Erratic EMS voltage sensor

[cbretana]

I have Skyview HDX with EMS-220 computer. Recently I had errratic voltage indications on my HDX. The voltage would intermittently jump up from it's normal 14.4-14.5 volts reading to 14.9-15.2, with associated audible "high voltage" warnings. At first this was intermittent. The high voltage readings behavior would last for 5-10 minutes and then it would go away, settling back down to 14.4 or so. After a few flights where it was like that, it became continuous, oscillating between 14.8 and 15.3 volts. I initially thought it was the regulator and sent it back to B and C Electric to be checked out/repaired, but they said it was operating normally and to check the wiring, I did that and rewired all the pins on the regulador, but the behavior was still there.

When I relayed all this the B and C technician, he asked me how I was reading the voltage and when I told him it was from a Dynon HDX / EMS, he suggested I check the accuracy of the readings.

So I hooked up a multimeter between battery bus and the negative (ground) terminal on the battery, and started the engine. The multimeter indicated a stable 14.4 volts at all engine rpms from idle (650 rpm) up to 1800 rpm. The HDX, otoh, indicated 14.7 volts at idle, and oscillated between 14.8 and 15.3 volts at 1800 rpm. So I interpret this to mean that the aircraft electrical system, including the alternator/regulator is okay - that the problem must be in the Dynon avionics, either in the EMS or the Skyview HDX, or associated wiring.

Is this an issue with the EMS-220?
Or do I need to calibrate something in the EMS engine settings?
Or is it a wiring issue?

Has anyone else experienced this? ... Or a similar issue?

 

[jakej]

"Or is it a wiring issue? " more than likely. I've never seen this issue with the Dynon EMS/system.

 

[cbretana]

Yes, I think I agree. That is where I was going next. I was going to check all the relevant wiring to the EMS. How (from what), does the EMS generate the voltage value? Pin 1 (red), on the D37F harness is connected to the battery bus, but this needs to be compared with ground. I would assume that is done using one of the pins labeled Ground? (Pins 3, 5, 13, 16 or 17), or with the ground wire attached to the EMS case? Or are all these connected to each other inside the EMS?
Is there a preferred way, (place), to connect these (battery negative terminal, engine case, etc.)?

 

[Stevec]

What about moving pin 1 to pin 2. I don’t know if they are just bridged internally or if they sense independently but could be worth a try.

 

[cbretana]

Well! That's an awesome idea. I'd have to change the wiring in the engine compartment, (and I'd have to find the yellow lead in the DB-37 for pin 2), but both pin 1 and pin 2 can be used for a voltage input. Perhaps I should add another widget to the HDX Screen hooked up to pin 2, and wire that to a different point in the electrical system. I would have two voltage widgets on HDX screen, from different points, and could compare them.

As to whether they're bridged internally, the way I read the docs, you can use both, for different voltages... on page 7-44, under Para. "Voltmeter Inputs", it says:
"Pins 1 and/or Pin 2 of the EMS D37 connector can be used to monitor voltage in your electrical system (or any other DC voltage source) from 10-30V DC. Connect either of the pins to a point in your electrical system that you wish to monitor."

As an aside, pin 1 is currently connected to the lead that connects the alternator output to the battery bus, and I was planning on splicing a separate lead to that and hooking it up to a multimeter, so I would have a separate, independent reading from the same point. I may do that to pin 2....

 

[maartenversteeg]

But if there is any concern with the ground wiring that provides the reference value for this measurement then adding/changing to pin 2 would show exactly the same behavior as pin 1. Maybe first verifying with an external meter if the current pin 1 wiring and one of the ground wires on the EMS show the same incorrect behavior. To verify that the EMS gets the right inputs you shoudl verify not just the voltage on the battery but the points where the EMS performs it's measurement, so the pin 1 and one of the EMS ground pins. Measuring between battery bus and the negative (ground) terminal on the battery shows that the battery bus relative to the battery ground is the right voltage, but if there is any issue with the wiring then this may not be identical to the voltage the EMS is seeing

 

[djones]

One of the EMS grounds must be tied to the battery ground bus. They are connected to each other internally and the remainder are used to ground various sensors as needed. Grounds are often the causes of weird behaviors such as this.

 

[cbretana]

The only ground connected to battery ground is the case ground, (attached to a ground post on the SV-EMS-220 case). Does one of the DB-37 pins labeled Ground also need to be connected to aircraft ground? Dynon tech told me all these were for output of ground signal for individual engine sensors.

And the system has been wired this way for two years with no issues. This problem only started a month ago. Also, initially, it was intermittent and has only become a constant behavior in the last week or so. So it can't be due to mis-wiring.

Nevertheless, to eliminate this as possible cause, today I will attach the one remaining DB-37 ground to aircraft ground.

Also, is it possible that an inaccurate or otherwise bad ground signal could cause EMS to generate voltage value that is consistently high by an amount that varies with engine RPM? Although multimeter attached directly across (between), aircraft ground and battery bus remains constant at 14.4 v, the Dynon HDX voltage widget varies up and down as engine RPM is adjusted. At idle, voltage reads 14.7. At 1900-2000 RPM, voltage reads 15.2-15.3 volts.

To isolate this further, today I will attach multimeter at exactly same point that pin 1 on EMS gets its signal from and compare HDX values with direct readings with engine at varying RPMs.

