I have recently been getting an issue where the landing light is blowing the fuse a few min after turning it on. Few minutes being 2 or 3 min after takeoff or getting in the pattern for landing. It doesn’t matter if it’s on steady or pulse. Then when the fuse goes I get a squealing in the radio when I transmit. Put the new fuse in squealing goes away. All connections seem to be good
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Landing light blowing fuse
- Thread starter jmo3789
- Start date
Full Throttle
Well Known Member
What size are the wires going to the lights and how many amps is the fuse??? Maybe you just need a slightly larger fuse if the wires can accept the current.
bbaggerman
Well Known Member
There are separate wires coming out of the Switch & Fuse Module to the landing light for steady and pulse operation. So that kind of eliminates most of the suspect wiring to the light. It is hard (but not impossible) to see a failure mode in the Switch & Fuse Module that would cause this. So my money would have to be a failure of the landing light itself.
As I am sure you know fuses don't have a sharp "knee". That is, everything is fine at 7.4 amps but blows instantly at 7.6 amps. You've probably seen this plot of current vs. time to blow for fuses...
So 2 to 3 minutes to blow tells me you are only slight over current. Bad wiring is usually either open or shorted to ground, but rarely "slightly" shorted to ground. So again, I think that points to a bad light.
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bbaggerman
Well Known Member
On the RV-12 there are separate wires for pulse and steady landing light modes so if the fuse blows in either operating mode then it probably is not a single wire chafing.
On the other hand, the audio squealing is an intriguing clue. When I've had squealing problems it was the due to the intercom and not the radio. Power for the intercom comes from pin 22 which on a DB-25 connector is pretty close to (but not exactly next to) the landing light power pin 11. It seems like a long shot but it may be worth poking around the wire harness near the Switch & Fuse Module and inside the connector backshell.
Some landing lights have a switching power supply. One characteristic of switching power supplies is that if the input voltage goes down, the input current goes up. The vast majority of electrical problems are due to to bad connections. Bad connections can cause a voltage drop which in turn can increase the input current to a switching power supply. Increased current can blow a fuse.
Check the voltage as close to the load as possible using the load negative wire as the reference point. Voltage measurements without a load are meaningless. If the voltage at the load is low, find the bad connection.
Check the voltage as close to the load as possible using the load negative wire as the reference point. Voltage measurements without a load are meaningless. If the voltage at the load is low, find the bad connection.
bbaggerman
Well Known Member
I like the switching power supply / bad connection theory. The landing light connector is a cheap Molex that doesn't really stand up well to the elements. Here is my landing light connector I replaced last year...
You can see where one pin got hot because of a bad connection due to corrosion. Of course it is worth taking a look at the grounding lug also.
