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Erratic Ammeter Readings

Doug Rohrer

Well Known Member
I have a -9A with an O-320. Recently I have noticed erratic negative amp readings on the G3X screen. Negative readings seem impossible because the shunt is in line with the alternator output lead, not the battery output lead. The alternator is a B&C unit with the B&C voltage regulator. The output is normal after starting and takeoff. It starts to jump around between -1 and -59 amps rapidly when the engine is backed down below 2100 RPM. The voltage is rock solid at 14.3 and the amber alternator failure light does not come on, so I am confident it is a sensor/display error rather than a real negative output. If the problem was a loose shunt connection, I would think the readings would bounce between normal and zero. I plan to call the Garmin guys, but I thought I would check with the forum first to see if anyone else has experienced this problem.
 
If the problem was a loose shunt connection, I would think the readings would bounce between normal and zero. I plan to call the Garmin guys, but I thought I would check with the forum first to see if anyone else has experienced this problem.

This would be my expectation as well. Might want to check that you don't have fraying of the sense wire insulation, allowing a path to ground. Beyond that, I would be guessing the issue is in the G3X. Might be wise to put an ohmmeter between the shunt and frame ground to insure the shunt's insulation isn't failing somehow.

While a problem in the alt can create a short and drain power from the buss, I don't think that the voltage would remain at 14.3 in that case.

Larry
 
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You checked the sense wire connections at the shunt, did you check them in the connector into the G3X? Might be a loose of intermittent pin connection.
 
I had a very similar condition on my Dynon ammeter. Current going from normal to 60 amps and back to normal. It ended up being the crimp on the sense connection at the shunt. Over-crimping and broken strands of the ring connector/24 gage wire led to erratic readings.
 
Excess resistance wil result in inaccurate readings, but always the same polarity(i.e. charge or discharge) or zero (no connection). The op is seeing a change in polarity which cannot happen with excess resistance or loose connection.

Larry
 
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Had similiar swing in amps and solid voltage on a Dynon set up. Found that the signal wire that attaches to the shunt had bad crimp connection. Actually the ring crimps were to large for the wire size and were not crimped sufficiently. Although it Worked for 700 hrs before it showed up!. It actually got a new alternator in the trouble shooting exercise.
 
Larry nailed it. I don't see how a frayed wire or bad crimp could cause a polarity reversal on the G3X screen. I will be flying again tomorrow and will do some more in-flight troubleshooting. If no new clues are found, I will be calling the G3X Experts. They have been very helpful and responsive in the past. BTW, the aircraft has 120 hours on it and is coming up on three years old.
 
Well, now that the G3X screen knows I am looking closely at the amp readout, it is behaving normally. Amps stayed positive and in the expected range during a two hour flight. Nothing's easy!
 
Electrical

I would check every connection between the battery and the main Buss including grounds. Have seen weird behavior caused by loose or intermittent connections more than once. Agree that a negative indication and the voltage staying at 14.3 sounds impossible.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
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