copper-clad aluminum cables and thinking of the best way to test for excessive resistance.  I am thinking:



	I could test each connector joint by isolating the cable & connector and probing between the clean bare cable and the connector, or



	Alternatively, perhaps I could test the entire cable run and the solenoids by disconnecting the cable it at the battery and the starter, activating the master and starter solenoids with a portable battery and probing between the battery-end and the starter-end.



	Or could I disconnect the starter cable at the starter, active the master and starter solenoids with the usual switchology and probe between the battery post and the starter-end of the cables.  Does connection to the battery affect the test?



	What’s your preferred strategy here?  Thanks.



-Kent

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user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Tue Aug 02, 2016 1:11 pm Post subject: Testing starter circuit

Disconnect the starter and replace it with a high wattage load.  Connect the negative side of the high wattage load to the starter housing.  Turn on the battery contactor and the starter contactor.  Connect the RED voltmeter lead to the POSITIVE battery post.  Then using the black voltmeter probe, measure the voltage drop at various points between the positive battery post and the high wattage load.  Harbor Freight sells a 100 amp battery load tester for $22.

http://www.harborfreight.com/100-amp-612v-battery-load-tester-61747.html

Read the reviews before buying it.

Repeat the above test measuring voltage drop between the negative battery post and the starter housing.

Disclaimer: I have never had to perform this test.

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Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Tue Aug 02, 2016 4:06 pm Post subject: Re: Using the

Kent,

Of the three methods that you posted, I like #2 the best except that I suggest you disconnect the aircraft battery negative terminal, not the positive terminal.  That will allow you to test the positive battery connection too.



Method 3 has the problem of aircraft battery current flowing through the cable under test between the battery and the master contactor.  That current will affect the test.

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Joe Gores

user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Wed Aug 03, 2016 5:51 am Post subject: Milli-ohm resistance test

Another option:  Disconnect the aircraft battery negative terminal.  Assuming that the master contactor has a single coil terminal and the starter contactor has two coil terminals, unbolt the master contactor from the firewall and isolate it from ground.  Energize both contactors with an aux battery.  The aux battery positive should be connected directly to the battery contactor positive stud, not to the nuts.

  Now the resistance of the starter circuit can be measured at any location between the aircraft battery positive post and the disconnected negative battery-cable end.

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Joe Gores

kjashton(at)vnet.net
Guest

Posted: Wed Aug 03, 2016 6:58 am Post subject: Using the "Poor man's 4-wire milliohmmeter"

 	  Quote: 	
		   On Aug 3, 2016, at 9:51 AM, user9253 <fransew(at)gmail.com> wrote:

 

 

 

 Another option:  Disconnect the aircraft battery negative terminal.  Assuming that the master contactor has a single coil terminal and the starter contactor has two coil terminals, unbolt the master contactor from the firewall and isolate it from ground.  Energize both contactors with an aux battery.  The aux battery positive should be connected directly to the battery contactor positive stud, not to the nuts.

  Now the resistance of the starter circuit can be measured at any location between the aircraft battery positive post and the disconnected negative battery-cable end.


	


	Yes, I can see that.  This is a Long-EZ with battery & master solenoid in the nose on fiberglass structure and a starter solenoid on the aft metal firewall.  I don’t see the need to dismount the solenoids but your suggestion would allow me to test the whole circuit from the battery area.



	I don’t know the internal resistance of my Skytec starter.  I suppose I can measure it separately.



	However, I think today I’ll just measure the resistance of some assembled joints on various large cables I have in the shop and compare that to some of the roughly soldered joints in the airplane cables.  If that doesn’t reveal a bad joint, I’ll try something else.



	Thanks for the help.

-Kent

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user9253

Joined: 28 Mar 2008
Posts: 1927
Location: Riley TWP Michigan

Posted: Wed Aug 03, 2016 9:00 am Post subject: Re: Using the

The reason that I suggested isolating the battery contactor from ground is because its coil is connected in parallel with the starter motor when the starter contactor is energized.  Your fiberglass airplane might be wired differently than metal aircraft.

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Joe Gores

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