since 1940

Bendix Magneto Frequently Asked Questions

How to order a Bendix Impulse Coupling

You can purchase either the Impulse Cam Assembly    shown here (less expensive, spring not included).   or the entire Impulse Coupling Assembly   shown here (more expensive) but with spring already installed.  The Impulse Spring (10-51324) is shown below   should always be replaced. Springs are in tension and any corrosion pitting causes them to break. Usually there is some corrosion in the impulse coupling area. The impulse coupling operates in a corrosive environment . When a spring gets corrosion pits it soon breaks and most springs will have some corrosion on them. So you should always replace the spring when replacing the impulse coupling.      Example of corrosion where the impulse coupling operates. Many customers would rather purchase the Impulse coupling Assembly rather than the Impulse Cam Assembly  because of the difficulty in installing the spring. If you have the D2000/D30000 (dual magneto) then we highly recommend purchasing the Impulse Coupling Assembly as the spring is almost impossible to install without expert guidance.   There are dozens of different impulse couplings used on Bendix magnetos. For us to determine the correct impulse coupling we need the magneto part number. This is a number that starts with 10-XXXXXX The magneto model number (example S4LN-25) does not help us as each model is made up of many different part numbers with different impulse couplings. The magneto part number is on the data plate.   Bendix magneto showing data plate

Purpose of the distributor gear

The distributor gear distributes the spark energy to each ignition lead. It consists of a shaft, gear, and electrode finger. As the gear turns the electrode finger comes close to each distributor block contact that is connected to each ignition lead. The spark jumps the small gap between the electrode finger and the block contact.

How to test for a "hot" magneto

Continental Service Bulletin SB653 (or latest edition) contains information for checking to see if the magneto is "hot".

SB653 in Acrobat format

In addition, the following is provided by Hartzell Propeller

Magneto Check
Periodically ensuring that your magneto’s p-leads have not broken is a good defense against unexpected starts on the ground. These leads, which are connected to the ignition (or magneto) switch in the cockpit, are responsible for grounding the mags to keep the engine from running. Sometimes the wires or connections between the switch and the magnetos break or come adrift. In this case, one or both mags may be "hot," or ready to deliver spark whenever the prop is turned. To test them, instead of shutting down the engine in the usual manner with the mixture control, use the key. Allow the engine to cool normally and idle down. Move the key slowly through both Right, Left, and then to the Off position. The engine rpm should drop slightly at both of the individual-mag positions and shut down completely in the Off detent. Allow the prop to stop and then move the mixture to the idle-cutoff position. Do not try to "catch" the engine before it comes to a stop because a dangerous backfire might occur. If the engine does not stop when the key is in the Off position, shut it down with the mixture, prominently mark the prop as being "hot," and contact maintenance personnel immediately.

There are a few other caveats to consider when in the vicinity of the prop.

Avoid pulling the airplane around by the prop. Yes, this seems the perfect solution to a vexing problem of how to change the airplane’s position without having to walk around and get the tow bar, but it’s worthwhile to make the extra effort. Neither the engine nor the prop particularly benefit from the loads imposed by horsing the whole airplane around.
Avoid pushing the airplane by the spinner. The spinner and backing plate are built to be light, so they’re quite fragile. Pushing on them can cause the backing plate to crack and can lead to spinner failure.
Avoid contact with prop deice boots and associated wiring. If you want your hot prop to be toasty when it counts, stay clear of the boots.

When do you overhaul or replace your magneto - how long should one last

Continued Airworthiness and overhaul information is contained in the manufacturer's maintenance (overhaul) manual.

In the case of magnetos it is specific to the magneto manufacturer, and in some cases, to the aircraft. For example, some Slick Magnetos on some Lycoming IO-540 engines have 200 hour impulse coupling inspection requirements mandated by the manufacturer and an airworthiness directive

Generically, there is a 500 hour inspection requirement for both Bendix and Slick magnetos and a overhaul at engine TBO.

Do I need a magneto noise filter?

The magneto noise filter MF-3A can be used on Bendix S20/200 magnetos. It is not used on the newer Bendix SC series magnetos. The MF-3A is also not used on the Slick series magnetos. One reason these magnetos do not use the noise filter is because the magneto capacitor inside the magneto is a feed-thru capacitor and acts as a noise filter.

To quote from Lycoming Service Letter L134 dated October 1960 (referring to the -200 and -600 Bendix magneto: "A feed-thru capacitor is incorporated in thee magnetos, which reduces the arching of the main breaker points, suppresses magneto radio interference and eliminates need for the external filter in the magneto ground circuit."

 

Any noise filter added to the magneto reduces magneto output.
 

What is the impulse coupling?