 

[jakej]

Yes, 1 x ground goes directly to airframe ground regardless - if more sensor grounds are needed than what's available in the supplied harness then it's acceptable to split a ground.
Re "And the system has been wired this way for two years with no issues" - you were lucky it lasted that long. Obviously there was grounding through one of the sensors or elsewhere however it broke down enough in time to cause you grief.
I always use the supplied grounds for sensors & use 1 of those for a dedicated EMS ground - suggest you do the same & the 'issue' should go away.

 

[cbretana]

Connected one remaining ground to aircraft ground, and still getting over voltage (up to 15.4 volts), out of alternator. When I wired up new high quality multimeter to same place that Dynon EMS was sampling voltage, I discovered that readings were within 0.1 volts of one another. So Dynon EMS is apparently reading accurately. That, and the fact that the higher than expected voltage varies consistently with engine RPM makes me think this has to be an issue with alternator.

Next step is to send alternator back to B&C Electric to be bench tested.

 

[Marc_J._Zeitlin]

Next step is to send alternator back to B&C Electric to be bench tested.

Charlie - I'd suspect the B&C regulator first - have you troubleshot that? And adjusted it correctly? They don't fail often, but they can fail.

 

[cbretana]

Marc,

Yes to both, that was my first assumption as well.
Nathan at B&C was awesome in helping me diagnose and test it, but nothing was obvious.

I had an LR3C-14, (Rev C), and B&C offers free upgrades to the LR3D-14 (Rev D), so I spoke with Nathan about getting it upgraded and bench-tested to see if they could reproduce my symptoms.

So I sent it back for the upgrade, and to have them test it. In addition to the upgrade, they did a through functional check. They tested and upgraded the regulator, verified that it was okay and sent it back to me.

I have reinstalled it, but am still getting the same symptoms, only now, because I have attached a new, high quality multimeter at the same points that the Dynon EMS-220 is using to sample the voltage, I now know that
1. The Dynon EMS is accurate, the alternator actually is outputting more voltage than it should.
2. That the output voltage is inexplicably but consistently varying in synch with engine RPM.
3. The regulator is okay. One of the regulator checks to to measure the field voltage signal it is sending to the alternator, (It should be about 10.7 volts). and this check shows 10.71 volts on my airplane.

So I believe it is the alternator.

I spoke again with Nathan at B&C yesterday, and he agrees. He said there is a diode in the Alternator (one of rectifier components that converts AC to DC), which controls output voltage, which if it failed, could cause these symptoms.

The fact that these symptoms, when they first started a month ago, were initially not constant, but only occurred intermittently, and gradually got worse over several flights and 5-6 hours of operation, until they were constant, also seems to me to be consistent with this hypothesis.

So, although it means the plane will be down for a few weeks, I am sending alternator back to B&C in Kansas for Nathan to check it out.

 

[Marc_J._Zeitlin]

So, although it means the plane will be down for a few weeks, I am sending alternator back to B&C in Kansas for Nathan to check it out.

All makes sense, and it sounds like you're on the right track. B&C is always great for support.

 

[cbretana]

Well, I have identified and resolved the issue. I sent the alternator back to B&C, they checked it, and it bench-tested fine. The only issue they found was a weak diode in the bridge, but they said this could not cause my issue. They replaced it anyway, and sent it back to me, but the problem still persisted.

However since then, based on the lack of any other easier ideas to pursue, and the guess that I was losing voltage because of excessive impedance between the alternator output and the battery bus (in Long EZ, these are separated by 7-8 feet) I decided to make some other changes in the electrical system. First, I wanted to see what the voltage at the battery bus was. I was monitoring the voltage right at the output of the alternator. It was displayed on my HDX and was oscillating between 14.8 and 15.3 volts as rpm changed. But the regulator was set to output 14.2 volts. It occurred to me that the input signal to the regulator that it was trying to control was being read from the battery bus in the nose, and that it might just be okay. SO I disconnected the voltage sensor from the alternator, and attached it to the same lead that the regulator was using. Sure enough the voltage now consistently read 14.2-14.3 volts at all RPMs. But now I could not see the alternator output voltage. So I temporarily (for testing) put a multimeter on the alternator output, and it was indeed still doing the voltage dance with RPM, between 14.8-15.3 volts. This convinced me that I had a voltage drop between the output of the alternator and the point on the battery bus where the regulator was sensing bus voltage, which, if it was due t os short could be an issue, so...

My battery bus was aluminum, and there were two sections, connected by a short AWG8 wire. Additionally, when I purchased the aircraft it had a 28 volt electrical system, which I upgraded to 14 volts. So the wire that carried the alternator output to the battery bus 7 feet away was also only AWG8 - (I guess with 28 volts that was sufficient). If the AWG8 was simply too small a wire to carry the amperage from the alternator, this would explain the problem. Also, the conductivity of my aluminum bus(s) is only 60% of that of copper, so that may have also contributed to the voltage loss. So I upgraded the wire to an AWG6 wire, and replaced my aluminum buses with copper ones.

Because the Dynon EMS-220 has two pins, (pins 1 & 2), that are capable of measuring voltage, I decided to add another widget to the HDX screen to measure voltage at the alternator output. So now I have both a battery bus and an Alternator output voltage widget. (Unfortunately, the DB37 EMS-220 wiring harness comes without a wire installed on pin 2, so I first had to modify that).

Once I had made these changes, I reran the engine, and voila, all was well. Now the battery bus voltage was 14.2 volts - stable at all RPMs and the alternator output voltage was stable 14.3 volts at all RPMs. Now that this is all fixed, I really don't need to see two voltage readings on my HDX, but now that it's there ...

But I learned a hell of a lot about electrical systems, and how to program and modify the HDX Engine widgets display... It's really flexible, and neat.