JPI reading voltage

[Ragsf15e]

22 minutes ago, AdamJD said:

Still stumped. On my last flight, JPI reported volts in mid-12s most of the 2.75 hour flight.  Event went to 11.8 when I was low power and descending. 

Took a multimeter out today with a friend.  With engine OFF and master / alt field ON, measured ~12.5v at the field connection alternator.  Also checked at the VR even though it shouldn't be less than what it was at the field connection at the alternator.  My battery was fully charged as well.  

We then ran the engine and measured at the power buss.  It showed ~14v steady on the meter at the power buss, but showed 13.4v. on the JPI.  The JPI is powered from the avionics buss.  Ran out of daylight to measure voltage with the meter on the avionics buss while the engine was running.  I may go out and do that tomorrow.

1) If I have ~12.5 volts on the multimeter at field terminal on alternator with master / alt field ON engine OFF, then field path from battery through everything to the alternator is good, right?

2) If I have ~14 volts steady on the multimeter at power buss with master and engine ON, the alternator and VR are doing their job, right?

My JPI was installed in Nov 22 and had shown ~14volts in flight until 2 months ago after my annual.  Post annual, it's been fluctuating between 13.5 and 11.8 and trending downward.  Last flight was the first time it dipped below 12 on the JPI.

Open to any thoughts or ideas.

One other thing that’s kind of coincidental… mine ended up very similar.  Theres a thick jumper between the power bus and aux bus bar right near it.  Mine had a bad connection on that big jumper, so I had 14v on one bus bar and 13v on the one next to it.

[EricJ]

2 hours ago, AdamJD said:

Still stumped. On my last flight, JPI reported volts in mid-12s most of the 2.75 hour flight.  Event went to 11.8 when I was low power and descending. 

Took a multimeter out today with a friend.  With engine OFF and master / alt field ON, measured ~12.5v at the field connection alternator.  Also checked at the VR even though it shouldn't be less than what it was at the field connection at the alternator.  My battery was fully charged as well.  

We then ran the engine and measured at the power buss.  It showed ~14v steady on the meter at the power buss, but showed 13.4v. on the JPI.  The JPI is powered from the avionics buss.  Ran out of daylight to measure voltage with the meter on the avionics buss while the engine was running.  I may go out and do that tomorrow.

1) If I have ~12.5 volts on the multimeter at field terminal on alternator with master / alt field ON engine OFF, then field path from battery through everything to the alternator is good, right?

Yes, that's good.

2 hours ago, AdamJD said:

2) If I have ~14 volts steady on the multimeter at power buss with master and engine ON, the alternator and VR are doing their job, right?

Yes, that's good.

2 hours ago, AdamJD said:

My JPI was installed in Nov 22 and had shown ~14volts in flight until 2 months ago after my annual.  Post annual, it's been fluctuating between 13.5 and 11.8 and trending downward.  Last flight was the first time it dipped below 12 on the JPI.

Open to any thoughts or ideas.

FWIW, my JPI always reads battery voltage a bit low, too.    I have a connector for my battery tender that is wired directly to the battery, and once in a while I'll check the voltage on that just to see where the rested battery open circuit voltage is.   It's always higher than what the JPI is indicating, but when the JPI is on the battery is loaded, too.   I should check the batt while the JPI is on to see how they compare.

My JPI does indicate 14.1 V when the engine is running, which is where it should be, so there's a small voltage drop between the front of the airplane and the back where the battery is.    I'm not too worried about it because the battery has stayed healthy for the last four years, and the JPI will alarm properly if the alternator dies.   For me it's like a lot of things, once you get used to what "normal" is, you just look for changes.

[Ragsf15e]

1 hour ago, EricJ said:

Yes, that's good.

Yes, that's good.

FWIW, my JPI always reads battery voltage a bit low, too.    I have a connector for my battery tender that is wired directly to the battery, and once in a while I'll check the voltage on that just to see where the rested battery open circuit voltage is.   It's always higher than what the JPI is indicating, but when the JPI is on the battery is loaded, too.   I should check the batt while the JPI is on to see how they compare.

My JPI does indicate 14.1 V when the engine is running, which is where it should be, so there's a small voltage drop between the front of the airplane and the back where the battery is.    I'm not too worried about it because the battery has stayed healthy for the last four years, and the JPI will alarm properly if the alternator dies.   For me it's like a lot of things, once you get used to what "normal" is, you just look for changes.

Eric, do you happen to know if there’s a separate “sense” wire to the jpi for voltage or is it displaying its own input voltage?  It might help narrow down where the loss is?

[EricJ]

14 minutes ago, Ragsf15e said:

Eric, do you happen to know if there’s a separate “sense” wire to the jpi for voltage or is it displaying its own input voltage?  It might help narrow down where the loss is?

I just looked through the installation manual and there is no separate system voltage sense wire.    There is the input supply wire (which is probably used for Voltage sensing), and the two wires for the shunt for ammeter/load meter sensing.    There's nothing other than those.

