Bob wrote:
I'm looking for some advice on a voltmeter (preferably digital) that
will plug into the "cigarette lighter" of my airplane. I found one on
the J. C. Whitney site, but $50 is too much for what I want to use it for.

I've got a JPI analyzer which reads voltages off the avionics bus, and
also a Davtron clock which does the same. Curiously, they differ, with
the JPI reading 0.3 to 0.5 volts lower! Furthermore, the voltage on the
avionics bus seems to be less than I'd expect, anywhere from 12.2 to
13.0 (instead of the 13.2 I'd expect). Further-furthermore, the voltage
drops during flight (I guess as the voltage regulator cuts off the
alternator as the battery is recharged after starting) to the point
where I get low-voltage warnings from the JPI.

No other symptoms of electrical problems. Been doing this for many
months (since I had the JPI installed) but still cranks fine.

Anybody know of a reasonably priced gadget that will let me monitor the
MAIN bus feeding the cigarette lighter?

Rich

The problem is you need accuracy and cheap and accuracy don't go together.

I would be surprised if the accuracy of the existing voltmeters are much
better than about 2%, which would easily account for the difference.

Multimeters in the $50 to $100 range have a typical accuracy of 1% for
DC. That translates to +/- .13V at 13V, which gets you close, but is
a bit much to pay for a one-shot measurement (unless you have other use
for the multimeter).

The cheapest option would be to find someone that already has a decent
meter (HAM, repair guy, etc.) that will let you borrow his so you can
make a calibration chart for the meters in the A/C.

As to the voltage, it does sound a little low.

One check you can make is to note the voltage with everything turned
on at runup RPM. Then turn everything you can off and note the voltage
again. There should be no change.

If there is a change, I would check, clean and tighten the connection(s)
from the battery to the avionics bus.

While you're doing that, I would also check to see if the JPI and Davtron
have their power and grounds connected to the same things.

Some installations also have a ground bus where all the avionics grounds
come together with one heavy connection to the A/C ground. If you have
one of those, check, clean and tighten the connection between the bus
and the A/C ground.

Then run the check again. If it still changes you probably need a trip
to the avionics shop to fix it unless you happen to be good at this
sort of thing.

--
Jim Pennino

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