Kill switch problems

by Had Robinson
updated September 4, 2020

Failure of the kill switch is more than just a nuisance, it is a safety issue, as well.

A. How it operates

The principle of the switch is to ground the primary winding of the ignition coil which will kill the spark and stop the engine.  The actual kill switch is a simple button.  Though cheaply made, it is relatively immune from failure.  The circuit uses the steel Bowden cable that runs to the throttle as the ground.  A wire is run up to the throttle and kill switch (button) from the primary wire coming out of the coil.  There may be a short extension of the wire so that it reach the connector on the wire going to the throttle.  The kill switch wire enters the wire loom near the throttle cable's connection to the motor frame.  It then extends up to the throttle between the loom and Bowden cable.  The wire continues inside the throttle and is soldered to one of the terminals on the kill switch.  A wire runs from the other terminal on the switch to the throttle end of the Bowden cable.  It is secured by a narrow piece of polyvinyl tubing.  The connection is not particularly sound but it works.

B. Points of failure

a. Coil primary wire

Refer to the Primary wire test in order to check this.

b. Kill switch

Only rarely is the kill switch in the throttle handle defective.  It is easy to test with a digital voltmeter.  Disassemble the throttle handle by removing all of the screws and carefully opening just the kill switch end.  Using test leads, connect the leads from the digital voltmeter across the switch and measure the resistance while pushing the switch and releasing it.  The resistance should go from a few Ohms to infinity, respectively.  If it is defective, it will have to un-soldered and a new switch installed.  These switches are common-sized and are available on eBay and places like DigiKey.

c. Defective throttle cable wire

The most common failure of the circuit is a defect in the wire itself when it was manufactured.

This is the kill switch wire from a Miniplane with less than (10) hours on it.  We could feel the break in the wire, which was about a foot from the throttle.  We sliced the insulation open and exposed the broken ends which, for a time, worked until they separated only a tiny bit.

This continues to be a quality control (QC) issue with the Italian manufacturers.  During manufacture, the wire is likely stretched to the point of breaking or the wire is stressed over a sharp object essentially cutting it as the outer insulation is being applied at the factory.  It seems to happen in batches.  One pilot's new Miniplane had (2) separate breaks in the kill switch wire.  What is particularly annoying is that the circuit will work for awhile because the broken ends of the wire will touch.  However, when the throttle cable is moved repeatedly, it will stretch a small amount which is just enough to cause the circuit to fail.  The pilot has no warning except when he attempts to shut off the engine.

To improve safety and convenience, it is highly recommended that all pilots install a choke control or an emergency kill mechanism if there is not already one installed e.g. on the Polini's and Minari's.

If the motor is new, a warranty replacement of the entire throttle cable can be done.  If your kill switch has failed and you are past the warranty period, plan on spending at least an hour to fix the problem.  Having the right tools is essential.

d. Frame attachment point for the kill switch circuit

Another point of failure is where the throttle cable attaches to the frame.  Miniplane recommends that the cable not be connected tightly so that it can move.  Unfortunately, rust and grime will spoil the connection between the throttle cable and electrical ground.  The result is that the kill switch will work intermittently.  It is a trade-off.  I suggest that pilots firmly tighten the nuts which connect the throttle cable to the frame.

Below is a Top 80 that had intermittent kill switch failure.  It was traced to the rust around the (properly) loose fitted cable connection.  Tightening the nut securely eliminated the problem.  Make sure that the cable is lined up properly if you tighten this fitting.  For the obsessive-compulsive: add a second thin nut to the end of the fitting and install an electrical ring connector with a wire to a fitting on the engine between the two nuts.  Then you can keep the throttle cable a tiny bit loose so that it can rotate. 

e. Damage to the cable and the enclosed wire

Miniplane does little/nothing to protect various parts of the paramotor from normal wear and tear.  This includes the throttle cable.

