914 Inconsistent Charging

I checked the connections and they seem good, which wires are you most concerned about?

My A&P thinks perhaps a weak regulator?

Dave,

The internal alternator on the 914 will produce 250 watts at 5800 RPM, which is about 22 amps.  At 5000 RPM that drops to about 18 amps.  The engine functions, including the fuel pumps take 8 amps with both pumps running, and 5 amps with one pump running.  So at 5000 RPM you get 10-13 amps for the airframe loads depending on if both pumps are running.  So that is something to think about as you add up (or measure) your total loads.  One easy way to check the total airframe load is to turn everything on inside the aircraft with the engine OFF, and look at the (negative) reading on your amp meter.  A reading of -7 would mean you have a 7 amp total airframe load.  

Assuming that this is a new problem, not a systemic one….  The regulator/rectifier is a solid state device, so you are correct that it typically works or does not work.  I’ve yet to see a “weak” one.  Not saying it can’t happen, but it’s not likely.  What I HAVE seen is a failing alternator Stator coil that is still partially functioning.  The Stator coil insulation can partially fail and develop a high resistance ground that slowly gets worse until it finally becomes a short circuit and stops working completely.   In this case you could have a “weak generator”.  The first thing I would do is to disconnect the two yellow wires that run from the generator to the two “G” terminals on the regulator.  Use an ohm meter to test between the two yellow wires and you should get between .1 and .8 ohms.  Then, test each yellow wire to the engine case (ground) and your meter should read infinite (no continuity).  These two tests are very indicative of the condition of the Stator.  A complete and more comprehensive charging system test is outlined in the Heavy Maintenance Manual, section 74-00-00, paragraph 3.7 (page 16). 

My A&P gave me this chart. 

I flew the other day and at 5000rpm and with one fuel pump and 2 screens (2 radios) I’m only getting about +1amp and battery stays about 13.2. The issue is when you come back to the pattern and pull the power pack it’s starts dropping 

another question, shouldn’t those wires pull out of the regulator connector? 

With what you have told us it’s hard to know if we are chasing a charging system problem, or if the alternator is simply undersized for your electrical loads.  It’s hard to go much further until we know the total power requirements for your aircraft to compare with the output of the internal alternator at a given RPM.

As per my last post, check the amp draw (as displayed on your G3X) with all airframe loads ON and all engine systems (including fuel pumps) OFF.  Then, add that number to the published engine electrical load (8 amps) and that’s your total connected electrical load.  Then it’s a simple matter of comparing that number to the alternator output graph to see at what RPM all loads are covered.  For instance, if your total connected load is 14 amps, then you would expect your amp meter to go negative below 3000 RPM.  If this exercise predicts what you are experiencing while flying, then the single internal alternator is likely too small to keep up at lower RPM - a design issue.  

As far as your question regarding the two yellow wires from the alternator,  it might be easier to remove the entire connector from the regulator, and test the yellow wires while still inserted in the connector housing.   These continuity and insulation tests are done with the engine and all power OFF. 

I plan on getting those loads tonight.

My question on the Voltage Test with engine running, since these wires are connected to the adapter for the regulator, can I test the voltage with the assembly unplugged from the regulator (engine running generator voltage test)