IGNITION SYSTEM

 

TTP Cessna 172	Cat.B1	Level -3	ATA05	Ignition System
	Cat.B2	Level -3

Engine uses two Unison/Slick 4371 series, impulse-coupled magnetos to fire two spark plugs in each cylinder.
 Lycoming Direct Drive Engine Overhaul Manual, 
Lycoming Operators Manual, 
Lycoming Service Instruction 1437 
Unison 4300/6300 Series Magneto Maintenance and Overhaul Manual.
Inspection time requirements -  Chapter 5, Inspection Time Limits.
Inspection procedures - Unison 4300/6300 Series Magneto Maintenance and Overhaul
Manual.

PTP Cessna 172	B1	LOC	FOT	SGH	R/I	MEL	TS
	B2	LOC	FOT	SGH	R/I	MEL	TS

741001 Magnetos - Examine the external condition and for correct installation and condition of the electrical leads. Complete a check of the engine timing (external timing).05-12-02 120

741002 Magnetos - Clean, examine, and adjust as necessary

742001 Ignition Harness and Insulators - Check for proper routing, deterioration, and condition of terminals. 

742002 Spark Plugs - Remove, clean, analyze, test, gap, and rotate top plugs to bottom

and bottom plugs to top.

743001 Ignition Switch and Electrical Harness - Inspect for damage, condition, and security.

743002 Inspect and lubricate ACS brand ignition switch. 

Magneto Removal/Installation
A. Remove the Magneto .
(1) Remove the engine cowl.
WARNING: Make sure that each magneto P lead is grounded.
WARNING: Before you rotate the propeller remove a minimum of one spark plug from
each cylinder to prevent the start of the engine.
(2) Remove the screws that attach the high tension outlet cover to the magneto.
(3) Disengage the high tension cover from the magneto.
(4) For a reference point when you install the magneto, turn the propeller in the normal direction until each impulse coupling releases near Top Dead Center (TDC) on the number one cylinder compression stroke.
NOTE: You will hear a click sound from the impulse couplings when they release.
(5) The crankshaft position can be found by the marks on the front or aft face of the starter ring gear support. Refer to the Lycoming Service Instruction 1437 or latest revision for more instructions.
(a) When you use the marks on the front face of the ring gear, they must be aligned with the small hole that is found at the two o'clock position on the front face of the starter housing.
(b) When you use the marks on the aft face of the ring gear, they must be aligned with the engine case parting line.
(6) Turn the propeller in the opposite direction of the normal propeller operation to approximately 30 degrees BTDC (Before Top Dead Center) on the number one cylinder compression stroke.
(7) Turn the propeller in the normal direction to 25 degrees BTDC on the number one cylinder compression stroke.
(8) Disconnect the P lead and ground wire from the magneto.
(11) Remove the magneto from the housing.
B. Install the Magneto.
(1) Apply a small quantity of silicone grease such as DC4 to each side of the new magneto base gasket, which will help future timing adjustments.
(2) Make sure the magneto drive gear is installed correctly, the nut torqued correctly and the cotter pin is installed. Refer to the Lycoming Service Instructions 1437 or latest revision and the Unison 4300/6300 Magneto Maintenance and Overhaul Manual Instructions.
CAUTION: Make sure you remove the T-118 timing pin immediately after you attach the magneto to the accessory case and before the magneto or propeller is turned.
(3) Insert the T-118 timing pin into the L timing hole in the magneto distributor block.
(4) Turn the magneto rotor in the opposite of normal direction until the timing pin is engaged fully into the distributor gear.
(a) If the magneto rotor does not move freely and the pin will not go into the hole in the gear, the pin has hit the pointer on the gear.
(b) Pull the pin out far enough to continue to turn the magneto freely in the opposite direction of normal movement until the pointer has passed the pin, then insert the pin.
1 Turn the magneto rotor until the pin engages the gear.
(5) Do a check of the crankshaft to make sure the propeller has not moved and is still set in position with the number one cylinder at 25 degrees BTDC (Before Top Dead Center) on the compression stroke.
(6) If the propeller as been turned and only one magneto was removed, it will be necessary to engage the impulse coupling on the magneto that is installed, and establish the crankshaft position.
(7) With the number one cylinder at 25 degrees BTDC on the compression stroke, do the steps that follow.
CAUTION: Make sure you remove the T-118 timing pin immediately after you attach the magneto to the accessory case and before the magneto or propeller is turned.
(a) Install the magneto with the new base gasket and the T-118 timing pin in position.
(b) Engage the magneto drive gear with the engine gear, in a position that will give a range of magneto timing adjustments in each direction.
(c) Hold the magneto in position against the accessory case and install the nuts, flat washers, clamps and new lock washers.
(d) Finger tighten each nut by hand.
(e) Remove the timing pin.
(8) Before you continue, you must adjust the magneto timing. Refer to Magneto-to Engine External Timing Adjustment .
(9) With the magneto set in position, first tighten each nut to 8 foot-pounds (10 N-m).
(10) Tighten each nut from one side to another, to a torque of 17 foot-pounds (23 N-m).
(11) Connect the P lead to the magneto.
(12) Attach a ground wire to the magneto.
(9) Examine the magneto angle to help make sure you put it in the same position for installation.
(10) Remove the nuts, washers and clamps that attach the magneto to the engine housing.
(13) Attach the high tension outlet cover to the magneto.
(14) Tighten the P lead nut to a torque of 13 to 15 inch-pounds (1.5 to 1.7 N-m).
CAUTION: Make sure you remove the T-118 timing pin before the magneto or propeller is turned.
(15) Install the spark plugs.
(16) Install the cowl .
(17) Complete a engine preflight operational check of the ignition system.