 |
Matronics Email Lists
Web Forum Interface to the Matronics Email Lists
|
| View previous topic :: View next topic |
| Author |
Message |
dlj04(at)josephson.com
Guest
|
 Posted: Mon Mar 24, 2014 9:38 am Post subject: P-leads |
 |
|
The concept of a "fuse" to keep faults in a tachometer from disabling an
ignition needs to be stamped out, it is a dangerous misconception. I
know of at least one incident where this almost caused a fatal crash. It
takes very little current through a P lead from a magneto or points wire
in a Kettering ignition to disrupt performance of the ignition. Can you
depend on this amount of current blowing a fuse? Is the resistor going
to open if there is a fault? Is the engine going to keep running well if
the far end of the resistor is shorted to ground or +12? Not unless it
has been tested to do so.
There are many tach designs that work without an ignition pickup. VDO
and other companies make programmable tachs that can be set to take a
signal from one phase of an alternator output, or an inductive pickup
from a flywheel or magneto case. Proper design of a tach could also
yield failsafe operation but that's still putting a lot of faith in
something you haven't tested. The "proof by assertion" that I've seen
from a few instrument vendors isn't really confidence-inspiring.
--
David Josephson
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
| Back to top |
|
 |
kleh(at)dialupatcost.ca
Guest
|
| Posted: Mon Mar 24, 2014 11:25 am Post subject: P-leads |
|
|
Yes "Fuse" is not the terminology I'd use for such a resistor.
Agree that the resistor definitely needs to be large enough value that
it won't interfere if grounded and testing it should not be overlooked.
Where I've seen this go bad was when a wire was run to the resistor
mounted in the tach. With age and moisture the wire insulation started
breaking down. The resistor should be physically at the ignition pick
off point, NOT remotely located. I apply the same concept to picking off
a fuel flow signal from an electronic fuel injector.
Ken
On 24/03/2014 1:37 PM, D L Josephson wrote:
| Quote: |
<dlj04(at)josephson.com>
The concept of a "fuse" to keep faults in a tachometer from disabling an
ignition needs to be stamped out, it is a dangerous misconception. I
know of at least one incident where this almost caused a fatal crash. It
takes very little current through a P lead from a magneto or points wire
in a Kettering ignition to disrupt performance of the ignition. Can you
depend on this amount of current blowing a fuse? Is the resistor going
to open if there is a fault? Is the engine going to keep running well if
the far end of the resistor is shorted to ground or +12? Not unless it
has been tested to do so.
There are many tach designs that work without an ignition pickup. VDO
and other companies make programmable tachs that can be set to take a
signal from one phase of an alternator output, or an inductive pickup
from a flywheel or magneto case. Proper design of a tach could also
yield failsafe operation but that's still putting a lot of faith in
something you haven't tested. The "proof by assertion" that I've seen
from a few instrument vendors isn't really confidence-inspiring.
--
David Josephson
|
| | - The Matronics AeroElectric-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?AeroElectric-List |
|
|
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