Impulse coupled magnetos use a spring loaded latching device called an impulse coupling. The impulse coupling helps latches to retard the timing of the magneto and unlatches at the optimum point to start the engine. The impulse coupling spring serves to accelerate the magnetic rotor shaft to produce a high-intensity spark. Engines can be configured with a dual or single impulse coupled magneto. Many Lycoming's use one impulse coupled magneto on the left and thus you would start the engine on the left magneto. Continental engines more typically have both magnetos with impulse couplings and on these engines you start the engine on both magnetos.

Impulse couplings optimize the starting performance by delaying (retarding) the spark event during starting.

Rivet Style Bendix Impulse Coupling Broken rivet on the left. Replaced with snap-ring style impulse coupling. (shown at top of page).

Do I need to change my impulse coupling from rivet to snap ring style

Basically here is the problem: The impulse pawls are held onto the coupling by a rivet. Being a rivet it has to be somewhat soft in order to be formed. Some engines (especially Lycoming) shake the impulse coupling and cause wear on the rivet shank. Wear causes the pawl to extend further outward. Eventually the pawl can extend outward enough to hit (engage) the stop pin. When this happens during flight the impulse explodes apart and sends pieces of the coupling down into the accessory gear train. When these pieces fall into the gearing it strips the crankshaft gear from the crankshaft causing instant engine failure.

These failures have occurred well within the inspection period allowed.

As a fix, the rivet is replaced with a hardened pin with a snap-ring. Wear and safety is greatly reduced. Personally, I would replace all rivet style impulse couplings with snap-ring style impulse coupling on my Lycoming BEFORE FURTHER FLIGHT.

See Airworthiness Directive 96-12-07 or latest edition plus the latest Teledyne Continental Service Publications on the subject for the latest information.

How do I time the magneto

See article on subject

Wiring diagram for Shower of Sparks

Here's one I found at Bob Nuckolls excellent website www.aeroelectric.com

Shower of Sparks Wiring in Adobe Acrobat format

What Magneto Gasket do I use?

The following article is a copy of an article printed in the Transport Canada Aviation publication titled Feedback, Issue 1/95. This article is reproduced with the kind permission of Transport Canada Aviation.

"Transport Canada (TC) has recently been informed of certain Piper and Mooney aircraft, powered by Lycoming engines, which had magnetos installed with incorrect gaskets. The magnetos were Bendix S-20 and S-200 series from Teledyne Continental Motors (TCM). The first report was a Piper PA23-250, powered by a Lycoming IO-540 engine, which had a magneto fall off in flight in the USA. According to the A&P mechanic who repaired the aircraft, the failure was caused by the installation of an incorrect gasket which did not provide proper support for the mounting flange, causing the flange to fail. It is thought that the thickness of the gasket had a bearing on the failure. The aircraft's remaining three magnetos were inspected and were also found to have incorrect gaskets installed and, although two of the magnetos were cracked, none had progressed to failure. This particular magneto application calls for the installation of two Lycoming gaskets (P/N 62224), one on each side of the adapter plate. The failed magneto on this aircraft had one P/N 62224 gasket between the adapter and the engine case, but had an LW 12681 gasket, and possibly a TCM P/N 534750 gasket, between the magneto and the adapter plate, which is incorrect.

The repair shop operator, who reported this problem to Transport Canada, has since found two aircraft with the same problem: a Mooney powered by a Lycoming O-360 engine, and a Piper PA24-250 powered by Lycoming O-540 engine. Although incorrect gasket installation was found, none of the magnetos had progressed to failure. None of these magnetos were installed by the engine manufacturer, but had been removed and reinstalled.

TC contacted TCM/Bendix about this problem and was told the magneto gaskets were furnished and installed by the engine manufacturer at time of build-up, and that different engine manufacturers use different gaskets for the same model magneto, e.g. Lycoming engine magneto gaskets differ from Continental engine magneto gaskets. TC also contacted Textron Lycoming, who confirmed that proper installation for the S-20 and S-200 magneto incorporates two P/N 62224 gaskets. These magnetos have an eyelet on the top and bottom of the magneto case, and are mounted using two of these gaskets, one on each side of the adapter plate.

The Lycoming LW 12681 gasket is used in certain applications when the magneto is secured by clamps, as in the case of S-1200, S-2000 and S-2200 magnetos. It is possible that through misunderstanding of the parts catalogue, or a simple mistake at re-installation of a magneto, that incorrect gaskets were used. AMEs are reminded that extra precautions should be taken when new parts are drawn from stores and used during re-installation. The gaskets were all authentic Bendix magneto gaskets. It was their particular application that was faulty in these instances."

How do know if the magneto capacitor is good or bad

See my article on Checking the Aircraft Magneto filter

I can't rotate the magneto because it is at the extreme end of the mounting slots

See my article Can't rotate magneto to get timing

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