Avionics question
 

Hello,

In a DR400 (engine running), the alternator warning light is on constantly, the low voltage light is flashing and the ammeter is reading positive charge (all immediately after staring the engine).

What does this situation indicate?

Thanks

Depends on the age of your aircraft, what year and engine size are we looking at?

Possibly the battery not accepting a charge? or a diode failure in the alternator. Is the belt tight?




will explain


http://www.knowyourparts.com/technic...-misdiagnosis/

At what engine revs is the light on?...Just tickover?
also
What electrical stuff, avionics are on when the light is on. In other words what is the elctrical load?

Another vote for a bad diode pack.

Need more data to solve the problem. What does (all immediately after starting the engine) how do they change with time and revs. Does cycling the alternator side of the master switch make any difference to the lights. The alternator warning light normally means the alternator is of line as with the alternator switched off. When you say a positive charge, how many amps. There should be a large charge that reduces as the battery recovers from engine start. The avionics should be off at engine start. Even starting with the alternator off is a good idea reduces load on battery. Would not expect alternator warning light on and a positive charge or the diode pack to be bad.

So a bit more info would help.

The alternator light and and low voltage warning light mean that the alternator out put is below battery voltage. That information doesn't gel with the positive charge on the ammeter. I would be taking the alternator to a work shop for checking/overhaul.

The alternator may have a dropped winding on the stator, a bad diode or two, or worn brushes, also could be a failed regulator.

P.S. An alternator does not put a load on the engine or battery on start.

Link to some rather good troubleshooting advice from Cockney Steve when I had a similar issue a few years back

http://www.pprune.org/private-flying...ml#post8570411

It will place a load on the battery on start.

The battery voltage will drop, the regulator will call for high output from the alternator and to do so will increase the current flow in the field circuit. Though the current draw in the field circuit will be very small compared the the draw from the starter motor. It's probably not worth turning off the field circuit during the start.

27/90 as soon as you switch on the alternator the field windings take max current on a 60amp that is about 3.5aps. So the alternator is trying to produce max output as it spins on start up the load, because the starter is pulling down the voltage the regulator still pushes 3.5amp in the field winding so the alternator output is 60 amps or a one horse power load on the starter motor. 720watts/hp so best to turn off the alternator on start up.

Horizon Flyer,

There's no way an alternator will give anything like it's full output at cranking speeds, I very much doubt there will be anywhere near the output/load from the alternator during the time the starter is engaged. By all means switch it off if you want to.

Red123
I fly an earlier version of the DR400. It has a generator.
The low volt warning light comes on at low revs. What it indicates to me is that the plane should never have had a low volt warning light retro-fitted.

It means you (we) have got low volts according to the lamp.
In reality, I know if I've got low volts, because the engine didn't start. After that there's plenty of rotation to keep enough life for a flight.

It probably means that the battery is not in great condition. Diode pack failure relates to an alternator. You presumably have one of those.

I was told that Maintenance need to do a battery capacity check annually (totally drain and full recharge). On a lead acid battery this is the fastest way to kill them that I know. They are supposed to be kept fully topped up.
There are other elements of maintenance that I feel simply contribute to wearing out components. But that's another topic.

I think the low volt warning is a good idea. For example it will let you know of a charging failure (broken drive belt etc) once airborne thus giving you a warning you will lose electrical power within a shortish period of time allowing you to plan to land without losing any critical items (e.g. radios if in controlled airspace).

The light comes on at low revs as at low RPM a generator cannot produce enough output to match the electrical load on the battery. It doesn't mean the battery is not in great condition.

A battery capacity check doesn't totally drain the battery nor does it kill them. The battery is discharged at a predetermined rate till the voltage reaches 1.85 volts per cell, (so 11.1 volts for a 12 volt six cell battery) and the time is noted. The time taken is checked against specs for the discharge rate used to determine the health of the battery.

A test discharge can in fact be good for batteries that don't get drained or used often. The test discharge helps prevent sulphation on the plates

There was a little "known issue" that some of the warning lights on the warning panel of DR400-135 CDI light up when you press the PTT switch. From memory, the low voltage light.

There seems to be a little gremlin hiding in the wiring loom somewhere. I don't know if it's been fixed by now. We just learned to ignore it.

I used to fly a Cessna 172M on which the low voltage light flashed dimly in time with the beacon. Engineering couldn't fix it and it caused no problems in the 20 years I flew it.