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Fuel pump fuse blows out at each start

hthaman

Active Member
Hi all, can someone please point me in direction of troubleshooting of at each start fuel pump fuse blows out, starter feels sluggish in rotating the prop, battery is sufficiently charged. Thanks in advance.
Rds
 
Fuel pump fuse blows when? At master switch on, or when key switch is turned to rotate engine at startup?
 
Hi all, can someone please point me in direction of troubleshooting of at each start fuel pump fuse blows out, starter feels sluggish in rotating the prop, battery is sufficiently charged. Thanks in advance.
Rds

Check your grounding cables...
 
Something is pulling your battery voltage low.

Either it isn't as fully charged as you think, or you have a poor battery cable connection (power or ground).
 
Check all battery and starter cables and connections, both positive and negative.
What size fuse is blowing, 5 amps?
It is strange that the fuel pump fuse would blow unless the pump draws more current as the voltage drops.
 
It is strange that the fuel pump fuse would blow unless the pump draws more current as the voltage drops.

Power formula - P (watts) = E (voltage) X I (amps of current)

If P is somewhat a constant, as in the power in watts required to make a device operate under it's normal load (in this case , the fuel pump), and the voltage dips, the current will increase (within the limits of the ability for the battery to supply it anyway)
 
Love this forum. Its like when I was a little kid watching Mr. Wizard... :D
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Is this an initial startup, or did the problem develop. Initial startup after construction suggests a wiring error or wiring damage (cut though insulation to ground?). If it developed at least you know it was right when you finished construction and I?d check for a ground with the fuel pump motor in the circuit and then out to see if it?s the fuel pump motor or wiring.
 
We had the same problem .. first fuse blew immediately after takeoff although cranking seemed to be fine. Everything we checked seemed to be ok including the battery voltage with the pump on. After a few blown fuses, we finally discovered it was actually the Odyssey battery which was delivered with the kit and only about 6 months old. Replaced the battery and have not had a recurrence for the past 2 years. Not sure what the battery problem was unless it was an intermittent internal short causing the voltage to drop.
 
You are right Dick, tested battery by advanced auto store and CCA value came out for 33 ( rated 170). New battery is same PC680 ? I heared there is a lighter battery available now ??
 
You are right Dick, tested battery by advanced auto store and CCA value came out for 33 ( rated 170). New battery is same PC680 ? I heared there is a lighter battery available now ??

EarthX LiPi battery (pretty much a drop in replacement. Save somewhere around 10 lbs. Your engine will crank faster/better. They advertise here on this site.
 
Electrical loads that have a switching power supply will draw less current as the voltage goes up and will draw more current as the voltage goes down. Examples are modern avionics that are designed to operate on 10 to 30 volts. I have read about a strobe switch that got hot, even though the strobe kept on working. Evidently that strobe has a switching power supply. When the defective switch dropped voltage, the strobe compensated by drawing more current, which made the switch even hotter.
It seems unlikely that a fuel pump would have a switching power supply. I conducted an experiment with a spare Facet 40105 fuel pump. I ran it with a 12 volt battery and measured the current at about 3/4 of an amp. Then I switched to a 6 volt battery. The pump still made noise but it quit pumping. The current dropped to about 1/2 amp. The 3 amp fuse in series with the pump never blew. I know the experiment did not duplicate the conditions in an aircraft. But it seems to me that the fuel pump does not draw more current when the voltage drops. Maybe the avionics cooling fans, that are on the same circuit, caused the fuse to blow. This is a good example of why each electrical load should have its own fuse. A discrete fuse for each item would make troubleshooting much easier. An even more important reason to have a separate fuse for each load is to prevent a bad component that blows a fuse from disabling an unrelated component.
 
I know the experiment did not duplicate the conditions in an aircraft. But it seems to me that the fuel pump does not draw more current when the voltage drops.

Testing a pump with no fluid load or back pressure?.... your right, it is no where close to a valid test.

The little cooling fans draw just about nothing for power. They would have a very insignificant influence on current draw with a dip in voltage.

If you want to do a real test, disconnect the fans and try starting your engine with an under charged battery and let us know what happens.
 
The pump was pumping gasoline, albeit out of a can and right back in. You are right, simulating the conditions with a weak battery would be a better test.
 
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 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....
 
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.
 
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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.
 
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