[Tandem46]

I was looking for some advice or a different angle on why I'm blowing the 2amp fuse (external, not internal) on my GX-60 GPS/COMM during takeoff only. Here's the background.

1. Just over a year ago during takeoff/climbout full throttle, full rpm, fuse blew. I'd flown the plane for three yrs at that point with no problems. Replaced 2amp fuse, and everything worked fine for 13 months, many, many hours and takeoffs.

2. Last Wednesday during takeoff, full throttle, full rpm, fuse blew again. (thirteen months elapsed since last fuse blown)

3. Last night (first flight since Wednesday), new fuse(radio working 4.0), blew during takeoff, full throttle, full rpm.

I figure it's one of three things. The radio, the wiring, or the alternator. I flew 13 months after the 1st fuse blew with no problems so I'm not sure about it being the radio.

Since it only blows during takeoff, full throttle, full rpm, this leads me to believe that my alternator is putting too much power on takeoff and blowing my fuse since takeoff is the only time I'm anywhere near full rpm. However I have other components that have 2amp fuses and none of them have ever blown. Also, monitoring the voltmeter has shown nothing out of ordinary. BTW, I have a new denso alt from Van's, purchased Aug '06 after original Van's alt went south.

Any ideas appreciated.

 

[lucky333]

Is 2 Amp enough?

It might be some intermittent short/high load issue but..

Check the actual current draw under full load (transmitting, display bright etc). If its close to 2A, you might want to consider a larger fuse (consistent with wiring ratings etc) or one with a different blow time i.e. moving from a fast delay to a medium delay etc. Fuses do age and lifetime is shortened as it approaches its rated value (google Littelfuse and or Bussman and get their databook).

BTW, 'aging' does not mean that you have to do periodic replacement. When run well below its rating a typical fuse is not a problem. But you will see specs showing for how long a fuse will carry a higher than rated load. For small excesses, it can run into hours/days depending on the fuse. Of course for large hits (shorts) the time is much faster as it should be.

As far a blowing on takeoff, I think you are on the right track. As the fuse weakens over time, the hopefully not too much higher voltage combined with the vibration is consistent with my suspicions.

Hope this helps
John

 

[Tandem46]

John,

Thanks for the reply. I should mention that there are two fuses called out in the install manual, one for the GPS and one for the COMM. The manual says there is an internal 3amp fuse and calls for a maximum 2 amp external fuse(the one that keeps blowing). The comm is 7amp internal, 5amp external.

 

[craigvince]

Hmmmm......

Another thought: It's only on takeoff (one specific attitude), full throttle (one specific vibration level). Maybe it's a bad connection or wire that only causes a problem in this specific physical attitude, when vibrations are the highest? Something else that's loose might be pushing against a wire or connector, causing it to short?

 

[Tandem46]

UPDATE

Just a quick update on why my GX-60 fuse kept blowing. After much trouble shooting (problem was intermittent), found this wire (photo) chaffed down to bare wire. It was the power input to the GPS side of the GX-60. Unfortunately while up under my panel I found other chaffed wires's as well. I have about 540hrs tach time on the plane. My advice (kinda common sense) make sure you tie wrap the heck out of your wire bundles so that nothing can move even a micrometer and obviously ensure nothing is resting on any kind of metal edge. I also found a ground wire chaffed through just from rubbing on a flat metal surface, no edge. It's amazing how easily something can get chaffed.