[AdamJD]

2 hours ago, EricJ said:

I just looked through the installation manual and there is no separate system voltage sense wire.    There is the input supply wire (which is probably used for Voltage sensing), and the two wires for the shunt for ammeter/load meter sensing.    There's nothing other than those.

So if I put the meter on the avionics buss with ALT field on, the meter should match the JPI, right?  Seems like an easy check.

[EricJ]

13 minutes ago, AdamJD said:

So if I put the meter on the avionics buss with ALT field on, the meter should match the JPI, right?  Seems like an easy check.

If this is a JPI 900 or 930 it should be wired to the main bus, which might be a little higher than the avionics bus.  Ideally they'd be the same, but since we're talking about where drops can happen, that's one. 

Remember also that there's a breaker between the bus and the JPI, so if there's a drop across that it'll influence the reading on the JPI.

[Ragsf15e]

26 minutes ago, AdamJD said:

So if I put the meter on the avionics buss with ALT field on, the meter should match the JPI, right?  Seems like an easy check.

100% what eric just said.  Test both sides of a breaker as the breaker could be bad.  Additionally, it depends on where the jpi power is wired.  The jpi comes on with the master, so it’s wired after your master solenoid (mine is right next to the battery).  Some installs have a wire run from the solenoid to the jpi, so I’m just saying that it isn’t necessarily connected to the main bus, but that’s a reasonable place to check.  You really want to check the install guide to give you an idea.  Depending on what type jpi, it might have a fuse but no circuit breaker.

[AdamJD]

Round 2 of testing.

Tested power buss again with engine running, just in case - 13.9 steady.  My JPI 830 is powered of the avionics buss.  Tested avionics buss with engine running, and it was steady at 13.4, so there's a 1/2 volt drop between those two busses.  The JPI fluctuated between 13.3 and 13.5.  HOWEVER, throughout this issue, it has read in the low 13s during taxi and runup until after takeoff and then degrades to the mid 12s in flight.

Since the battery seems to have a full charge, and the power buss is steady in the high 13s, I'm convinced I don't have an alternator issue and I now have an unnecessarily new voltage regulator.  However, to confirm a good alternator, my next step is to test the power buss with the multimeter in cruise flight when the JPI drops to the 12s.  If the power buss stays steady in the high 13s, I assume I have either a grounding issue with the avionics buss?  Or a bad JPI voltage reading?  The whole avionics buss is new as of Nov 22.  And the voltage was near 14 steady on my EDM data until about 2.5 months ago, after annual.

Thanks for all of the advice!

[Ragsf15e]

1 hour ago, AdamJD said:

Round 2 of testing.

Tested power buss again with engine running, just in case - 13.9 steady.  My JPI 830 is powered of the avionics buss.  Tested avionics buss with engine running, and it was steady at 13.4, so there's a 1/2 volt drop between those two busses.  The JPI fluctuated between 13.3 and 13.5.  HOWEVER, throughout this issue, it has read in the low 13s during taxi and runup until after takeoff and then degrades to the mid 12s in flight.

Since the battery seems to have a full charge, and the power buss is steady in the high 13s, I'm convinced I don't have an alternator issue and I now have an unnecessarily new voltage regulator.  However, to confirm a good alternator, my next step is to test the power buss with the multimeter in cruise flight when the JPI drops to the 12s.  If the power buss stays steady in the high 13s, I assume I have either a grounding issue with the avionics buss?  Or a bad JPI voltage reading?  The whole avionics buss is new as of Nov 22.  And the voltage was near 14 steady on my EDM data until about 2.5 months ago, after annual.

Thanks for all of the advice!

I’m betting it’s pretty simple.  A loose or corroded connection between the power bus and avionics bus.

You need to test several points along the line back from the avionics bus to find that big drop.  Could be at the avionics master or at the connections to either bus or at the circuit breakers.

0.5v drop is pretty good resistance at one point!

Also, only having the unnecessarily new vr is a win!  I got new vr, alt and field wiring before finally doing this testing!

[AdamJD]

56 minutes ago, Ragsf15e said:

Also, only having the unnecessarily new vr is a win!  I got new vr, alt and field wiring before finally doing this testing!

+1 for MooneySpace.  Your experience kept me from turning this over to the mechanic who could have possibly convinced me the alternator was bad.  They obviously went right to a bad VR without testing it.  So thank you for sharing!

When I get more time, I'm going to put the tester on both sides of each breaker on the avionics buss and test both sides of the avionics switch.  Will report back.

[EricJ]

30 minutes ago, AdamJD said:

+1 for MooneySpace.  Your experience kept me from turning this over to the mechanic who could have possibly convinced me the alternator was bad.  They obviously went right to a bad VR without testing it.  So thank you for sharing!

When I get more time, I'm going to put the tester on both sides of each breaker on the avionics buss and test both sides of the avionics switch.  Will report back.

The good news is that a 0.5V drop should be reasonably easy to find.   It may be worth just checking the connections between the main bus and avionics bus, the relay, etc.