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Master Switch Voltage Drop

MartinPred

Well Known Member
Here's a nagging electrical problem:

I've been having low voltage issues for a long time now despite putting on a new internally-regulated alternator and a new Odyssey battery. I finally got around to hooking test leads all over the place and here's what I found.

Alternator puts out a solid 14.5 volts and that voltage registers just fine all the way to the master switch and the main bus...until I throw the master switch. When I do that the whole system drops to about 12.8 volts. And once I start loading up the system, the voltage continues to drop until I show a discharge.

But of course the master switch just grounds the master relay and connects the charging circuit. So what could be sucking up my voltage?

I've checked all the wires and found no obvious signs of wear or shorting.

Any ideas?

-Matt
402BD
 
Now that is an interesting set of symptoms -

If the alternator is working (and it appears it is with no load) and not working when the master is closed, its voltage regulation is not functioning.

I would have the alternator checked at a suitable shop.

If the alternator is OK, the only other situation that could pull the voltage down is a serious leak to ground somewhere in the system. It will in a short time cause a hot spot and probably a fire.

Definitely do not fly this airplane until things are sorted out and the voltage does not drop below 14 with everything working.
 
Check the engine to airframe ground connection. I've seen this before in a little Grumman AA-1. The ground strap connections were severely corroded, thus inhibiting the alternator from delivering good volts/amps under any kind of load. The alternator might have been finding ground through the tach cable.
 
Silly as it may sound, check the alternator belt for tightness. If it is too loose it will slip with load application and the charge rate will drop, even to the point where it is not doing anything.
 
sporadic alternator

Sounds familiar! two weeks ago, same problem with my -8. Traced the problem to a 5 amp breaker for the alt. field. New breaker -- alternator works like new. A&P advise: "Either an alternator works or it doesn't":
 
Elimination

Pull all your circuit breakers or fuses (except for the alternator field) and try your test again. If you see the same results, you've eliminated loads as a possible culprit.

Check your grounds. Even a low resistance ground (instead of near zero) can do things like this.

Bypass the battery contactor with a jumper (be careful). No change? Eliminates that as a culprit (bad contacts).

My money would be on a bad ground.

Good luck. Let us know what you find out.
 
Check the engine to airframe ground connection. I've seen this before in a little Grumman AA-1. The ground strap connections were severely corroded, thus inhibiting the alternator from delivering good volts/amps under any kind of load. The alternator might have been finding ground through the tach cable.

And to add to this...

There was a case of where the alternator brackets were anodized, painted, or something along those lines. The alternator just wasn't grounded to the engine well enough.

L.Adamson --- RV6A
 
My split master switch was bad,

after 1450 hrs the rivets holding the spades for the alt wires were a little loose and caused an intermittent problem. Replaced with quality pole switches. The original was the classic cessna dual switch.
Dale
 
Will check for a bad ground

Thanks for all the advice. I can tell you that it's definitely not the alternator, belt, bracket, or engine mount. All have only about 300 hours on them and are in pristine condition.

Plus I get the same indications when I disconnect the alternator and replace it on the ground with a 14.5V 10A power supply.

Good tips on pulling all the breakers and bypassing the master relay. I'll try that this weekend and let you know how it turns out.

-Matt
402BD
 
Not an RV but at about 200 hours also in a pristine condition, I had strange problems on my Jabiru. Found the problem to be galvanic corrosion under an eye terminal of the ground wire on the crank case.
 
Getting Better

Well, after chasing every line and isolating every connection, I found no obvious culprit. So I cleaned every connection and that brought the voltage drop down to about 1 volt.

I'm going to clean everything again, maybe replace some switches and CBs and see how it goes.

-Matt
402BD
 
Well, after chasing every line and isolating every connection, I found no obvious culprit. So I cleaned every connection and that brought the voltage drop down to about 1 volt.

I'm going to clean everything again, maybe replace some switches and CBs and see how it goes.

Flew a rather new RV once, that dropped to battery voltage in flight. The belt was tightened a bit more, and voltage checked out on the ground. But once in flight, it dropped again. When pulling the alternator to have it checked, I could see that the connector on the end of the B-lead had been arching. It just wasn't crimped tight enough. A new connector & crimp solved the problem. It just couldn't pass enough current, when loads were applied.

L.Adamson --- RV6A
 
Measure voltage drop across connections

Instead of using ground as a reference, measure the voltage drop along each wire, touching the voltmeter test leads to both sides of every connection and switch and contactor. The bigger the voltage drop, the bigger the problem. This test can be performed on both the positive and negative wires in the circuit. If the voltmeter says zero, try setting the meter to a lower range.
Joe Gores
 
Good idea

Joe's idea is a good one, low range on the voltmeter. Especially check across the battery contactor, which may have some burn/pitted contacts. You didn't mention if you tried jumping across it during your isolation tests.

If you pulled all of your circuit breakers and still had the same indication, then that eliminates most (if not all) of your loads. The only other load on the system is... your battery. Maybe swap it out for a quick ground test.

Don
 
Not the loads

So no change with all loads isolated. Bypassing the battery relay got me about .5 volts back. Didn't think to isolate the battery but I'd be surpassed if that's it. I replaced it about two years ago and I keep fitfully charged.

I'll try the battery check and measuring line voltage this weekend and let you know how it goes.

Thanks,

-Matt
402BD
 
Success!

Well, after cleaning every connection, re-installing the main ground wire to the engine mount (which was loose), and tracing every other wire in the plane, I was still getting a one volt drop every time I threw the master switch (this was using the ground power supply).

So I was already to replace the battery, but before doing that I decided to run the engine one more time just to make sure I was getting the same indication from the alternator. I fired her up and, what do you know, 14.0 solid volts on the meter! And it held even after I turned on every piece of gear I have. Huzzah!

The only thing is I did so much before running the engine, I'll never know exactly what the problem was. Oh, well. At least it works (and I didn't have to buy a new battery).

My guess it was the ground wire. Not sure why I was getting the voltage drop with ground power. Maybe a 10-amp power supply can't carry the load.

Thanks everyone for all the advice. I'm sure at least one of you was right. ;)

-Matt
402BD
 
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