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06-26-2018, 08:03 AM
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Join Date: Aug 2014
Location: Bowie MD
Posts: 886
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Quote:
Originally Posted by Toobuilder
Great it appears to work now, but can anyone explain why? If the primary symptom is the inability to crank over the first compression stroke, then how does an intermittent contact on the key switch do that? The key activates the starter solenoid, and the starter solenoid sends the big power to the starter. the starter solenoid is either on or off. Bad contacts in the key switch will not alter the amount of power running through the main plunger contact in the starter solenoid. The only way I can see this happening is if the solenoid makes/breaks contact because of the poor signal from the key. But this would manifest as a buzz or chatter of the solenoid, which would be obvious to the pilot.
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Yes of course bad contacts will cause this. Its all about resistance. Chatter only comes about if the solenoid has just enough to engage, but not enough to hold. That power for the solenoid comes from and through the key switch. Its a narrow case of poor contact. In all cases, the contact points in the key switch get burned and pitted by repeated arcing. This really doesnt matter - you discovered the problem, replace the switch and fly on. Maybe take it apart there after and then you'll understand the failure modes and how to deal with them or alleviate them.
__________________
Mani
Busby MustangII (FoldingWing) Pending DAR.
Don't be a hater; I'm a cousin with thin wings! 
N251Y (res)
Last edited by maniago : 06-26-2018 at 08:06 AM.
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06-26-2018, 08:31 AM
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Join Date: Jul 2009
Location: Mojave
Posts: 4,652
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Are you suggesting that the ability of the solenoid to pass hundreds of amps through the massive contact points are determined by the resistance in the small solenoid plunger coil circuit? How is this possible? The two circuits are electrically isolated.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
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Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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06-26-2018, 08:33 AM
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Join Date: Aug 2017
Location: Zeeland, MI
Posts: 25
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My Key-Switch Theory
Quote:
Originally Posted by Toobuilder
The only way I can see this happening is if the solenoid makes/breaks contact because of the poor signal from the key. But this would manifest as a buzz or chatter of the solenoid, which would be obvious to the pilot.
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Michael,
This may be a bit premature, but I now believe that my key switch is breaking the 12V circuit to the starter contactor, when it is turned to the extreme right. During the transition to this position, my prop starts moving, because the starter contactor is energized.
Then, it's de-energized due to the faulty switch. I never heard starter contactor chatter during my starting attempts.
Previous to last night, it would take 3-4 attempts at turning the key to get the prop past the compression stroke. My theory is, once past the compression stroke, the prop finally had enough spin momentum to start the engine, despite the key eventually breaking the starter contactor circuit.
BTW, My key-switch is ACS P/N 11-03170 (A-510-2 FAA-PMA Approved).
The key assembly was installed by the builder 12 years ago.
Dale
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Dale DeWeese
RV-9A Owner
EAA Chapter 211 Member
Zeeland, MI (Z98)
VAF Dues Paid
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06-26-2018, 08:43 AM
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Join Date: Aug 2014
Location: Bowie MD
Posts: 886
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Quote:
Originally Posted by Toobuilder
Are you suggesting that the ability of the solenoid to pass hundreds of amps through the massive contact points are determined by the resistance in the small solenoid plunger coil circuit? How is this possible? The two circuits are electrically isolated.
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Yes. Despite being isolated, correct function of the first, produces a correct function of the second. Its a mechanical relationship, more than likely brought about by an electrical "wear" relationship wrt the contacts in the key switch.
IOW, what Dale said above ought to make sense if you consider that the switch worked correctly for a while, but then started "acting up". This kinda stuff happens all the time with sliding contacts - they get burned and oxidized at the edges. And I suspect youre in an edge condition.
Eitherway, if you can make it function as Dale describes, that ought to clue you that its the switch internals themselves, and again, take apart your switch and you'll discover exactly whats going on. Noone else can say for sure with your particular switch, just speculate.
__________________
Mani
Busby MustangII (FoldingWing) Pending DAR.
Don't be a hater; I'm a cousin with thin wings! 
N251Y (res)
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06-26-2018, 08:44 AM
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Join Date: Jul 2009
Location: Mojave
Posts: 4,652
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Quote:
Originally Posted by Dale D
Michael,
This may be a bit premature, but I now believe that my key switch is breaking the 12V circuit to the starter contactor, when it is turned to the extreme right...
