Low Voltage Warning

[Mpatrickw]

Has anyone seen this alert on a diesel DA40? I have been battling with solving this problem for two months now. Under full electrical load, at any rpm, a low voltage alert is triggered for a second or so, on the annunciator display of my DA40, with the switching on of an additional load like transmitting or switching on fuel transfer pump. New Centurion 2.0s and alternator were fitted 300hrs ago. All batteries good following recent annual. We thought we’d solved it by replacing the main relay – all fine for 8 hrs IFR with pitot heat on then the problem returned intermittently. The problem only appears when close to all equipment on, inc pitot heat. Without pitot heat the load isn’t great enough to be triggered by anything else. We’ve ruled out the pitot heat and it’s been checked twice for operation and any likely short to earth. Total system load is less than 50% on the engine display ammeter.

I would greatly appreciate the advice of anyone with aircraft electrics experience. I and my engineers are running out of ideas. I’m also close to IR test so this is double trouble as any examiner won’t be happy to fly with the problem. My aircraft is therefore effectively AOG for IFR. The details of voltage measurements I’ve taken are:

Alternator output measured at alternator: power Idle + avionics on - high 14.5, low 13.5. 1400rpm + avionics only 14.5 – 13.8. 1400rpm full load inc pitot heat 14.8 – 13.6. 1900rpm full load inc pitot heat 14.7 – 13.3.
Voltage at on-board volt meter (i.e. post voltage regulator) Engine off 12.1v. Engine Idle high 14.5 low 14.0. 1900 rpm full load inc pitot heat 14.7 – 12.9.

Is the alternator output normal? Is the voltage regulator suspect? My engineers initially condemned the annunciator display (CED). The part costs £2.5k…! Until focus moved to replacing the main relay. Voltage either side of the new relay is identical.

I don’t know where to turn or know whether I need Diamond DA40 expertise, aircraft engineer with electrical expertise, Avgas or Jet A1, or an avionics expert. No, Diamond Austria weren’t much use I’m afraid. Neither is the AMM. Technify haven’t answered. Many thanks if you have any ideas!

[Rich]

If I understand correctly, you're seeing substantial voltage swings with the engine and load at steady state. That's not normal at all. Based on this limited information it would seem that there is an intermittent high-impedance short somewhere. That's no comfort, because it could be anywhere. Do these fluctuations occur with all load that you can remove turned off?

FWIW my non-D steam gauge DA40 system voltage runs 28.1-28.3 volts - always. (there are two volt meters in most of these planes and they sometimes disagree by 0.1 volt.

[Mpatrickw]

Thanks Rich, Yes - With no load, no avionics at all, at 1400rpm it fluctuates between 13.8 and 14.5. The heavier the load, the greater the fluctuation, as low as 13.3. Where do you start tracking down a short behind my packed panel?! Are there some logical places to start, and are these resistance checks? You can tell my electrics knowledge is at its limit...! Two weeks ago we could easily simulate the problem, in the air and on the ground, but now it is intermittent, its even more difficult for the engineers. Automotive advice is that alternator output is normal between 13.5 and 14.5. Does that apply here?

[Rich]

I think that 13.5 volts applies at lower RPMs. In the normal operating RPM range both my motorcycle and truck show 14.1-14.2 volts, steady. Same with the plane, at 28.1-28.3 volts.

But the plane's voltage does drop off when the RPMs are reduced below about 1100 RPM and give a low-voltage warning (<26 volts I believe) at idle. This is normal.

In my plane if I had this behavior and the alternator were beyond suspicion the first thing I'd look at is drive belt tension and security of the mountings and tension adjustment arm. Is yours belt-driven? Is the pulley securely fastened to the alternator shaft? (I had the pulley of a newly-rebuilt alternator in my Cherokee depart the airplane in flight.)

To help track a short down I'd pull all possible circuit breakers and see if you still get this behavior. Failing that, you may have to resort to pulling connectors.

Don't discount the voltage regulator. The short could be in there.

As background I'll describe the way the alternator and voltage regulator in most motorcycles (emphasis: not all, but my FJR is a typical example) work.

The alternator runs full out, producing everything it can at the current RPM. There are no field wires, the rotor being populated with permanent magnets. It's a glorified magneto. In modern motorcycles they usually put out 3-phase AC power. This is rectified to a pretty flat DC output, whose voltage would rise to some highly undesirable maximum, given no regulation. The regulator is what was once called a "crowbar circuit". Once the voltage reaches the desired setting (usually 14-ish volts) the regulator shunts current as necessary through a resistive load to keep the voltage where it needs to be.

This is not how most plane or automobile systems work, instead controlling the field current (hence the magnetic strength of the rotor) to limit the voltage. But it illustrates how a high-impedance short can come to cause your voltage to be lower than the desired values. The bad news is it could be anywhere.

Here's another possibility to consider: If the field wiring between the regulator and the alternator were compromised it could inhibit the current actually being delivered to the rotor and thereby cause the voltage to be lower than the regulator was controlling for. You would need a DC ammeter to measure this. If the amps to the field coils rose as the voltage dipped, this would be normal. If the amps followed the output voltage then likely the wiring, brushes or regulator are at fault. Deterioration of the insulation of the wiring or having a few broken strands of the wiring somewhere could be at fault. Not common, but I've had both happen to me in different vehicles.

[Valera]

Relay regulator changed? It looks very similar to him.

Надіслано від мого Lenovo A680_ROW, використовуючи Tapatalk

[Valera]

Normal voltage should be at least 13.8 for rechargeable batteries

Надіслано від мого Lenovo A680_ROW, використовуючи Tapatalk

[Mpatrickw]

Valera, could you explain further what you mean by the relay regulator please? Thanks.

[Valera]

I have an AE engine. Look MM for your engine, it says where the regulator relay is located

[Valera]

You also have a similar electrical circuit. This applies also to cars. The relay regulator is surrounded by a circle that may cause a malfunction

[Karl]

I have seen a similar thing, though not exactly the same.
For us it was a low voltage indication when selecting flap.

We found a reasonable heavy duty wire behind the left hand side of the instrument panel was the problem.
Sometimes it showed overheating at the crimp insulation.
Replacement of this cable resolved the issues.
Pictures are better than a lengthy explanation so see attached for identification.

Having difficulty attaching the pics will try again shortly.