With normal use, the cable will rub on various parts of the frame.  If the cable has not been protected at these contact points by wrapping it with electrical tape or a piece of split hose, the kill switch wire can short to ground.  This will cause an engine shutdown at unexpected times.  In extreme cases, the cable itself can wear through to the inner wire and the throttle can jam or cease to function

Carefully examine the entire length of the cable for damage.  Apply tape or hose to those areas where the cable rubs any part of the frame.

C. How to fix the kill switch wire

1. Items needed

• 3/4" heat shrink tubing (HST) for newer models that have the new style kill switch. (available from Harbor Freight)
• #18 gauge hook-up wire (use ONLY quality hookup wire.)
• soldering kit or "red" butt connectors and a tool to crimp them  Note: the connectors from Harbor Freight are garbage.  Get good ones from Home Depot or Lowes.
• right angle pick tool (available from Harbor Freight)

To easily tell what is wrong, separate the connector where the wire enters the throttle cable at the carburetor end.  With the engine running at idle, touch the connector that goes into the ignition coil to a ground point on the engine.  If the engine stops, you know your kill switch circuit (what is in the throttle cable) has a problem.  You can use a short test lead to do this.  At this point, you must disassemble the cable.  It requires quite a few tools and some quality #18 wire (available from auto parts stores).  If you not know how to solder, you will need some "red" butt connectors and a crimping tool (available from Harbor Freight).  Remember: you must use quality wire.  The throttle cable shakes and flexes constantly when in use and cheap wire will fail quickly.  Quality wire is made up of many very thin wires instead of a few thick wires and why quality wire costs substantially more.

Because there may be multiple breaks, I recommend that the entire wire be replaced.

If you do not replace the entire wire, carefully check the full length of the wire for breaks.  Where there is a break, the wire will easily bend sharply.

2. How to fix

Remove the shrink wrap from the connectors and separate the ends (see the photo below).  This is where the kill switch wire goes into the wire loom covering the throttle cable.  If you are handy with a digital voltmeter you can find out if you have a break in the kill switch wiring or if it is a problem with the button switch.  The wire going into the cable should connect to ground when the kill switch button is pressed.

In order to repair/replace the wire you will have to remove the compression ring that holds the wire loom to the end of the Bowden cable fittings.  In order to repair/replace the wire, remove the ring at the carburetor end.  The rings can be re-used if you are careful.  If they do break, you can go to a bike shop and have them apply new ones.

You can insert a right angle pick (available from Harbor Freight) into the small hole where the ring is compressed.  Using a pair of pliers, you can force the pick into the hole and it will spread the compression area just enough to remove it.  Be sure to force the end of the pick into both sides of the ring.  To re-affix it, just squeeze the ring tight with a pair of wire cutters.  Do not squeeze too hard or you will cut right through the ring material.  You can re-use the compression ring repeatedly if you are very careful not to stretch/squeeze it excessively.  Alternatively, you can purchase the special tool which clamps down the ring but this is the only way I know to release a ring for re-use.

The small piece of vinyl tubing that holds the other wire from the switch to the fitting on the Bowden cable is visible in this photo.  The other wire from the kill switch must be firmly connected here or the kill switch will not function.

The loom can now be pulled back and the kill switch wire removed from the throttle cable assembly and examined.  Breaks can often be manually felt in the wire.  The best way is to connect a digital voltmeter (in resistance mode) to each end of the wire and measure the resistance.  It should be less than a few Ohms.  Now, stretch the wire.  If the resistance goes to infinity, it is obvious that the wire is defective and must be replaced or repaired.  I do not recommend a repair because there may be other areas of the wire that are ABOUT to fail and fixing just one area will be a waste of time.

However, we got ambitious with this wire (below) and repaired it anyway.  So far, the repairs were sufficient and there were not any additional defects in the wire.  The pilot and I stripped the bad ends back a bit, slipped on a piece of shrink-wrap, soldered the ends together, and then slid the shrink wrap over the joint and applied hot air from a heat gun.

The completed repair.

Use a digital voltmeter to check the kill switch circuit after you have made any repairs.  Reassemble everything, including the shrink-wrap which holds the kill switch in place on newer models.  The shrink-wrap, however, is optional and is merely cosmetic.