From the Depot – C-1 Booster Coil

LOCATION

The booster coil may be mounted in any position, and in any location that permits routine servicing. However, it is recommended that the high tension lead be of the minimum possible length and that unshielded units be mounted in a junction box.¹

^{1 For more information related to mounting specifications and installation see AN 03-5-79, Op, Serv & Ovrh Inst w PC for High Tension Booster Coils A-1, C-1, 512, 513, 1313 & 1497 (Eclipse-Pioneer), 10-June-1945, (AN 03-5-79).}

MAKEUP

The Spec. 94-32182 Army Air Forces (AAF) Type C-1 unshielded booster coil consists of the complete coil assembly, mounting bracket assembly (4), and clamp / clamp assembly (5). The complete coil assembly constitutes the operating section of the booster coil. It consists mainly of the coil and housing assembly, contact spring assembly (11), stationary contact assembly (8) and the contact bracket assembly (7). The coil and housing assembly consists of an insulated molded housing with a primary and secondary wound wire core condenser sealed inside. The high tension lead extends into a spout (1) formed at one side of the housing, and is terminated in a metal cap that is molded into the spout. A grounding strip extends from one end of the coil housing and contacts a metal base (3) that is permanently assembled onto the housing. All other coil leads are soldered into hollow studs that are molded into the top of the housing. The contact spring assembly is mounted on one of the coil housing studs. The contact spring assembly extends over the wire core, and the moving contact (10) is riveted to the free end of the contact spring. The stationary contact (8) assembly is screwed into the contact bracket assembly (7), which is mounted on two of the coil housing studs. In all units the position of the stationary contact (9) is fixed by means of an adjusting wheel and a ratchet clip that is riveted to the contact bracket. The complete coil fits into the mounting bracket assembly (4) and is held in place by the clamp (2) and clamp assembly (5). The mounting bracket assembly and clamp assembly both have a padding strip (6).

BOOSTER COIL FUNCTION

The low rpm of the engine during initial starting fails to actuate the magneto with enough speed to generate the required high-tension current for engine ignition; consequently, the booster coil is actuated at the same time the starter switch is closed for cranking, and supplies the high voltage needed for safe, clean engine start up by the following process. The booster coil and magneto are separate components, and the former can generate a series of sparks on its own. During the start cycle, these sparks are routed to the trailing finger on the distributor rotor and then to the appropriate cylinder ignition lead. The primary winding has one end grounded at the internal grounding strip, and its other end connected to the moving contact point. The stationary contact is fitted with a terminal to which battery voltage is applied when the magneto switch is placed in the start position, or automatically applied when the starter is engaged. The secondary winding, which contains several times as many turns as the primary coil, has one end grounded at the internal grounding strip and the other terminated at a high-tension terminal. The high-tension terminal is connected to an electrode in the distributor by an ignition cable.

ADJUSTMENT OF BOOSTER COIL

Tampering with the setting of the booster coil is not recommended without training; adjustment of the setting requires special test equipment and must be made in accordance with the procedures outlined in section VII of Service and Instructions for High-Tension Booster Coils, AN 03-5-79, 10-June-1945

ELECTRICAL CONNECTIONS

Refer to the installation drawings in the manuals prepared by the aircraft manufacturer for connections and correct wire sizes. 

Since the booster coil is designed to operate only during actual engine cranking, its positive terminal must be connected so that the primary coil circuit is energized only when the “START” switch is closed (in direct cranking systems), or when the “MESH” switch is closed (in systems having inertial type starters). 

Since the return connection to the secondary winding of all the booster coils is made through the grounding strip at the base of the coil housing, all booster coils must be securely grounded. In the event that the selected location does not provide an adequate ground though the mounting feet of the unit, connect a ground strap between one of the mounting bolts and a structural member.

Make certain to use standard 7 mm high tension cable for making the connection between the high tension spout of the booster coil and the booster connection on the magneto.

VARIANTS OF BOOSTER COILS

The 94-32182 Army Air Forces (AAF) Type C-1 unshielded booster coil was primarily manufactured by Eclipse and General Electric. 

The Eclipse components seem to be more prevalent and for that reason we will focus upon their part numbering system and nomenclature. Similar in look, the AAF Type A-1 was given the Eclipse Type 512 part number for 12 volt system voltage. The AAF Type C-1 was given the Eclipse Type 513 part number for 24 volt system voltage. 

Styles are commonly referred to as A or B, with style B having a contact spring assembly of lighter construction than that used in corresponding style A units. Lighter spring construction decreases vibration frequency and results in lower leakage and higher output.

INTERCHANGEABILITY

Spec. 94-32182 Type C-1 unshielded booster coils produced under the AAF specification should allow for interchangeability between components produced by wartime manufactures of various part numbers.

REMOVAL OF BOOSTER COIL

Remove the box cover, high tension, battery and ground wires from the Booster Coil. Remove Coil.

IMPORTANCE OF BOOSTER COIL BOX

It is imperative to ensure that the coil is installed within a housing or box and mounted in the appropriate place with a cover installed.

When a coil is openly mounted on the frame, beams or engine mount the aircraft is exposed to an increased chance of fire. The manuals clearly specify that the booster coil should be installed in a box or housing. The purpose of the enclosure is to prevent radio frequency interference or RFI.  The enclosure provides an electrical shield to prevent such interference transmitted during operation.The box also reduces the risk of fire if the fuel pump, primer lines or fuel lines should develop a leak putting fumes inside the cowling and the points then igniting those fumes.

INSPECTION & TEST

This integral component requires frequent inspection and attention.

Some key questions in determining if your booster coil needs to be inspected / tested / replaced:

• Has the booster coil ever been inspected or tested? Have the contacts been inspected for pitting or burning?
• 50 hour inspection – After every 50 hours of airplane operation, inspect the booster coil for housing cracks; replace the unit if cracks are found. Make certain that all mounting bolts are secure. Check that all shielding spout caps are tight. Should the high tension spout cap be loose, check that the high tension connection is secure.
• Know your A/C so any change will draw your attention. Typically most airframes are wired to power the booster coil when the starter is energized regardless of the position of the magneto switch.  If your aircraft is equipped with a booster coil and will only start with the magneto switches turned to the on position you likely have a failed booster coil.
• 100 hour inspection – After every 100 hours of airplane operation,inspect the vibrator contacts. For access to the contacts of shielded units, loosen the shielding spout nuts, remove the housing cover screws and lift off the housing cover. Use crocus cloth to polish dirty, burned, or slightly pitted contacts; replace the booster coil if the contacts are badly pitted or burned. 
• Engine Change – At the time of engine overhaul or change, remove the booster coil and overhaul.  
• Thoroughly inspect during each annual inspection

As a reference, the Service and Instructions for High-Tension Booster Coils, AN 03-5-79, 10-June-1945 outlines a more thorough inspection of booster coils, and maintenance manuals within the electrical system sections have comprehensive instructions for pre-flight, after flight, daily flight, and at 25, 50, 100 hour inspections.

WHEN PURCHASING

If you question whether your booster coil is functioning properly, or you are planning to replace it all together, ensure airworthiness prior to purchasing. AirCorps maintains an inventory of Type C-1 booster coils overhauled and in stock. We also have the booster coil on our Certified FAA Repair Station (8ORR202D) accessory capabilities list. 

We can perform inspections, overhaul, installation, and answer questions related to these important components. 

Type C-1 – Booster Coil with 8130 Authorization- AirCorps Depot. 

ERIC TRUBLOOD