Aging Mooney Aircraft Spark Concerns
Bendix Magnetos
April 2015
This month’s ShopTalk will pinpoint a common problem of the M20 series aircraft with Bendix S20 series magnetos. A sharp bend in the P-lead from the right magneto stresses the insulation, often causing cracks. This leads to a short or reduced spark energy at the spark plug. 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.
With the introduction of the M20J, Mooney stopped using the S20 magnetos. The M20J used a Bendix dual magneto that does not have this problem. 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. Slick magnetos might also be on some pre‑model M20J 360 engines as Lycoming uses them on factory overhauled and remanufactured engines. Also, Slick sold an after-market kit replacing the Bendix magnetos. Nevertheless, the bulk of the older Mooney fleet (M20A through M20G) has Bendix S20 magnetos.
As wires age, the insulation surrounding the lead wire looses its flexibility and is easily cracked when flexed. 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. On Slick magnetos it is not necessary to remove the P-lead to check the timing.
The shield that covers the insulated wire for the P-lead is often torn or broken. This, in itself, is not normally a problem for proper operation of the magneto. The outer shield is grounded to the airframe to limit radio noise issues. 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 .
One common item on all these aircraft is the rear mounted propeller governor and the triangular mounting bracket that secures the prop control cable to operate the prop governor in the engine compartment. This is where an important complication occurs. The P-lead from the right magneto must make a ninety-degree bend to get past this bracket. During magneto maintenance care must be taken to avoid cracking the insulation at this bend.
A partially grounded P-lead can cause a rough running engine that can be especially difficult to troubleshoot. It is not uncommon to assume a magneto going bad when the culprit may be a partially or completely grounded P-lead.
Take a look at the photo in this article; one can clearly see the P-lead inner core wire exposed to the outer shield. Once this defect is discovered the solution is a simple one; cut back the shield to where the insulation is still intact. Terminate the shield and using a new P-lead kit and Mil-Spec wire, replace the damaged P-lead and wire. No wire comes in the kit. Use only Mil-Spec aircraft wire.
In the late 1990s, Teledyne Continental Motors purchased the Bendix line of magnetos, so the P-lead kit is a Continental Motors part. It still uses the old Bendix part number but must be purchased from a Continental Motors distributor.
The main P-lead part number for either left or right on most S20 magnetos is 10-157208. This will not fit the retard points P-lead receptacle on the left magneto, but this P-lead is rarely damaged; it's mainly the right magneto P-lead that is the culprit.
A quick way to tell if your airplane has the P-leads in question is to look at the back of the magneto. Where the wire (P-lead) attaches to the magneto, there will be a small knurled round nut that tightens onto the back of the magneto.
If your magnetos have a 10-32 nut or a large nut that takes a wrench to remove it you are probably OK, but have your mechanic examine the P-leads carefully at the next annual by removing them and bending them near the magneto attach point as you see in this photo.
If you find any cracked or cracking insulation replace that damaged P-lead. As our airplanes age, their service manuals becomes less informative and the need to pass on our discoveries in the field more important.
With this article we hope to save your mechanic time and you the owner money by doing informative trouble shooting and maybe even preventing a future problem. So take a careful look at those P-leads
As always, if you have a question about this article, we can be contacted via e-mail or by phone at our aircraft repair facility in Evanston, Wyoming, 307-789-6866. Until the next ShopTalk, enjoy flying your Mooney.
