The International Cessna^® 170 Association

I've got an issue that I am sure is the ignition switch. I'm sure that the majority of you have pull starters but I have a 0300D in mine with the key start. It appears that while in the start position, it must be shorting out the mags because it won't fire until I stop cranking. Hand props right off, first piston over the top. I haven't tried disconnecting the p-leads yet, or cranking it with the plugs out and checking for spark but seems to me the switch has to be the culprit because you can crank away and as soon as you release from start position, it fires right off. This will be the third switch that I've replaced in the 18 years that I've had it for the same reason. Anyone hear of this problem or had it happen? Seems like a $225.00 aviation approved switch would be a little more dependable than an auto store special for the 'ol Ford or Chevy.

Joe

Try disconnecting the battery and checking if you have continuity from the p-lead to ground with the switch in the "start" position. If so you have a switch problem. If you decide to replace the switch you might try an ACS switch, only $89.00 from Spruce.
http://www.aircraftspruce.com/catalog/e ... itches.php
This switch came with my B&C starter conversion and been working great for 9 years. They also sell a rebuild kit for this switch for $15.

Thanks Karl, I'll check it out.

Joe

Also, be aware that some model/installation key-start switches ground out (kill) one magneto while allowing the other magneto to be "hot" during start. This arrangement (most common with "retard breaker" or "shower of sparks" systems, but is sometimes used when only one magneto has an impulse coupling installed) may cause the sort of failure which you are describing. Be certain to install your switch according to non-grounding installation instructions. The many terminals on the back of the switch must be installed according to the individual aircraft mfr's drawings...which may not be how your mechanic thinks it should be installed.

There is ample opportunity to install switches incorrectly as you can see the terminals on the back of some switches:

Terminal #1 is only used for retard-breaker starting systems and many installation instructions can get confusing.
Example:"Terminal 1 must have a jumper installed when left magneto has impulse coupling or retard breaker.
Connect wire from RIGHT MAG to terminal 1 of drawing.
Connect wire from LEFT MAG to terminal 2 of drawing.
Connect wire from STARTER SOLENOID to terminal 3 of drawing.
Connect wire from Positive terminal of power supply to terminal 4 of drawing. Connect wire from terminal 5 of drawing to nearest structural member."
The problem is, some mechanics misunderstand those instructions, expecially when BOTH magnetos have impluse couplings like our airplanes should have. (They see the left mag with an impulse coupling and then incorrectly make the installation according to instructions which ground out the right mag during start.)

Joe, are you certain you have a BENDIX switch? ACS issued some defective switches and there is a $15 Service-Kit (A-3650-2) to cure it. The description states: "Consists of replacement internal switch contacts, lubricant, diode assembly, and terminal board as required for compliance with ACS Ignition Switch Service Bulletin SB92-01. Kit A-3650-2 contains diode assembly for starter solenoids with one or two coil contacts.

(JUMPER USED WHEN LEFT MAG HAS IMPULSE COUPLING OR IS RETARD BREAKER MAG)

Joe, contact Jim Wildharber (Spelling) in Tx. Jim had problems and had to put a ^^&& (deal that lets electricity go one direction). He was having problems killing the battery and shorting instruments connected to his buss bar. I put a push button in mine and have had no trouble in lieu of the mag switch w/start. Since it's been working something must be sticking in the switch? Ron

Sounds like Jim may have had a switch installed incorrectly. If so, adding a homemade solution with a "diode" is probably not the best way to correct it because it would require some fancy paperwork.

Notice how this problem seems to arrive along with the installation of "key-start" systems?

/the terminals are anchored to the body of the switch by rivets which , in time or due to poor construction, can become loose and rotate and come in contact with the adjacent terminal.

sphillips wrote:/the terminals are anchored to the body of the switch by rivets which , in time or due to poor construction, can become loose and rotate and come in contact with the adjacent terminal.

(nice website at Teton!)

Good point. Those terminals are riveted onto a phenolic board which is attached to the body. Overtightening those terminal connections (such as when installing the wire terminals) can loosen them and allow them to move/subsequently fail.

Thanks all for the info. I'll check it out.

Joe