Aerospace Logic Volt/Ammeter

Intermittent problem with an Aerospace Logic VA-100 Volt & Amp
indicator.
Volt side works at all times and is accurate, amp side works properly
occasionally but more often than not displays -0.0 amps. When it does
work it indicates charges and discharges properly.

Unit supposedly shipped with a 60A/50mV shunt. Aircraft has a 60A
alternator. Unit measures mV change across shunt.

Red side of shunt from battery / Black side to buss
Twisted pair from indicator, red to red side of shunt, black to black
side of shunt.
Black wire from indicator to ground
White wire from indicator to rheostat for back lighting
Blue wire for 28v system not used.

Instrument set up for 60A, high and low limits within the proper
range.

When amps fail to indicate, battery only will not show a discharge and
engine running w/alternator output will not show a charge. Both cases
indicate -0.0. No change with a change in load, it will continue to
indicate -0.0.

Any experience or thoughts?

Thanks,
Jim

In rec.aviation.owning wrote:
Intermittent problem with an Aerospace Logic VA-100 Volt & Amp
indicator.
Volt side works at all times and is accurate, amp side works properly
occasionally but more often than not displays -0.0 amps. When it does
work it indicates charges and discharges properly.

I'd guess an open in one of the twisted pair wires to the shunt. With
the given wiring, it has to be reading the system voltage between one
of the twisted pair wires and ground, so both wires in the pair probably
aren't open.

If the shunt itself was intermittently open, you'd probably see the
displayed voltage rise quite a bit (alternator working without the load
of the battery) or fall quite a bit (battery not being charged by the
alternator) depending on which side of the shunt the voltage is read
from. Shunts are usually pretty stout mechanically, anyway.

For diagnostic purposes, you could disconnect the twisted pair at the
indicator and connect the pair to a multimeter. If the multimeter
sometimes indicates 0 mV even when you know there should be a charge or
discharge, check the twisted pair wire and the shunt. If the multimeter
always indicates correctly, then there may be a problem inside the
VA-100 indicator.

Matt Roberds

Disclaimer: This is based on experience with ground vehicles. I don't
have an A&P; I don't even have a TG&Y. Some of this may not be
allowable owner maintenance. Your mileage may vary.

In rec.aviation.owning wrote:
In rec.aviation.owning wrote:
Intermittent problem with an Aerospace Logic VA-100 Volt & Amp
indicator.
Volt side works at all times and is accurate, amp side works properly
occasionally but more often than not displays -0.0 amps. When it does
work it indicates charges and discharges properly.

I'd guess an open in one of the twisted pair wires to the shunt. With
the given wiring, it has to be reading the system voltage between one
of the twisted pair wires and ground, so both wires in the pair probably
aren't open.

I finally got Aerospace Logic's Web site to respond and the manual they
have there for the VA-100 shows that it does have a separate power wire -
it isn't powered through one of the shunt wires. What this means to you
is that either or both of the wires in the twisted pair could be open,
not just one as I said above.

Matt Roberds

Thanks Matt. I tried to post a link to the site, but it was down.
Sorry I failed to mention the red power wire.

There's a note in the install manual that says if you need to shorten
the twisted pair, to cut the pair coming from the shunt, do not cut
the pair that is attached to the indicator. We didn't cut either, but
what difference would it make?

It also mentions routing the twisted pair away from other wire bundles
"if possible". Well, in this install, the indicator is on the
opposite side of the panel from the shunt, and unless we leave it
hanging, the twisted pair has to be routed along the other wire
bundles. There is simply no room or other route available. I would
think that interference problems would be consistent and repeatable.
This appears totally random and either -0.0 or accurate.

Thanks again.
Jim

In rec.aviation.owning Jim Burns wrote:
There's a note in the install manual that says if you need to shorten
the twisted pair, to cut the pair coming from the shunt, do not cut
the pair that is attached to the indicator. We didn't cut either, but
what difference would it make?

I don't know for sure. Possibly the connector used at the indicator end
is harder to take apart and reassemble than the connectors used at the
shunt end. Or maybe they prefer for the wires to be untwisted or more
loosely twisted at the shunt end, rather than the indicator end.

It also mentions routing the twisted pair away from other wire bundles
"if possible".

Mainly this is to prevent other wires from inducing a current into the
twisted pair. The signal in the twisted pair isn't very large; at 60 A
charge or discharge it will only be 50 millivolts (0.05 V) at maybe a
milliamp or two. This is comparable to microphone level audio, and you
might be familiar with how easily that picks up electrical noise.

I would think that interference problems would be consistent and
repeatable.

Probably true. The catch is that to create interference, you'd need to
turn a big electrical load on or off, which will change the ammeter
display anyway even if things are working right.

One way to check for interference would be to disconnect the twisted
pair wires at the shunt and connect them directly to each other and to
nothing else. In this case the ammeter should always read 0.0 A; any
other reading indicates interference.

This appears totally random and either -0.0 or accurate.

I agree that this isn't consistent with interference; it's more like a
broken wire or loose connector that's either making contact or not.

Matt Roberds

Disclaimer: This is based on experience with ground vehicles. I don't
have an A&P; I don't even have a TG&Y. Some of this may not be
allowable owner maintenance. Your mileage may vary.