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Tech Tips from Flightline Technical Services: Troubleshooting a Flaky Ammeter

Tech Tips from Flightline Technical Services: Troubleshooting a Flaky Ammeter

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Last month, we published a letter from Piper Flyer member Rich Schwartz. In addition to a stubborn prop vibration, he is having difficulty with the ammeter reading on his Arrow III.

Here is the second half of his letter:

Q: The ammeter [in my 1978 Piper Arrow III] consistently reads about one-half of a needle width off the zero line. The picture of the Seneca panel on page 45 of the October 2016 issue [of Piper Flyer] depicts this very well.

The indication is the same no matter what the load is, the needle will jump one needle width when a high-load item is first turned on (such as the fuel pump), then returns to zero.

The ammeter should work like a load meter and show the total load on the electrical system.

The shunt has been installed according to the Service Bulletin that came out. The original ammeter was replaced with a new unit from Air Parts of Lock Haven, and the results are the same.

This leads me to believe that the wrong shunt may have been installed and the ammeter is, in a sense, being bypassed.

I have used a multimeter to check the battery and alternator parameters, and all were normal.

I have my A&P [certificate], but do not work professionally in that capacity. I work exclusively on this airplane and a Stearman.

Any assistance you can offer would be most appreciated.

Thanks again,

Richard Schwartz

 

A: The original configuration of the ammeter in the PA-28R-201 was an internally-shunted meter, thereby all the power from the alternator went directly through the meter.

Piper decided that was not the best way to do things so they issued Service Bulletin 811A which changed the meter to an externally-shunted meter. This way all the power went through a shunt and the meter is hooked up in parallel to the shunt and actually reads minivolts.

The ammeter is calibrated to display amps.

 

I suspect that the connections at the shunt may be the issue, since you have already replaced the meter.

The shunt in this case is just a long piece of #6AWG copper cable.

I have several documents showing the before and after wiring, specs on testing the shunt and a copy of SB811A.

 

Hope this helps,

Tom Pentecost

Distribution Service
Administrator (DSA),

Flightline Technical Services

(a Piper Aircraft Dealer and Service Center)

The information noted in this exchange can be found on the forum at PiperFlyer.org under “Magazine Extras.” Members can download wiring diagrams and Service Bulletin SB811A there. —Ed.


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