On Sun, 23 Sep 2007 13:31:10 -0700, Jay Honeck
wrote:
I still haven't had time to work with my A&P to remove my (newly)
flakey JPI EDM-700 unit. (As you may recall, some of the LED segments
that make up the number display are going in and out.)
Someone here suggested that it might be due to low voltage, possibly
indicating an alternator problem. I've started watching the voltage,
and have noticed something odd:
The JPI has its own built-in voltage indicator. It will usually read
13.4 or 13.5 volts. We also have a stand-alone, panel-mounted digital
voltage/amperage meter, mounted a few inches below the JPI. It
usually reads a few tenths of a volt different than the JPI. (I can't
remember if it's up or down...)
a) Is this normal? Why would they read different voltages?
b) What voltage is normal? Is 13.4 too low?
Thanks!
I have found that the JPI is very sensitive to voltage. I have a
persistent voltage drop in my system which is exacerbated by putting the
landing gear up or down. This causes a large current draw by the gear
pump motor, which due to resistance somewhere, causes a voltage spike
downwards. My JPI reboots as a result of this, and is the only actual
symptom. Other avionics are unaffected, so this problem is not severe,
but it does bug me.
This effect has been noted by many members of Cardinal Flyers Online
over the years, as I found when I was researching it.
I used to have an inflight voltage of under 13.0 when all electricals
were on, or maybe 13.3 with the "usual" load. I have had some work done
on my electrical system, including replacing old components and wiring,
which cleared the problem up for a time. Now my voltage is a steady
13.7, but I think this is still somewhat low, and the JPI reset problem
is starting to occur again.
In principle this is not hard to troubleshoot. With the engine running
and a full electrical load, simply measure the voltage starting at the
alternator and moving back to the avionics buss and battery, at all
possible points. At the alternator it should be over 14.0 I think. This
should isolate where there is a voltage drop, and therefore too much
resistance.
So I think 13.4 is too low, at least it is in my plane. I would like to
see 14.0. And you should determine exactly where the JPI and your panel
voltmeter are connected, because a few tenths of a volt difference is
certainly pointing to excess resistance between them.
randall g =%^) PPASEL+Night 1974 Cardinal RG
http://www.telemark.net/randallg
Lots of aerial photographs of British Columbia at:
http://www.telemark.net/randallg/photos.htm
Vancouver's famous Kat Kam:
http://www.katkam.ca