Voltage difference

[Oscar Avalle]

After my two very long flights some squawks appeared that I am somehow puzzled... May be somebody could shed some light on what is going on.

I have a JPI 830 and a EI digital voltage gauge. The voltage on the EI indicated 12.5 volts while on the JPI voltage was down to 10.4 before turning on the alternator. Once the alternator was on voltage on the EI went up to 14.2 while the JPI staid at 10.4 (went even down to 9.9) and only after a while it went up again to 12.5 and it staid there. Once the alternator was off, it went down to 10.5 and EI staid at 12.4. Any idea what is going on????

[carusoam]

Sounds like...

1) voltage is being generated and properly controlled. Voltage around 10V would show many signs of everything not working properly.

2) JPI is probably not grounding properly. Resistance is high between the ground wire and plane.

3) What is the range of operating voltage of the JPI? Probably pretty wide range compared to other devices.  Allowing it to work when other devices would normally not.

4) check the power and ground lines to the JPI.  Looking for loose or broken wires.

Ideas that come to mind.  Hope that may spark a real answer from somebody.

as usual, I am a PP. Not a mechanic.

Best regards,

-a-

[mikerocosm]

Either way, it sounds like your battery is on the slippery slope to failure . .  .

Best wishes, M

[N201MKTurbo]

When you have one voltmeter you know what your voltage is. When you have two voltmeters you have no idea what your voltage is.

The first thing to do is get a calibrated voltmeter and see which one is telling the truth.

[N601RX]

Find out where each one gets its voltage from.  They may not be fed from the same buss.  The busses should not have anywhere near this much difference between them unless there is a bad connection between them.

Also look at the ground strap to the engine.  The JPI should be grounded to the engine, not the airframe.  The EI will be grounded to the airframe.

[Browncbr1]

I have found that my JPI shows consistently about .1-.2 volts less than actual.  I figured it was because of some kind of hard wiring on the circuit board that takes a voltage reading after the unit draws power instead of before.  This might be a false understanding of how electronics work, but I have a cigarette adapter that has a voltage reading that is the same as what my voltmeter tells me.    However, the larger difference you are explaining seems like something may be wrong with your JPI ...

[Oscar Avalle]

On 11/24/2015, 11:38:48, Browncbr1 said:

I have found that my JPI shows consistently about .1-.2 volts less than actual.  I figured it was because of some kind of hard wiring on the circuit board that takes a voltage reading after the unit draws power instead of before.  This might be a false understanding of how electronics work, but I have a cigarette adapter that has a voltage reading that is the same as what my voltmeter tells me.    However, the larger difference you are explaining seems like something may be wrong with your JPI ...

Yes, that was also the case until I had my 430 replaced with a 650. Also my oil temp and oil pressure a bit off compared to the EI. Nothing major. But since I upgraded the 430, the voltage indication became not reliable. I believe that the unit's grounding may have been affected by the installation. Let me see what happens and I will report back. Thanks!!