Intermittent low voltage problem

[pfactor]

I have a 1977 M20J. For the last month or so, my low voltage light would blink just for 2-3 seconds and then it fixes itself and it's fine. This use to happen maybe once or twice in a 1-hour flgiht. It never lasted long enough for me to really worry about it.

When I went in to Top Gun for a 100-hour inspection and I mentioned this issue to them. They suspected a bad electrical connection somewhere, so they checked and cleaned up a bunch of contacts. In the logbook entry they noted that they 1) cleaned the engine-to-airframe ground, 2) repaired the alternator ground and 3) relocated the CGR-30P ground. Unfortunately that didn't fix things. The problem was still happening intermittently. We decided to watch it for a bit to see what happened.

Last week, it got worse. I went up for a flight and about 10 minutes after takeoff, the low voltage started blinking and stayed blinking for a few minutes. Per the POH, I tried pulling and then resetting the ALT Field breaker. That didn't make any change. The voltage was below 12V long enough that my Aspen PFD switched itself over to battery backup power. I came in for a landing, and on short final the low voltage light turned off, and the voltage was closer to normal (around 13.5V). It was even fine while taxiing back to parking.

I took the plane back to Top Gun last week. On the flight there, I saw a few quick blinks of the low voltage light, but nothing significant. They checked and cleaned all my major electrical connections and ground, tested the battery, the voltage regulator and alternator appeared to be working ok for them. They improved the ground connection for the voltage regulator. My Concorde sealed battery is strong, measuring 12.8V at the terminals. They did swap out my master switch (I think it was the original 1977 one), because there was about 50 ohms resistance present when the switch was on. In short -- no smoking gun found. On the flight home, I unfortunately had a few more blinks of the voltage light.

I'm going to call Top Gun tomorrow and figure out what to try next. I wanted to seek out advice from this group. Looking at the engine monitor data, it seems like the voltage is all over the place, which makes me suspect the voltage regulator, but I'm no A&P.

I have attached graphs of the low voltage from the engine monitor data.

1) 2015-08-30.png is the flight where the low voltage light first blinked for a few minutes
2) 2015-09-03 RHV-SCK.png is the flight to Top Gun where I had a couple of blinks of low voltage.
3) 2015-09-03 SCK-RHV.png is the flight home, where again I had a few low voltage blinks.
4) 2015-09-05.png is local flight where the voltage was very erratic.

Phil

[OR75]

Was the alternator belt recently adjusted for tension or changed ? 

[pfactor]

Was the alternator belt recently adjusted for tension or changed ? 

No.

[ArtVandelay]

If the Aspen and the the engine monitor and annunciator all indicate low voltage, then I think the voltage regulator or circuit breaker for the alt field breaker is where I would look next. I assume you can't recreate on the ground, so I would do the cheap fix first:

Tighten and clean connections for the Alt field at the voltage regulator and alternator.

I would replace the alt field circuit breaker.

I had circuit breakers go bad on my J, this is exactly how they failed.

[Guest]

How about worn alternator brushes? Easy to check on most alternators.

Clarence

[tony]

Did you check the alternator belt to see if its loose?

[jetdriven]

Definitely check the field wire.   It breaks right at the connector at the alternator. Also, the insulation inside the wire breaks down and it shorts the field conductor inside to the shielding jacket which grounds it. You can't see this but you can check for it by turning the master on, hooking a voltmeter up to the field wire and wiggling the field wire and bending it.  Anything isn't right t will lose the 12v at the ring terminal.   I had to replace mine from the cannon plug on the firewall to the alternator.  

Edited September 8, 2015 by jetdriven

[pfactor]

Thanks for the suggestions!

It sounds like the next place to focus is the field wire or the alt field circuit breaker. I'll talk to Top Gun about doing that next, maybe just replacing the field circuit breaker as a fairly cheap thing to try.

The alternator belt is tight, so I don't think it's that. I believe Top Gun checked the alternator brushes, but I'll make sure to ask about that.

[FBCK]

I had a similar problem on my 231, replaced the voltage regulator and the problem went away.

[pfactor]

Top Gun replaced the voltage regulator, and things are much better. I did see one brief flash of the low voltage light on the flight home. We'll try replacing the field wire at annual next month.

I'm going to attach two files showing volts & amps. The first is from my flight to SCK with the old voltage regulator. The voltage is inconsistent, but generally above 13.5V. For the flight home, it was rock solid at 13.8V, with just two brief dips down to ~12.5V.

Thanks for all the helpful replies!

Phil

[rrbeck11]

Phil -

I had the same symptom on my 1978 M20J.  Low Voltage indication on the annunciator panel flashes intermittently.  The problem was in the annunciator itself.  The guys at LASAR put me in touch with the original designer of our annunciator panel, who told me to spray contact cleaner on the potentiometer (center, rear of annunciator circuit board) that sets the low voltage threshold.  My avionics guy opened the panel, sprayed contact cleaner, cycled the potentiometer several times, then reset the low voltage threshold  (see Manual 24-33-00 par. 6).   I have not seen the light blink (with the engine running) since.

May not be the cause of your problem, as your analyzer plots brief dips below threshold voltage of 12.5, but I don't know where your monitor picks up the bus voltage.  

It seems clear from your plots the regulator helped a lot, but if it flashed once after the new regulator, my bet would be that you'll see it again.

\Good Luck

 

Randy

 

[pfactor]

I forgot to follow-up on this thread. We ended up overhauling the alternator. The brushes were worn (as someone suggested earlier in the thread).

I think I had two problems: a bad voltage regulator and worn brushes on the alternator. Everything has been peachy since I did both those things.

Phil

[carusoam]

Nice follow-up Phil.

There is a great benefit to having a data collection device on all the various electrical devices.  Thanks for sharing your findings.

the challenge was hard to spot without the JPI.  The solution was even more of a challenge without MS.

This is the second mention of worn brushes this week.

the landing gear motor was the victim of another pilot.

Best regards,

-a-