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Old 05-23-2018, 09:20 PM
rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 7,868

Originally Posted by Mich48041 View Post
I conducted an experiment today on my RV-12. Not wanting to ruin the PC680 aircraft battery, I disconnected it. In its place, I used the riding lawn mower battery connected with jumper cables. I thought that the lawn mower battery might have trouble cranking the Rotax 912ULS, but it cranked just fine. I left the ignition switches turned off so that the engine would not actually start. I turned on all electrical loads, including landing light, to run the battery down. I periodically cranked the engine as the battery slowly ran down. It took an hour and a half for that cheap $35 lawn mower battery to drop its voltage down to 11.5 volts. The engine still cranked, albeit noticeably slower. The fuel pump fuse never did blow. I was not able to duplicate the symptoms reported by others. Perhaps there is some magic voltage that causes the fuse to blow and I never happened to crank the engine at that particular voltage. In my airplane, the fuel pump is the only load on that 5 amp fuse.
I imagine it is possible that the .15 amp reduction in load by not having the two fans on the circuit could have made the difference in your fuse not blowing....
Any opinions expressed in this message are my own and not necessarily those of my employer.

Scott McDaniels
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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Old 05-23-2018, 09:52 PM
hthaman hthaman is offline
Join Date: Jan 2014
Location: IA
Posts: 56

Originally Posted by Mich48041 View Post
hthaman, did a new battery cure the fuel pump fuse blowing problem?

New battery fixed the issue. Thanks Joe. I bought another PC680 from autozone as did not want to spend close to $ 400 on a "life" battery and extra for another charger as I usually fly alone so I am ok with the added weight of PC680. But did not notice the battery from " autozone "does come with outer shell and it a pain in the butt to take the battery out from it in order to fit in. Should have ordered online would have saved few $'s as well.

thanks all and regards.

Last edited by hthaman : 05-23-2018 at 10:00 PM.
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Old 05-25-2018, 12:31 PM
Mich48041 Mich48041 is offline
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 2,737

Bob Nuckolls commented on the fuel pump fuse blowing HERE.
This is my interpretation of what he posted:
The solenoid coil that drives the pump piston can draw up to 12 amps. But since it is only turned on for a very few milliseconds out of each pump cycle, the 5 amp fuse does not get hot enough to blow.
The fuel pump internal solid state controller temporarily fails at some low voltage point while cranking the engine with a nearly dead battery. Instead of turning on the solenoid coil for a small fraction of each pump cycle, the malfunctioning controller turns on the solenoid coil and leaves it full on. The duty cycle is then 100 percent at 12 amps which blows the fuse.
Joe Gores
RV-12 Flying

Last edited by Mich48041 : 05-25-2018 at 02:08 PM.
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