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So that should be very easy to see. Previously, when the blade stopped against the compression stroke, was it de energized, or was it still engaged but "slow"?
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
______________
Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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06-26-2018, 09:09 AM
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Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
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None of us can see/hear what you do. But low starter voltage will present somewhat different symptoms from a bad switch. If it's the switch causing the problem, the start contactor will either be engaged, or not engaged, or chattering. It *won't* cause a low voltage condition at the starter. So there will either be full start voltage, no start voltage, or pulsed voltage. Pulsed voltage caused by a chattering switch isn't likely to effectively move the prop.
If the switch is making and then breaking, the engine will likely still come up against a compression stroke; once moving, 'flywheel' effect will try to keep it moving. But you'd likely see the prop recoil slightly after hitting the 'spring' of compression. You can see the same effect if the battery is actually too low to push a blade past compression; the prop recoils when you release the start switch.
Low voltage at the starter would likely drive the prop against the compression, and hold it there (until the switch is released; see above).
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06-26-2018, 09:26 AM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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Quote:
Originally Posted by rv7charlie
It *won't* cause a low voltage condition at the starter. So there will either be full start voltage, no start voltage, or pulsed voltage.
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Unless the high current contacts in the start contractor have deteriorated.
This is one of the failure modes of poor starter performance and is why the starter troubleshooting guide walks you through measuring voltage drop across all connections and components while passing a lot of Amps (while cranking).
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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06-26-2018, 09:44 AM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,596
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Quote:
Originally Posted by Toobuilder
Are you suggesting that the ability of the solenoid to pass hundreds of amps through the massive contact points are determined by the resistance in the small solenoid plunger coil circuit? How is this possible? The two circuits are electrically isolated.
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Yes.
Keep in mind the voltage sag when the starter engages. That same voltage sag will be present on the starter solenoid. If your start switch has bad contacts, the voltage getting to the starter solenoid my be adequate to hold it shut with no voltage sag, but once the starter drops buss voltage, the starter solenoid drops out from under voltage.
I hate Cessna style legacy key igntion switches. I alway use toggle switches for the ignitions and a push button for the stater solenoid.
Carl
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06-26-2018, 11:11 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by Toobuilder
Great it appears to work now, but can anyone explain why? If the primary symptom is the inability to crank over the first compression stroke, then how does an intermittent contact on the key switch do that? The key activates the starter solenoid, and the starter solenoid sends the big power to the starter. the starter solenoid is either on or off. Bad contacts in the key switch will not alter the amount of power running through the main plunger contact in the starter solenoid. The only way I can see this happening is if the solenoid makes/breaks contact because of the poor signal from the key. But this would manifest as a buzz or chatter of the solenoid, which would be obvious to the pilot.
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As I mentioned in my post, I suspect that high resistance in the coil activation circuit will prevent enough current flow to fully extend the solenoid, but enough current to keep it from chattering. This could create a partial contact of the two plates and greatly reduce current flowing to the starter. It's a theory, but makes sense. If I recall correctly, the moving plate is not rigidly attached to the plunger and slight tipping of the plate without full pressure from the solenoid could create a partial contact. If the solenoid was actually dropping out, I struggle to see how the starter could have enough energy to overcome the initial resistance from compression.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 06-26-2018 at 11:19 AM.
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06-26-2018, 11:19 AM
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Join Date: Jul 2009
Location: Mojave
Posts: 4,652
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Quote:
Originally Posted by Carl Froehlich
If your start switch has bad contacts, the voltage getting to the starter solenoid my be adequate to hold it shut with no voltage sag, but once the starter drops buss voltage, the starter solenoid drops out from under voltage.
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I get that completely. But the starter solenoid "dropping out" is a discrete event. The OP indicated (to me) that the starter was slow to get over the compression stroke.
If it tried to climb the compression stroke and gave up (solenoid dropped out), that's completely different than really slow to climb the compression stroke (low amperage at the starter), and you drop the key to try again.
That's the trouble with diagnosis by internet - Words have very specific meanings to different people.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.
Michael Robinson
______________
Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
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