Yesterday I was asked to look at a cranking problem. First thing, I grabbed the big cable at the start contactor, and gave it a wiggle and pull. It pulled right out of the terminal crimp. Here you can see the discoloration due to overheating in the bad crimp.
However, the big problem was on the ground side of the circuit. On this particular model, there is a "ground tab" welded on a motor mount tube just aft of the lower right dynafocal ring. The builder had made a short ground cable to connect that tab with a 1/4-20 stud on the sump flange. A stranded tefzel cable three or four inches long is not flexible enough to accommodate engine movement, so in very few hours, here is the result:
So, what are the lessons? First, a cable that is expected to flex must have some length, and standard aircraft wire is generally too stiff anyway. Use welding cable (very flexible), or better yet, order a braided ground strap from B&C:
http://www.bandc.aero/braidedbondingstrap.aspx
Second, in my opinion, grounding to a welded motor mount tab is dumb, as it adds resistance to the circuit, and another potential failure mode (a broken tab). Run the ground strap from the dedicated ground lug on the case (if your engine has one, just above the RR motor mount) directly to the single point airframe ground or the battery.
If your engine is electrically dependent, run two ground straps.
However, the big problem was on the ground side of the circuit. On this particular model, there is a "ground tab" welded on a motor mount tube just aft of the lower right dynafocal ring. The builder had made a short ground cable to connect that tab with a 1/4-20 stud on the sump flange. A stranded tefzel cable three or four inches long is not flexible enough to accommodate engine movement, so in very few hours, here is the result:
So, what are the lessons? First, a cable that is expected to flex must have some length, and standard aircraft wire is generally too stiff anyway. Use welding cable (very flexible), or better yet, order a braided ground strap from B&C:
http://www.bandc.aero/braidedbondingstrap.aspx
Second, in my opinion, grounding to a welded motor mount tab is dumb, as it adds resistance to the circuit, and another potential failure mode (a broken tab). Run the ground strap from the dedicated ground lug on the case (if your engine has one, just above the RR motor mount) directly to the single point airframe ground or the battery.
If your engine is electrically dependent, run two ground straps.
