The spark plug wires on my IO360 seem to be much too long; they are all coiled up on both sides. Do I have the wrong plug wire harness?
Search strings: spark plug harness, ignition harness, plug cables, ignition wires
Technical Editor:
My bet is that it is the correct harness, but incorrectly installed. The correct harness is a long harness. If you have plug wires that cross behind the engine, under the oil filter, the harness was put in wrong. This usually happens when the wrong harness was previously installed. There is another IO360 harness with short leads, which run behind the engine (which is not correct for our planes). Someone later ordered a new harness, the right one came, but was installed the same as the wrong one.
All four leads from each mag run outboard and forward from the mag. The LH mag’s two short leads go to the bottom #2 and #4 plugs. The two long leads go up through the plug wire bushing in the aft baffle, and cross over the engine in front of the baffle, feeding the top plugs for #1 and #3. The RH mag feeds the bottom plugs on #1 and #3; the two long leads cross over (forward of the baffle) to the top plugs on #2 and #4. The four leads that cross forward of the baffle are tied together. There is an insulated clamp that mounts on a bracket; the bracket goes under the last case-half bolt on the aft top, just forward of the baffle.
Once the harness is properly installed, you will likely find that it is the correct length. If it has not been badly damaged, it will probably still work fine, once ‘unfolded’, cleaned, and properly installed. If the cut insulation is the outside shrink-wrap or silicone-coat, no harm is done. In fact, you can ‘repair’ it with Plasti-Dip, if you like. If the outer copper braid has been pierced, that’s a different story; but if that were the case, I think you’d have a missing plug.
Make sure you have the correct high-tension grommets where the two wires go forward through the aft baffle on each side. They should be sort of an off-white, pale yellow, or pale green, and completely fill the round hole in the baffle. New harnesses tend to now be shipped with the new-style rectangular grommets, which do not properly fill the round holes. I have seen some cases where a mechanic had made and riveted in place a filler plate. I suppose that’s OK, but I prefer the better dielectric provided by the large round-hole grommet.
The IO360 plug harness that I have on record for the Bendix 1200-series magnetos is as follows:
M2420 Correct SLICK harness; includes:
LWM2421 Left harness (T1=65″, T3=59″, B2=44″, B4=36″)
LWM2422 Right harness (T2=65″, T4=59″, B1=42″, B3=36″)