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Toggle switches with magnetos - Help!

Fearless

Well Known Member
I have wired my plane using toggle switches to control the magnetos and starter on my O-320. I utilized the setup with the switches from the Z-11 drawing in the AeroElectric Connections manual. I am pretty sure that I have my magnetos and engine timed correctly. I don't seem to be getting any kind of spark at the cylinders. How can I find out if I am getting any current through the leads or what resistance I should be getting though the wires from the switches. How do you check the spark at the wires at the plugs? The switches seem to be working correctly. When I try my left mag switch to the start location the starter turns over. When I try to turn on my right mag switch with the left mag engaged the starter circuit is disabled which is correct(my left mag has the impulse coupling). Dealing with electrical systems is not my strong suit. I have new plugs and wires in the install and my magnetos were serviced before this engine was rebuilt. I am getting fuel up through the carb when I move the throttle.

If the timing was off on the magnetos and engine - if I was getting spark through the magnetos wouldn't the engine try to do something?

This is the last step for me to get to my inspection - to get this engine to fire off - I am so discouraged right now.

The plane is at an airport over an hour from my house so these empty trips is starting to wear me down.
 
Step 1, disconnect the p leads from the magneto terminal and spin the engine with no plugs installed. If you dont have spark at the plugs, you have a mag problem.

P leads "ground" to kill ignition, and "open" to run.
 
How can I find out if I am getting any current through the leads or what resistance I should be getting though the wires from the switches. How do you check the spark at the wires at the plugs?

Pardon but your post does not make it clear that you understand that magnetos only require a switch to ground them out and stop them. A magneto switch breaks continuity to allow the mag to run; grounds the mag to stop it. BTW, always consider magnetos hot because if the switch goes bad, they may not be grounded.

Check for spark by laying a spark plug on the engine and cranking it.
 
If you've cranked the engine with a spark plug out and found no spark, pull all the spark plug leads, for safety. Disconnect the P-lead and the shield from the mag. With the mag switch "off" check for continuity between the shield and the center conductor at the mag. You should have continuity. Turn the mag switch "on" and you should have an open circuit. Also, make sure the center conductor is open to ground (the magneto housing or engine case). Any ground to the magneto housing will shut off the mag. One of the builders I do Tech Counseling for had his keyswitch wired wrong and the mags were grounded all the time.
 
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Disconnect the left mag plead, at the mag, and try to start the engine with the plead disconnected. With the plead disconnected at the mag, the left mag is hot all the time and the aircraft wiring is eliminated. If timing is correct and mag is functional, the engine will fire assuming you have any fuel delivery. If you don't get the engine to fire then you have a mag or timing issue. Test for spark at one of the spark plugs that comes from the left mag as the others have suggested. If you get spark but the engine doesn't fire , you are timed way off or on the exhaust stroke or you have the ignition harness routed wrong, again assuming you have fuel to the cylinders. If you don't see spark at the plug then the problem is in the mag. I am assuming you don't have Bendix/tcm 20 series mags, which will ground out if you remove the pleads. If you do have that type mag, just insert your timing wiring dummies and make sure the end of the wire where you would hook the timing light is insulated away from ground and do the test listed above.
Good Luck,
Mahlon
 
The magnetos are Slick types. Thanks for all the suggestions. I will try to check for spark at the plug on the (top) number one cylinder (left mag wiring) and go from there.

Mahlon - great to see you are still monitoring the forum.
 
You probably know this, but to be sure, if you pull the plug to look for a spark, you will need to ground the plug. It won't spark if not grounded, even if your mag is functioning properly.

Larry
 
You probably know this, but to be sure, if you pull the plug to look for a spark, you will need to ground the plug. It won't spark if not grounded, even if your mag is functioning properly.
Larry

Another thing to note here is that a magneto spark at cranking speeds is pretty weak. You must look closely and preferably under fairly dark conditions.
 
THE most common issue with the use of toggle switches is that we mentally default to "up is on, and on closes the circuit". You would be surprised how many people, including proficient engineering types, get stuck in this mental rut. Heck, many switches, including MS-style "real airplane switches" are labelled "off" and "on", and this just helps compound the problem.

As mentioned by others here, magneto switches work BACKWARDS. When you flip the switch in the "up" direction, you want it to OPEN the circuit. Opening the circuit removes the ground to the P-lead. When the P-lead is ungrounded, the magneto is live and able to make spark. Again, this is totally backwards to most switch applications where we use the closure of the switch contacts to energize a circuit.

If you want to do a quick test, turn your mag switches to the OFF position, hit the starter and see if the engine starts. My money is on the engine starting with the mags in the "off" position. :)
 
THE most common issue with the use of toggle switches is that we mentally default to "up is on, and on closes the circuit". You would be surprised how many people, including proficient engineering types, get stuck in this mental rut. Heck, many switches, including MS-style "real airplane switches" are labelled "off" and "on", and this just helps compound the problem.

As mentioned by others here, magneto switches work BACKWARDS. When you flip the switch in the "up" direction, you want it to OPEN the circuit. Opening the circuit removes the ground to the P-lead. When the P-lead is ungrounded, the magneto is live and able to make spark. Again, this is totally backwards to most switch applications where we use the closure of the switch contacts to energize a circuit.

If you want to do a quick test, turn your mag switches to the OFF position, hit the starter and see if the engine starts. My money is on the engine starting with the mags in the "off" position. :)

+1

I almost wrote this, but assumed you would follow Mahlon's advice to disconnect the P lead from the mag to eliminate mis-wiring issues.

Larry
 
Still troubleshooting

It appears I am not getting any spark from my (left) impulse coupled magneto - I am getting a friend who is an A&P to give it a look over and will go from there. I had both of my magnetos serviced before I installed them on the engine.
 
Follow-up

Well after talking with my very patience support person at G3 Ignition (Thomas) we came up with a plan of attack to try to figure out what my electrical issue might be. I took the magnetos off the plane to check if they are still firing and they are (Thomas had already bench checked them when he modified my mags for the coil terminals needed for the G3 electronic ignition system). I reset the magnetos for my left rotation engine. I re-installed the magnetos with the number 1 cylinder at 25 degrees BTDC and hooked up the sychronizer to set the mags. To try to determine if I have a problem with wiring I left the wires off the left magneto to leave it hot (and grounded the right one so it wouldn't fire) and turned over the engine with the starter. Low and behold it fired on about the third rotation. I immediately cut off the fuel to stop the engine. My next step is to bring the wiring from my G3 ignition box to the magneto but leave the switches out of the loop and the G3 box turned off and try to start the engine again. If it starts then I will bring the switches back into the loop and try to start the engine again. If this all pans out then my problem was timing on the mags and engine all along. I did learn from Thomas that I need a jumper to run from the P-lead terminal to the coil terminal to make the magneto hot when bench testing which I hadn't thought of before. The G3 ignition box takes care of this when wired properly on the plane. Thanks for the support from this group.
 
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