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Amp meter Draw

Robb

Well Known Member
My Rv7 always shows about 7 amps when I key the mic on my Garmin 430. I also see my manifold gauge drop a lot at the same time. It has always done this but does not seem to be a safety issue. I am going to have it looked at when i have the ads b out installed just wondering if anyone has an idea of what's up


Dues paid gladly
 
1 - What kind of Ammeter? EFIS or Analog
2 - What kind of current sensor? Shunt or Hall Effect.

:cool:
 
The fact that an unrelated gauge is affected would indicate poor connections somewhere in the system, likely grounds, or ground path issues (ground loops).

What grounding philosophy did you use when wiring the plane?
 
Not really sure how it was grounded as I did not build it. The gauges are all analog
 
The fact that an unrelated gauge is affected would indicate poor connections somewhere in the system, likely grounds, or ground path issues (ground loops).

I don't think so. More likely he is getting radio frequency interference, with one or more wires acting as an antenna. Can be hard to chase this down. On my -7A I get a fuel pressure warning on the EFIS whenever I key the mic.

Erich
 
The fact that an unrelated gauge is affected would indicate poor connections somewhere in the system, likely grounds, or ground path issues (ground loops).

What grounding philosophy did you use when wiring the plane?
This is the highest probability and the hardest to hunt down. Start with the radio antenna cables, connections and attachment to the fuselage. Check for broken, loose wires or corrosion somewhere. After that check all wires (especially the ground wires) starting with the affected gauges.

Good luck. These kind of gremlins are a PITA to track down.

:cool:
 
This is the highest probability and the hardest to hunt down. Start with the radio antenna cables, connections and attachment to the fuselage. Check for broken, loose wires or corrosion somewhere. After that check all wires (especially the ground wires) starting with the affected gauges.

Good luck. These kind of gremlins are a PITA to track down.

:cool:

Another potential cause is leaky coax routed beside un or poorly shielded wire connected to an instrument.


RG400 coax is less likely to cause this than RG58.
 
Mine turned out to be "coiled" wiring

If you have extra wire coiled behind the panel, get rid of it by trimming the wiring, or laying out the wires in a "non-coiled" fashion. I was getting an ammeter spike when I keyed the mike.

Ron
 
Take a 4" square of aluminum foil, wrap it around a wire or a bundle of wires, and slide it along the wire(s) until the goosey readings stop. If you are using a bundle of wires, take them one by one and see which one kills the interference. Repeat, lather, repeat until you've checked every wire in the airplane.

At these frequencies, RG58 is every bit as good as RG400. That's the LAST resort, although a loose termination on the coax braid WILL cause the problem. The al foil will find this fault also.

Jim
 
Sounds like you have a monitor and or EFIS display of some kind, What is it?
My GRT EIS/EFIS had similar RF issues, only certain channels. Fixed with snap on ferrite cores on the fuel pressure sender wires (fuel pressure was the affected channel)
Tim Andres
 
Sounds like you have a monitor and or EFIS display of some kind, What is it?
The gauges are all analog
At these frequencies, RG58 is every bit as good as RG400. That's the LAST resort, although a loose termination on the coax braid WILL cause the problem.
EXACTLY! But go ahead and start with this first (loose termination on the coax braid or antenna ground) then check other wires/plugs for good contact and ground. Also, do you have more than one radio and if so does the problem happen with both?

:cool:
 
I'd look at the shunt first

The shunt for an analog ammeter only produces 50 mV full scale so a grounding problem there could easily cause false readings. I say "grounding", but the shunt may not be connected to ground. I think your first task is to determine how the shunt is connected to the electrical system.
 
The shunt for an analog ammeter only produces 50 mV full scale so a grounding problem there could easily cause false readings. I say "grounding", but the shunt may not be connected to ground. I think your first task is to determine how the shunt is connected to the electrical system.
Shunts are never connected to ground and neither are the two wires from the shunt to the meter. Grounding a shunt is a very bad thing and will blow something like the alternator. Also, the standard value for shunts in our aircraft electrical systems are 1mV per Amp.

:cool:
 
My Rv7 always shows about 7 amps when I key the mic on my Garmin 430. I also see my manifold gauge drop a lot at the same time. It has always done this but does not seem to be a safety issue. I am going to have it looked at when i have the ads b out installed just wondering if anyone has an idea of what's up


Dues paid gladly

The 430 has a separate transmit board. In my case it was a 28 volt setup and that portion of the unit draws 3 amps at 28 volts during transmit. That translates to 6 amps at 14 volts, so what you are seeing on the ammeter is expected.

Not sure why your MAP guage is moving. What kind of gauge. Many of the MAP sensors are actually transducers and send a variable voltage and not a resistive type like most of your other engine sensors. They have a power supply and should be immune to ground issues causing fluctuations. I would be looking more at RF noise impacting the signal. The transducers should have their 12v feed wire and return signal wire twisted together to help avoid noise issues induced onto the wire. The GRT EIS MAP sensor even ships with the wires twisted for you, presumably to help avoid service issues like you are seeing and the service calls they create.

Larry
 
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