[PCHunt]

I recently re-built my "fat wire" electrical system. Removed the old battery and contactors that were behind the firewall, and installed the new system fwd of the firewall, similar to modern RV design. I used a Van's ammeter (+/- 40 Amps), but wired it to act as a loadmeter, i.e.: only showing positive amps being produced from the alternator.

Everything worked as advertised on my first flight after the modification, with the following "weird" exception:

Each time I key the radio mike to transmit, the ammeter goes immediately Full Scale. And even "weirder", if the ammeter is reading very low amps, say +3 amps, it goes to -40 amps (full scale) rapidly. But if I turn on the nav lights, the panel lights, and the landing lights to create a load (+15 amps), then the ammeter goes full scale to + 40 amps when the mike is keyed.

I'm stumped! Any ideas? Every other gage in the cockpit works fine.

The needle indication mentioned above are immediate. A very sudden needle movement to either + or - 40 amps concurrent with keying the radio mike. Can't be good for the gage

 

[agough277]

It seems there is RF from your radio causing the needle movement. The ammeter is a sensitive device, requiring little current for deflection. Move the meter farther away from the radio or shield it better. I believe the case for the ammeter is plastic thus not a very good RF shield.

 

[647jc]

Also be sure and use a twisted pair of wires from the shunt to the meter. This will help cancel out any induced EMF.

 

[PCHunt]

Thanks, I'll try the twisted pair idea first, easier!

 

[Doug]

First off install the twisted pair.

What radio do you have? If you have a Garmin SL30 or 40 has it had the recent AD upgrade? I don't know but I suspect that these radios have radiated or conducted emission issues that may be the subject of the recent AD. Also, check the antenna is properly and soundly grounded.

Irrespective of the above, IMO the gauge should work properly! In another thread there has been discussion about the failure of Vans gauges and looking at the photos of the insides - it appears there is nill RF/HF bypassing on the circuit board.

May I suggest that you try installing a bypass capacitor ( 100nF Ceramic 50v - they are small in size and usually specifically designated for the purpose, but any ceramic type should be OK) directly across the gauge's supply pins. A second one across sensor input may also help.

Ideally the bypassing should be on the circuit board itself, but I am not in a position to know where to place them.

Whatever you do try - do one thing at a time so you can report what actually fixes the problem.

Doug Gray

 

[PCHunt]

Plan of attack

First off install the twisted pair. That will be step one.

What radio do you have? Honeywell/Bendix/King KY 97
Also, check the antenna is properly and soundly grounded. It is.

Irrespective of the above, IMO the gauge should work properly! In another thread there has been discussion about the failure of Vans gauges and looking at the photos of the insides - it appears there is nil RF/HF bypassing on the circuit board. I have seen those threads and photos.

May I suggest that you try installing a bypass capacitor ( 100nF Ceramic 50v - they are small in size and usually specifically designated for the purpose, but any ceramic type should be OK) directly across the gauge's supply pins. A second one across sensor input may also help. This will be steps 2 and 3.

Whatever you do try - do one thing at a time so you can report what actually fixes the problem. Good idea. I tend to take the shotgun approach!

Should know in a few days, say by Wed. BTW: all other newly installed Vans gages are working fine.

 

[PCHunt]

Problem appears to be solved!

Feedback: Today I twisted the wires from the shunt to the ammeter, and on a test inside the hangar, it appears to have solved the problem.

I will test it again tomorrow with engine running, etc.

Thanks to all for your inputs............. Love this forum!!