Electrical System Troubleshooting

 

A good story about troubleshooting the electrical system from a fellow Comanche owner.

I was flying back to work and shortly after lifting the gear and flaps my alternator breaker tripped.  OK at the time I thought it was the alternator breaker, but it was actually the field breaker.

I did half an hour of trouble shooting, but no time, I had to leave for work.  Left the plane and headed to work.  Ten days later with lots of time to research what the problem could be (I thought for sure I needed a new alternator) I got home and dug into the trouble shooting.

 

I was working through the Plane Power alternator trouble shooting chart (very good by the way). First thing is to measure the bus voltage. Well soon as I put on the master, the field breaker tripped.

Started checking what could be going to ground. Pulled the field off the regulator, yep the wire from regulator to alternator was going to ground. Pulled it off the alternator, still going to ground. Weird. This wire happens to be shielded. Was showing a dead short from internal wire to shield and to ground. Examined the wire in the engine bay and at one part it looked bad. I cut it off and the dead short went away. I guess living for over 55 years in the engine bay was too much for it. It was quite cracked and the coating on the shield was mostly gone.

I replaced the section in the engine compartment (will do the rest at annual or AP installation).

Flipped the master on, no problem. Fired the plane up and all is good.

Glad I chose the trouble shooting path instead of throwing parts at it (I do have an alternator and regulator on the shelf).  For many of us folks, this could be expensive.  I’ve been to shops that first answer is bad alternator, ok, $1200 or so later and the problem would have still been there.  Bet a shop would have then said, oh bad regulator...  The plane would have been down for sometime finding parts and really in the end the cost was around $5 + one hour work for troubleshooting and fixing.

The bad wire was around 18 inches from the alternator, and it's sitting on the trouble shooting guide in the attached photo.