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

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nuckolls.bob(at)aeroelect
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Posted: Wed Aug 03, 2016 12:17 pm Post subject: Using the "Poor man's 4-wire milliohmmeter"

 	  Quote: 	
		           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 donâ€™t see the need to dismount the solenoids but your suggestion would allow me to test the whole circuit from the battery area. 	


   What gage wire is used for

   battery and ground leads that

   run the length of the aircraft?



 

   Bob . . .

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nuckolls.bob(at)aeroelect
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Posted: Wed Aug 03, 2016 1:11 pm Post subject: Using the "Poor man's 4-wire milliohmmeter"

At 02:01 PM 8/2/2016, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Kent or Jackie Ashton <kjashton(at)vnet.net>



 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?



         Whatâ€™s your preferred strategy here?  Thanks.



 -Kent



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   Bob . . .

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kjashton(at)vnet.net
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Posted: Thu Aug 04, 2016 6:26 am Post subject: Using the "Poor man's 4-wire milliohmmeter"

AWG 2 Bob.  I am doing a few more tests and will post results.

-Kent



 	  Quote: 	
		   On Aug 3, 2016, at 4:13 PM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com> wrote:


	


 	  Quote: 	
		     What gage wire is used for

   battery and ground leads that

   run the length of the aircraft?

 

   Bob . . .

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nuckolls.bob(at)aeroelect
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Posted: Thu Aug 04, 2016 8:10 am Post subject: Using the "Poor man's 4-wire milliohmmeter"

One of my favorite tools for chasing this

   dragon (and similar issues) is this modified

   battery tester from Harbor Freight.



  http://tinyurl.com/jdtw499



  In this instance, the device is also used as a high

  current load for my test bench . . . you may not want

  to cut the leads so short. In any case, this mod

  allows you to bolt the load into the system being

  evaluated with good mechanical/electrical integrity.

  

  When you strip the wires to attach fat-wire lugs,

  be sure to find ANOTHER wire buried in with the

  strands of the fat-wire. This is the SENSE wire

  for the voltmeter. Strip the sense wire too

  such that fat-wire and sense-wire strands

  drop into the wire-grip of the terminal.



  To troubleshoot integrity of your cranking

  circuit, connect (-) lead to crankcase. (+)

  lead to the BATTERY side of your starter

  contactor.  With the master switch ON, you

  should read battery voltage. Then crank the

  puppy up to some handy value . . . say 150

  amps. The voltage should NOT be less than

  9v.



  The next handy-tool for one-man

  diagnostics is a very long set of leads for

  your multimeter that will let you clip to

  the (-) side of the battery and then take

  successive measurements of VOLTAGE at battery(+),

  battery side of battery contactor and crankcase

  while generating the same, test load as before.



  These readings will let you evaluate ground and

  supply segments of the cranking circuit INDEPENDENT

  of starter controls.



  The next test may be a two person test . . . disconnect

  the (+) lead from your starter motor and connect

  (+) lead of your HF tool to the starter

  motor power lead. While someone presses the start

  switch in the cockpit, repeat the votlage measurement

  under the 150A load. You need to do this with some

  dispatch . . . starter contactors should not be

  energized for more than 10 second or so without a cooling

  period of a minute or so.



  Once you have the numbers observed in these

  experiments, the combination of conditions that

  add up to your perceived performance issue will

  be easily deduced.



  Using this adjustable, relatively static load

  to emulate starter currents allows you to make

  meaningful voltage drop measurements . . .

  a nearly impossible task when using the starter

  as a load. 



  This tool is also handy for checking your alternator

  output. Bolt it into the system, run engine

  above minimum rpm for full output . . . crank

  up the HF load and demonstrate your alternator's

  state of health.  Far too many owner/operators

  have wasted a lot of time taking alternators

  off the airplane and transporting it to some

  test facility.



  it's a really good thing to KNOW what part

  is bad BEFORE you get out the wrenches and

  hammers.



 

   Bob . . .

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