Electrical Problem

[ScottHess]

I'm soliciting opinions as to how I managed to smoke my KX125, among other things today in my RV-4. My electrical system is really simple:

• ND 35 Amp Alternator with Ford regulator
• Ammeter shunt is in between buss bar and master relay
• Alternator output also connected to main buss bar with a 40A fusible link.
• Most everything is fused off the main buss (radios fed from a secondary buss that is switched)
• Alternator field circuit fed from main buss with 5A breaker, crowbar OV in place
• Dynon D-6 always on, fed from main buss and separate CB
• Cessna type split master switch

Yesterday I caught a slight whiff of smoke while on a short flight. I returned to the field and everything was fine with no signs of anything having caused it. Today, I took off and the smoke became obvious, about the same time all my electrical equipment signed off. I shut off the master and landed. On the runup prior to flight, ammeter was showing a charge, voltage was around 14v. The ammeter seemed to have a slight wiggle which may have been there, but no whine was heard. I recently installed new programmable fuel gauges (which are really nice btw) which have their own internal electroluminescent dials powered by a supplied inverter. They are powered by the dimmer circuits (2 Britta dimmers, separate, one for panel lights and one for an LED strip). Everything has been working fine, though the airplane has only about 50TT.

Upon landing and after shutdown, I flipped the master back on and everything seemed to work with the exception of my KX125 and dimmers (both). I opened the cover of the 125 and it looks like the fire went to two alarms, mostly on what appears to be the audio side of things. The internal fuse also blew, though the 10A supply fuse did not.

The field breaker never popped either. I did check the OV circuit before I installed it, and it did short at around 16V as I recall.

I've since checked the charging system which is still charging properly with proper voltage. Intercom still works, and still no alternator whine.

The master relay was not new when I installed it, neither was the Cessna master switch. I recall the master switch having been intermittent at first but I cleaned it and checked the resistance and it was fine, and has worked perfectly since. Of course I can't remember whether it was the battery or alternator side of the switch which had been flaky.

So here's the question, IF the master relay failed, or the master switch failed.. leaving the alternator to its own devices.. shouldn't the crowbar have tripped and taken the alternator off line? I clearly had some kind of event, and since the rest of the panel went out, it must have been because of a battery disconnect.

I'm really puzzled by this. In addition to the $$$ for the KX125, I'm going to replace the switch, master relay and alternator but I am not going to smoke another radio without finding the cause. The transponder, intercom, EFIS, etc. all seem fine. And the backlight does still work in the KX125.

Any and all opinions are appreciated!

Scott

[az_gila]

..a short (probably intermittent) in the field line to the alternator...

And perhaps your field breaker has also failed...

[ScottHess]

So it would have 'appeared' as though the battery disconnected as the alternator went to maximum voltage and things stopped working... ? I will check the breaker, since if the crowbar had been doing its job the breaker would have tripped, no?

[apkp777]

Scott,

For your KX125 to cook itself it had to demand or receive too much voltage. An internal short would cause it to want more voltage, but because of your failure of the other instruments tells me that it was delivered too much voltage. A bad voltage regulator will allow an over- volt output from the alternator however, a stuck master solenoid usually will either fail open or closed. Failed open, the battery doesn't come online, failed closed it won't go off. However, I don't see where it could cook the radio in either condition.

The master switch also fails either open or closed, but I don't see it cooking the radio.

The alternator field circuit is always an item to consider. However, the battery is the supply for the field and if there is a short in the field circuit, batter voltage is shorted to ground and the field breaker pops. An open in the field circuit and the alternator stops its output.

However, a short in the alternator field itself can cause a "runaway" alternator where the output becomes excessive..

The Crowbar (I don't have a diagram in front of me) should isolate a runaway alternator so now we have come full circle.

All this leads me to think as Gil posted the problem lies with an intermittent short in the field circuit which is not a hard fault (popping a breaker) rather faulting so that your are energizing your ground circuit with (probably 14.5V) ships power. Thus cooking the radio which for whatever reason didn't have protection for this sort of thing.

Pull all your breakers (or fuses) and see if you have a "hot" ground bus. Also, check your wiring to the master solenoid and make sure that it is open to ground with the master in the off position.

All of the above are just my thoughts out loud.

[ScottHess]

Thanks guys - I will troubleshoot further. Not sure I understand the hot ground scenario though. With respect to the failing master switch theory - if the master switch opened say under the vibration of the running engine and due to marginal contacts, would not the alternator start to bootstrap since it is still producing power and feeding the main buss... and the field circuit? Again though, the OV should have tripped the field breaker.

Again thanks for the input!

[rocketbob]

Have you checked the ground wire from the engine?

[Pat Hatch]

One other possibility and something to check in addition to all the other suggestions is a sticking starter relay. Consider that if the starter relay does not release, the starter could become a generator, unregulated at that. Somewhere in the recesses of my mind I seem to recall this happening to someone.

[aerhed]

One other possibility and something to check in addition to all the other suggestions is a sticking starter relay. Consider that if the starter relay does not release, the starter could become a generator, unregulated at that. Somewhere in the recesses of my mind I seem to recall this happening to someone.
__________________

Yeah, and also, I've seen where high resistance in a Cessna split switch (field side) is coincidentally the sense voltage for the Ford regulator setup which yielded a high field voltage to the alternator, thus cooking batteries, but usually not blowing radios. I've also seen the same problem with high resistance elsewhere in the feed to the regulator like corroded terminals or a too small/long wire run.

[Pat Hatch]

Scott,

Thinking about your problem some more, I wonder if what you have is just an internal failure of your KX125. I think I would pull this radio and then go out and see if everything is normal. Could be you just had an internal failure that caused that internal fuse to blow. Of course, this wouldn't explain the intermittent failure of your entire electrical system, or would it?

[ScottHess]

Thanks again for everybody's input. The engine has a good ground.. I just checked it mechanically and electrically (at the alternator). I do not have a unilateral ground wire going to the alternator however.

I did think of the hung starter possibility however I don't think that was it.

I guess the other possibility is that I shut the master off without thinking, after the smoke, and remembered things out of sequence... and it only was just the radio going TU.

I'm going to replace the master switch, on the chance that there was high resistance in the field circuit, which as aerhead mentions is also the sense voltage.

Still wondering however what happens when you disconnect the battery while leaving the alternator online though....

I'm just happy the radio was there to protect the fuses.