What the heck just happened ? (electrical)

christian

christian

Senior Member
So I’ve always been interested in test equipment, especially simple things like test lights but never went to the trouble of buying one. Anyhow I’m on Amazon and see a cheapo test light that has a built in readout that shows voltage so I buy it. It arrives and I’m eager to try it (even though nothing is currently broken). I pop open the drivers side hood and connect the ground to a head bolt and then the probe to the starter terminal. It lights up and reads 6.5v, nice. I then think, hmm what else can I try…. So I stick the probe on the cutout output terminal. Instantly it sparks, welds itself to the terminal nut and I’ve blown my 30a starter fuse….

What the heck? Any idea why this happened?
 
Yes sir, positive ground. What i dont understand is why it happened on the cutout output terminal, but everything was fine when i hooked up the main starter terminal. Just wondering if that spot is a general no-no for some reason.
 
christian said:
Yes sir, positive ground. What i dont understand is why it happened on the cutout output terminal, but everything was fine when i hooked up the main starter terminal. Just wondering if that spot is a general no-no for some reason.
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If your Model A has stock wiring, the connection on the starter switch terminal goes straight to the battery. The current path to the connection on the generator cutout goes from the lug on the starter switch up to the terminal box, through the ammeter, back to the terminal box, then to the lug on the cutout. That path should have more resistance (not much though) than the path directly from the starter switch lug.

Is it possible your test light has a "amps" or "current" setting and it accidentally got switched to that?
 
Thank you for the response, your hypothesis could certainly be true. I've had my A for 12 years now and I've probably used my fluke at the cutout terminal dozens of times without issue. I'm guessing this 20 dollar amazon test light must be junk, after the incident the unit is totally dead. Swapped out the fuse and the car is happy as can be. Letting you all know to avoid this item:

"Heavy Duty 3-65V Backlit Digital LCD Circuit Tester, Test Light with 140 Inch Extended Spring Wire, Car Truck Low Voltage & Light Tester with Stainless Probe"
 

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The other thing that may be a possibility, could you have touched both the terminal AND the case of the cutout?
 
Either you accidentally touched ground while probing the cutout terminal or there is a short in the circuit tester. Does the circuit tester still work? You could test for resistance of the circuit using the ohm setting on your Fluke. If very little resistance, it is shorted. That way you don't have to chance another welding experience.
 
It appears that the probe provided a connection of the battery to the generator, so the generator acting like a stalled motor, drew a large current to it.
 
I think eagle hit the answer. You could test the test light by using a battery like a 9V or AA and see if it works. The bulb and circuitry inside the tester would not allow that much current to flow and allow the welding of the probe.
 
christian said:
I'm guessing this 20 dollar amazon test light must be junk, after the incident the unit is totally dead. Swapped out the fuse and the car is happy as can be.
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Ah, yes... "CCC". :D


(Cheap Chinese Crap)

Although $20 is not so cheap. :mad:
 
Unfortunately the unit is totally dead, no further testing on it possible. I'll probably stick with the tried and true units that have just a bulb in them.
 
jrelliott said:
I think eagle hit the answer. You could test the test light by using a battery like a 9V or AA and see if it works. The bulb and circuitry inside the tester would not allow that much current to flow and allow the welding of the probe.
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I agree. Not to pile on here but if the car is positive ground your first connection of ground clamp to head bolt was reverse polarity and electronics usually if not always turn up their toes on that one. :)
 
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