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Blown Starter Fuse

sglynn

sglynn

Well Known Member
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
 
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.
 
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?
 
It could just be an old weak fuse. A bad fuse holder connection can generate heat which will blow a fuse.
 
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.
 
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.
 
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.
 
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.
 
snopercod said:
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.
Click to expand...

Make sure that diode or mov didn?t fry. There have been a few reports of cracked diodes. Momentary short before they smoke and open up could take out a fuse.
 
JonJay said:
Momentary short before they smoke and open up could take out a fuse.
Click to expand...
That was my thinking. On my installation, I have MOVs from the battery-disc-starter, as well as MOVs and a diode to ground on the coil. Overkill, maybe...
 
When Voltage is low - current will increase

The solenoid is an almost pure inductive load with a small amount of DC resistance provided by the cross section area of wire & the length of the wire. A significant portion of the "total impedance" is produce by counter Electro Motive Force - a solenoid is just a linear motor operating with a "locked rotor". This fuse should be a "Slow Blow" type typically used with inductive loads. You may also consider increasing the fuse rating to 10A - but definitely a slow blow.
 
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Carry a spare fuse

In the past I used a 10 amp fuse and carried a spare. If it blew more than once in several tries I changed the starter solenoid. The mechanical contacts in the solenoid get beat up and eventually start drawing a lot of current. Happened at least twice over the years, my boat did the same thing. I have just changed to circuit breakers and not sure how that will work out. I guess if it trips more than once I will change the starter solenoid. A good quality sealed solenoid seems to lengthen the service cycle.
 
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