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Author Topic:   Electrical Troubleshooting
newt posted 08-22-2011 05:05 PM ET (US)   Profile for newt   Send Email to newt  
I'll make a very long story short. I finally was able to fix a problem with my Garmin GPS2010 that would not power up. The cause of the problem was a bad fuse (I think), but took forever to figure out because I measured 12+ volts everywhere it should have been.

How is it possible that I had proper voltage, yet the device would not power up until I changed the fuse? The only other electrical connection between the GPS2010 and the rest of the boat electronics is NMEA 0183 data wiring to the VHF radio and another GPS unit.

dfmcintyre posted 08-22-2011 05:26 PM ET (US)     Profile for dfmcintyre  Send Email to dfmcintyre     
Was the fuse bad? And your saying that prior to changing the fuse, you were reading 12v at the plug?

Unless there is some sort of odd backfeed coming down the data wiring I could only guess that maybe there was a bit of corrosion on the fuse or bracket that allowed for minimal amps and the correct voltage, but when plugged in, the amp draw was enough not to allow full volts.....

Jim - Did that make sense?

jimh posted 08-22-2011 05:47 PM ET (US)     Profile for jimh  Send Email to jimh     
The "bad fuse" could have been in a semi-conductive state, with a high resistance. Say the fuse had melted but was still slightly conductive, so the resistance of the fuse was 1,000-ohms.

If you measured the voltage at the equipment connected to this circuit with a very high-impedance voltmeter, which would be typical for any modern digital multi-meter, it would read a voltage that was almost 12-volts, as long as the equipment was switched OFF.

When you turn the equipment to ON, the current drawn would immediately cause a very big voltage drop across the 1,00-ohms of the fuse, and the equipment would not operate.

I suggest you use your multi-meter to measure the resistance of the "bad fuse" (if you still have it) to see if there was any residual conductivity in the fuse.

You could also get this same sort of voltage reading if there were a bit of corrosion across the fuse holder terminals, and there was a high resistance across them. The fuse could have been blown to open-circuit, but there could be a voltage sensed across the high impedance of the corrosion.

newt posted 08-23-2011 08:20 AM ET (US)     Profile for newt  Send Email to newt     
Interesting comments. I did note that my old analogue meter always read lower voltage and more inconsistently than my modern digital meter.

I may have left the old fuse on the boat, and will measure the resistance next time can. Thanks for the insight.

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