I have an issue with the voltage and charging current being all over places during flight.
Instead of showing a steady voltage/current, my EDM930 engine monitoring shows a high spread, most concerning being the amps, between 5 to 25A.
Any idea what would cause this? A voltage or current regulator, the alternator/charger, the battery, or maybe the EDM930 sensor itself? On the ground there is a steady voltage indication, it behaves like this only when the engine is running.
Any idea on how to diagnose or fix this would be really appreciated…
I would start with cheap reasons that the current to the field coils might vary, check first for play I the alternator rotor bearings and then for worn brushes.
If that’s OK look at the cables to the field, especially the crimp connectors.
I had to replace 2 Voltage regulators so far on my alternator. I’d have it checked. It is fairly inexpensive to replace.
Can the variation be reproduced on a ground run?
With a battery across the bus, a small variation in the alternator voltage will create big variations in the charging current. An extra 1V could produce an extra 20-30A.
Does the charging current correspond with the voltage? For example at 24V there should be around zero current. At 28V you should see a few amps (if the battery is more or less charged). What is the peak voltage showing and what current does it correspond to, at that instant?
And finally does a peak in the voltage correspond to a peak in the current?
It does the same thing on the ground, if the engine is running. Wiylth the engine stopped, there is a constant voltage.
I have a video, unfortunately I cannot seem to be able to upload it here. However here are the values:
14.0V – 12A
13.9V – 22A
13.9V – 11A
13.7V – 25A
14.2V – 5A
All this in a few seconds. I have a good multimeter, but not an oscilloscope. What should I measure? Voltage and/or current at the alternator output or after the voltage regulator, with or without the battery connected?
You can’t upload videos to EuroGA. We have just 160GB allowance, the whole forum is 90GB of which 45GB is user uploaded objects (photos mostly), and videos would blow the allowance instantly 
You have use Youtube or (preferably) Vimeo and then you simply drop the URL into your post.
Those readings do not correspond to a normal charging circuit. The highest voltage produces the lowest current. Unless this circuit is wired in a way I don’t understand (I know there are others here who know much more) there is something weird going on. It looks like something is putting a big (and varying) load onto the bus.
But the voltage and current are both steady when the ALT CB is pulled?
Also could anything produce a draw of 25A?
@wigglyamp will probably know.
As an EDM 930 user I can maybe provide some insight.
Start by talking to JPI support. They know things we don’t and are friendly and responsive.
You may also want to post the graphs of the logs here for “our” electronics gurus to explore. I upload mine to savvy and then it is as simple as a screenshot
I had an issue with both fuel gauges showing stupid values at random.
JPI seemed to know about it and sent me a field software update package (!).
I have not yet dared start the daunting install…
Other hints:
- the EDM 930 is extremely sensitive to bad grounding. JPI are adamant that many problems are caused by grounding that does not strictly follow their guidelines.
- When more than one probe fails, check if they are connected to the same interface board (same connector in the back).
I have seen a short in the wire of one probe bring down an entire board and the 4 other probes. The short came straight from the factory, btw…
And if you are in the process of installing a 930, here my 2 cents.
Wherever possible, keep the steam gauges as backup and as a reference in case things like this happen.
Sadly, these all-in-one primary engine monitors are also an all-in-one generator of “not airworthy” hiccups…
What concerns me is that the voltage goes down when the current goes up. Yet, there is probably nothing that can draw the indicated current of 25A, especially in an intermittent manner.
Assuming this is not a new installation, I would check the connections on the shunt and make sure they are tight.
I agree about not removing old instruments. The build quality of the new stuff is not that great.
Peter wrote:
What concerns me is that the voltage goes down when the current goes up
This behaviour depends on where in the circuit you are measuring amps.
An oscillating voltage regulator is frequently caused by high resistance in the field wiring/connectors. A few sprays with contact cleaner and dis/re-connecting sometimes helps.
In your airplane this wiring probably goes from the Alt Field breaker to the ALT switch to the VR to the alternator.
Any resistance higher than a few ohms is cause for concern. If you can close the ALT switch with BAT off (some aircraft cannot), then measuring from breaker input (accessible in your airplane?) to VR input will factor ALT switch, breaker and associated connectors in one single measurement.
An alternate way is measuring voltage at the VR field input and output with engine stopped and BAT+ALT on (it should be max 0.5V less than bus voltage).
You can follow Hartzell’s troubleshooting flowchart assuming no/low ALT output and this will take you the long way down to the bottom of the chart. Further guidance here
In a TB20 (which is the type shown in the OP’s profile) they use a solid state Lamar regulator which PWM modulates the field current.
The ammeter displays the current going into or out of the battery (excluding starter motor current, for obvious reasons).
