JPI930 and indicated voltage

I just finished installing a JPI930 in a clients plane and when the system is active, the display shows 12.5V. The customer hooked up a volt meter to the battery and it showed 13.9V. He is insisting that the JPI should read the same. I explained to him that the reading on the JPI is from the "Bus" and the reading from the battery is directly from the alternator. I have installed three (3) of these units and have never had the JPI930 show the same voltage on the display as that coming from the alternator. The JPI is getting the voltage directly from the pick-up leads off the shunt. Is there anything else I can do and how do I explain to this pilot that the unit is displaying properly?
Thanks for the help.


  • He called JPI yesterday and didn't believe what they told him. The client then told me he went out to various Cessna 182 forums, and they told him his mechanic did not install it correctly. I really hate it when other pilots tell one another that the A&P is wrong! UGH!!

  • I installed a JPI930 in my 1964 C210 and had the same readings. I didn't think anything was wrong.

    Thanks for the help and advice.

  • Put a volt meter where the JPI is connected to system, that will determine whether the JPI is accurate or not.
    1.4V is a big difference.
  • I recently put a Plane Power voltage regulator on my 182P. Had an unrelated issue and sent an EDM-900 download of a flight to Savvy Aviator. They noticed the voltage readout was 13.6 and recommended adjusting the regulator to about 14.6 for better battery life. This thread brings into question using the reading of the battery voltage on the JPI screen to adjust the alternator output voltage. Should I use a voltmeter at the battery itself for this adjustment?
  • There are a few possibilities:
    The JPI is inaccurate, unlikely, but easy to check with a voltmeter, make sure you are measuring at the same place as the JPI.
    There is a bad connection and there is a 1.4V voltage drop across it, that would be BAD and dangerous.
    The JPI is connected in the wrong location, I cannot think of a location that would have that large voltage difference.
    The voltmeter is inaccurate, easy to check by using a different one.

    I would recommend figuring out the issue as a bad connection could cause a fire.

  • Sorry to FlightProf but I agree with your customer. Background, I am an Electrical Engineer. The JPI 930 installation manual requires the EDM930 Bus Wire be 20 gauge and connected to the Battery Contactor. This is for minimal voltage drop during engine start. After engine start, unless there is an issue with the bus wiring or the contactor itself, there should be very little voltage drop between the battery and the MAIN BUS or Alternator output.

    The wiring between the contactors and the main bus is going to be larger than 10 gauge, but I will use 10 gauge as an example. The resistance for a 10 ft length of 10 gauge copper wire is 0.01 ohms. Even at 50 amps, the voltage drop will only be 0.5 Volts.

    There is very little voltage drop across the ammeter shunt. Typically a few millivolts for digital meters. Even for analog meters it would be 50 milivolts for full scale.

    If you have that kind of voltage discrepancy, I would be looking at ground contacts and checking if there is a voltage drop across the contactor or between the contactor and the main bus.

  • An additional note to what I posted previously. The Bus voltage the EDM is reading is internal directly from the power wire to the EDM. The two wires at the external shunt are for measuring the voltage drop across the shunt which is displayed as current. It is not measuring the Bus voltage. I meant to add the following.

  • My JPI-900 is readzing fairly consistently 13.8v. Should I be concerned?
  • Scott ShererScott Sherer COO Forum Moderator

    Right on the money! Everything is good :) You're fine.

    Scott Sherer
    Wright Brothers Master Pilot, FAA Commercial Pilot
    Aviation Director, Cessna Owners Organization Forum Moderator and Cessna Owners Author.

    Need help? Let me know!

  • Thanks Scott.
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