By Don Maxwell
Mooney Airplane Pilots Association
MAPA Log, Volume 31, Number 32
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|Shower of Sparks|
Pre “J” Mooneys and M20K Mooneys that have the
Rocket conversion have one thing in common. Most all use the “shower of sparks”
The “shower of sparks” system is fairly simple, but widely misunderstood. Most folks think there must be a “genie in a box” that makes our engine start by magic. I’ll try and explain how it all works and why and also give some easy tips on how to check and see if your system is working properly.
I receive many calls from owners and mechanics that are experiencing “shower of sparks” (SOS) starting problems and asking for help in diagnosing their problems.
Why We Need the “Shower of
Our Mooney engines operate on the four-cycle principle: intake,
compression, power and exhaust. On the intake cycle, air and fuel are drawn into
the cylinder. The fuel/air mixture is compressed on the compression stroke. Near
the top of the compression stroke, the mixture is ignited forcing the piston
down. The power stroke and the hot
gases are expelled on the exhaust stroke, and once completed, it all starts
again and is repeated hundreds of times each minute.
Unlike automobile engines, aircraft engines have fixed ignition timing. Normally the magneto is installed to fire the spark plug at 25 degrees before the piston gets to the top of the compression stroke. This timing is for the best operation of the engine after it is running. The problem, with the timing set to 25 “top dead center” or “TDC”, the engine would try to run backwards. So, to get the engine running in the right direction, we must delay or “retard” the time of ignition. The best time for ignition to occur in starting is just as the piston is at the top of the compression stroke, TDC, or just a little past it. In this case we must delay the spark for 25 degrees of rotation.
The “Shower of Sparks” system is composed of a magneto with two sets of points installed, a vibrator switch, ignition switch and the aircraft battery.
The magneto has two sets of contact points. One set is adjusted to open 25 degrees before TDC used for normal operation. The other set; the “retard points” are adjusted to open 25 degrees later at TDC for starting.
The operation of choosing which points are used is controlled by the ignition switch. The vibrator switch is a box mounted on the firewall that houses a set of contact points that vibrate open and closed very rapidly when power is applied from the ignition switch.
Figure 2 shows external view of vibrator.
Figure 3 shows internal view of vibrator. "No genie installed."
The ignition switch has 6 positions. “Off” which grounds both magnetos and makes them both inoperative. “Right” allows the right magneto to operate and makes the left magneto inoperative. “Left” allows the left magneto to operate but makes the right magneto inoperative. “Both” allows both left & right magneto to operate.
The other two positions are “turning the key just to the right of the both” position activates the shower of sparks vibrator, and “pushing in” on the ignition key at this position activates the engine starter.
Figure 4 shows internal circuitry of ignition switch.
How it Works
When starting our Mooneys the ignition switch is turned to the far right. The starter vibrator is activated in this position and can be heard as a buzzing sound. The ignition key is pushed inward to activate the starter. At the same time, the ignition switch internally grounds the right magneto so it will not operate. The right magneto is set at the same 25 degrees as the left for normal operation, so we do not want it to fire and cause the engine to “Kick Back”. So during the starting process, only the left magneto is used for starting. The ignition switch also selects the “retard points” and disables the normal points in the left magneto.
The vibrator switch is now supplying interrupted aircraft battery power to the left magneto coil thru the retard points. When the piston reaches TDC on the compression stroke, the retard points open allowing the magneto coil to charge and discharge as rapidly as the vibrator points can open and close. The result is a continuous spark that resembles a lightning bolt igniting the fuel/air mixture. This event continues for a few degrees of rotation on each cylinder until the engine starts and the switch is released to the both position. In the both position, the right magneto is enabled as well as the left magneto’s main set of points.
Symptoms of a failing SOS system
1. Engine kicks back during starting
2. Hard starting
3. Engine starts when the key is released to the both position
Troubleshooting the Shower of
1. Check for a buzzing sound as the ignition switch is turned past the both position but before pushing IN on the key. If a buzzing sound is heard, most likely the vibrator is alright, but it can still be defective.
2. With the cowling removed, disconnect the starter relay from the starter so that the starter cannot be engaged.
3. Fig 5 shows the starter relay. Remove the small wire at top of the relay to disable the starter
Figure 5 shows starter relay.
Remove the spark plug leads and top plugs of all
cylinders. Remove the bottom spark plug leads.
5. Rotate the prop, by hand, until the number one cylinder is at Top Dead Center, TDC on the compression stroke.
6. Make certain that the starter has been disabled and cannot be activated by the ignition switch. Some early Mooneys (M20B and M20C) have a starter disengage switch located beside the starter vibrator switch. This switch was installed so that the starter relay could be disengaged to allow hand propping. NOTE: Whenever hand propping a Mooney or any aircraft with Shower of
Hold the #1 spark plug lead by the insulation and place
the spring at the end of the harness lead, very near the cylinder. Have another
party in the plane turn and hold the key to the far right past “both”. The
vibrator should be buzzing and a constant arc of electricity, “shower of sparks”
should be seen between the tip of the lead spring and the engine cylinder.
Sometimes it may be necessary to move the prop back and forth a few degrees to
find the exact spot where the “retard points” will open.
Note: This step varies a little depending on the manufacturer of the magneto harness, but we will assume we are working with a 4 cylinder Lycoming, Bendix magneto with a Bendix harness. In this configuration the magneto will fire the top left sparkplugs, #1 and #3, and the bottom right sparkplugs #2 and #4. Other aftermarket harness manufactures have the harness fire the opposite bottom left and upper right.
When the “shower of sparks” is visible, I rotate the
engine through its firing order (1, 3, 2, 4) to see that each cylinder is firing
If no spark is seen, it is most likely the retard points
in the left magneto. The magneto will have to be removed and the retard points
inspected, reset or replaced. If the retard points in the magneto are good, the
problem will be the vibrator or ignition switch.
Most starter vibrators in Mooneys are located on the
cabin firewall just to the right of the compass center post. If your Mooney is
original and still has the forward access panels, the switch is easily found by
removing the co-pilot side panel. If your Mooney has been modified with a 201
style windshield, the vibrator will be more difficult to remove unless it was
relocated by the modifier.
As our Mooneys age I am seeing more defective ignition
switches. After 40 years, many are just worn out. A few clues are:
keys that come out in any position; engines that will not start; and when
the key is released; and the engine starts.
Hopefully this article has taken the “genie” and magic out of the “shower of sparks” system and will help you diagnose its problems. As always we are available to help you with your Mooney questions.
To view and print: SI M20-39