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  #1  
Old 09-02-2019, 08:57 PM
sglynn's Avatar
sglynn sglynn is offline
 
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Location: Anacortes, WA
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Default Blown Starter Fuse

What would cause a starter fuse to blow? It is 7.5 amps in the Advanced Control Module used with a Quick Build panel. It is the older model before they went to logical breakers. I flew yesterday, no problem. But today it wouldn't turn over. Turns out the starter fuse was blown. But why? I examined it and it looks kind of cheap and flimsy. I put in a new one and everything is working fine. Would excessive cranking to start a hot fuel injected engine blow the fuse? Does 7.5 amps sound right for the starter motor? I was thinking it would be a 10 or 20 amp fuse. What else might cause the fuse to blow. I have only 38 hours on the airplane, RV-7A. Thanks
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  #2  
Old 09-02-2019, 09:01 PM
BobTurner BobTurner is offline
 
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The starter motor draws a huge current. The fuse in question most likely is for the starter solenoid (relay), and no, it should not blow from excessive cranking.
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  #3  
Old 09-02-2019, 09:12 PM
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Default starter contactor fuse

Bob
Thanks for your quick reply. Yes, I pulled the manual just now, it is the starter contactor fuse. That's interesting to think about. What would cause it to blow? Could it just be an old weak fuse?
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  #4  
Old 09-02-2019, 09:23 PM
Mich48041 Mich48041 is offline
 
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It could just be an old weak fuse. A bad fuse holder connection can generate heat which will blow a fuse.
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  #5  
Old 09-03-2019, 02:22 AM
Yen Yen is offline
 
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Location: Benaraby Queensland. Australia
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Default

I seem to remember that when the voltage drops in a circuit, the amperage goes up.
You hit the starter and the volts drop way down because the battery is not 100% charged, that may cause the amperage of the starter solenoid circuit to rise enough to blow the fuse.
Replace the fuse, charge the battery and it may be fixed.
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  #6  
Old 09-03-2019, 09:35 AM
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sglynn sglynn is offline
 
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Default starter contactor fuse

Yen
interesting idea, because just before this happened I was running the panel and lights for awhile with the engine off. I was loading new database and trouble shooting a tail light that burned out. So I may have run down the battery just before trying to start it. Fuse is changed. I'll see how it goes.
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  #7  
Old 09-03-2019, 12:17 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by Yen View Post
I seem to remember that when the voltage drops in a circuit, the amperage goes up.
.
This may be true some times, but for a dc relay, no.
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  #8  
Old 09-04-2019, 12:19 PM
gasman gasman is offline
 
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Quote:
Originally Posted by BobTurner View Post
This may be true some times, but for a dc relay, no.
OHM'S LAW only applies sometimes??......
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  #9  
Old 09-04-2019, 01:46 PM
BobTurner BobTurner is offline
 
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There are lots of non-ohms law machines. e.g., motors often rely on induction to limit the current. If the voltage is low and the motor fails to turn, it can draw a large current.
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  #10  
Old 09-04-2019, 02:19 PM
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snopercod snopercod is offline
 
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You should have a kickback diode or MOV between the contactor coil and ground to quench the voltage spike produced when the coil is de-energized.
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