Compromised insulator between the inner wire and the shield of the P-lead can cause internal arcing or in some cases become completely grounded thus squelching the high voltage to the plugs. The outer shield is grounded to the airframe to limit radio noise issues. This, in itself, is not normally a problem for proper operation of the magneto. The shield that covers the insulated wire for the P-lead is often torn or broken. On Slick magnetos it is not necessary to remove the P-lead to check the timing. At each annual inspection or when magneto work is accomplished (every 500 hours) the magneto P-lead is removed from the magneto assembly to check or set the timing of the magneto. Nevertheless, the bulk of the older Mooney fleet (M20A through M20G) has Bendix S20 magnetos.Īs wires age, the insulation surrounding the lead wire looses its flexibility and is easily cracked when flexed. Also, Slick sold an after-market kit replacing the Bendix magnetos. Slick magnetos might also be on some pre‑model M20J 360 engines as Lycoming uses them on factory overhauled and remanufactured engines. Mooneys saw the last of the Bendix magnetos as Lycoming abandoned the shower-of-sparks starting method and adopted Slick's impulse coupling magnetos on 360 type engines. The M20J used a Bendix dual magneto that does not have this problem. With the introduction of the M20J, Mooney stopped using the S20 magnetos.

Most Mooneys produced in the 1960s and up to the introduction of the M20J in 1977 had these common magnetos installed on the O360 and IO360 Lycoming engines. This leads to a short or reduced spark energy at the spark plug. A sharp bend in the P-lead from the right magneto stresses the insulation, often causing cracks. This month’s ShopTalk will pinpoint a common problem of the M20 series aircraft with Bendix S20 series magnetos.

Bendix magnetos for continental engines