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Master switch or contactor

kaweeka

Well Known Member
Yesterday I noted a long delay between switching on the master and hearing the familiar click of the contactor, somewhere around 10 seconds. I tried this again and it behaved normally. I cycled a few times and noted sometimes it would activate normally and others a very long delay. My first thought is the master relay is talking to me and should be replaced but with an irregular pattern of behavior, how do I test to understand if it is indeed the contactor or the master switch?

Thanks,
David
 
Connect a voltmeter to battery contactor with red probe on battery terminal
and black lead on small terminal. Turn on master switch and observe voltage.
If voltage across contactor terminals is less than battery voltage, then the
master switch circuit has high resistance. An analog voltmeter or 12 volt test
light might work better than a digital meter for this application.
 
It's most likely the battery contactor.

Use a voltmeter and place the red lead on the small contactor terminal and the black lead to ground. Flip the master switch on, if the voltage jumps up and the contactor has not activated, the contactor is bad. If the voltage jump is delayed, the problem is the switch or somewhere between the switch and the contactor (unlikely).
 
Last edited:
mfleming suggested another good test. Connect the red voltmeter probe to
the battery contactor small terminal and the black probe to ground. The meter
should read battery voltage with the master switch off. If not, the contactor is bad.
Turn the master switch on and the voltage should be zero. If not zero, then
there is a problem with the master switch circuit.
 
I appears to be the contactor. I ordered one from ACS and received it yesterday. Installing on Saturday.

David
 
I lost my starter contactor a few years ago like this, the coil grounds through the body of the contactor with the coil riveted to the frame, which is bolted to the firewall. The rivet started getting loose and was not conducting current back to the ground on the firewall, and would sometime work and sometimes not. This presented good voltage on the input to the coil measured to ground, but the coil was not seeing ground.
 
Keep in mind some master switches switch the ground terminal on the master contactor and others switch the signal/power terminal on the master contactor. Since these are EABs the builder could have gone either way. Switching ground is safer because it eliminates the possibility of a short or arc happening with the voltage feed line going from the battery to the master switch. However, most folks do switch power signal to master contactor. Also make sure you have a diode either incorporated internally in the contactor or externally. That keeps your master switch in good shape by eliminating the possibility of it arcing inside the switch like a spark plug when flipping it on and off.
 
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