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The horse is dead, but I'll beat it again - low voltage & alternator issues

MooneyMike

By MooneyMike
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MooneyMike Contributor

MooneyMike

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Greetings everyone.  Just before takeoff (whew!) last week I got a low voltage annunciator alarm and saw a discharge on the ammeter of ~ 25 amps.  I had seen this once before and it went away after cycling the master and avionics breakers.

But not this time.

The alternator tested fine and on a bench I saw it put out 69 amps.  

I measure 9.74 volts on the field lead of the alternator which is a bit low (Master switch on, engine off of course).  

Across the alternator field circuit breaker I measured 0.23 volt drop and after cycling the breaker a number of times it did not change.

From the field pin on the VR Cannon plug (pin B) to the field lead (removed from the alternator) measured 12 - 15 ohms.  

The VR Cannon plug (pin C) to ground was 1.2 ohms

VR Cannon plug input (pin A) measured 12.3 volts when the battery buss was reading 12.5 volts.

 From appearance, the OECO VR looks original so I've ordered a Zeftronics unit. If it is original, once cannot complain about 42 years of service!

What should be the resistance from pin B of the VR Cannon plug to the field lead?  It is a switched circuit that goes through one of the two poles of the Master Switch.  

The measured resistance seems a bit high to me but I don't know if it accounts for my low field voltage reading.  I've been working this problem with the advice/guidance/assistance of the local shop A&P so I'll ask them and maybe give LASAR a call too.

Thanks all,

Mike

N201XF

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carusoam Grand Master

carusoam

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There is a thread around here with the innards of the sticks and stones and springs voltage regulator exposed...

Read that thread on your way to the Zeftronics store... :)

The thread has all the details you need to set-up the old battery killer... :)
 

Ever think about buying a BatteryMinder?
 

If you had the slightest thought about buying the BatteryMinder...  go directly to the Zeftronics store... :)


There is only one thing that is better at killing a Gill Battery other than its brief natural age limit... the sticks and stones and springs voltage regulator... :)
 

Attempted PP humor only, not a mechanic...

Best regards,

-a-

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MooneyMike Contributor

MooneyMike

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Found this burnt connector (the blue one) on the Master switch.  The lead pulled free from the connector when removing so that may be the culprit for higher than expected resistance readings.  The burnt look is rather suspect. 

 But I was unable to make any resistance readings from pin B to the alternator field lead today.

Obviously I'm anxious to see if that fixed it.  Being unable to find a drawing for the switch I can only guess this lead is for the alternator circuit.  

I also replaced the ring terminal connector at the alternator but my resistance readings were with the new connector.

In the next couple days a package from Aircraft Spruce will be on my porch and I'll replace the Cold War -era voltage regulator and see if it will deliver 42 years of faithful service.

More to follow no doubt.

IMG_2141.jpg

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PT20J Grand Master

PT20J

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It doesn’t take much battery to excite an alternator. 

It sounds like you had the alternator tested and it’s fine. That leaves wiring or the voltage regulator. Since you measure voltage at the field terminal of the alternator, I’d bet on the regulator. Thirty years ago I had an OECO regulator fail on my ‘78 M20J. I replaced the power transistors and it worked fine after that. The hardest part was getting it in and out.

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MooneyMike Contributor

MooneyMike

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1 minute ago, PT20J said:

It doesn’t take much battery to excite an alternator. 

It sounds like you had the alternator tested and it’s fine. That leaves wiring or the voltage regulator. Since you measure voltage at the field terminal of the alternator, I’d bet on the regulator. Thirty years ago I had an OECO regulator fail on my ‘78 M20J. I replaced the power transistors and it worked fine after that. The hardest part was getting it in and out.

Skip

 

 

I'm replacing the VR, and yes it was a pain to get out.  No quite the shotgun approach to troubleshooting but not far from it either.  When the new one comes I'll measure the field circuit again for resistance and see if the new terminal rectified the problem.  (yeah, an electrical joke, yuck yuck.  A horse walks into a bar and the bartender asks ... )

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MooneyMike Contributor

MooneyMike

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2 hours ago, Paul_Havelka said:

With that burnt connector I would imagine that the connection inside of it was definitely suspect.  Most of the time I like to go ahead and flow solder into those connections.  Remove the plastic insulator and use shrink tubing after soldered.

 

That's a good idea.  

thanks,

Mike

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larrynimmo Experienced

larrynimmo

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Ironically...on my 1981 M20J replacement of the voltage regulator will go down in my book as the only easy change out of a factory part...it is so easily accessible on co-pilot side directly accessible in the “knee” area....even the wiring has a twist lock connector. (My AI had a Good used one) 

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N201MKTurbo Grand Master

N201MKTurbo

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The voltage regulator regulates the current through the field to control the voltage output of the alternator. 
 

The voltage at the field terminal is not as important as the current, but current is harder to measure. With the engine off, the max field current should be flowing, but that current isn’t widely known. I would think 9 volts would be plenty. If you measured the voltage at the field with the engine running, after the battery is charged, it would be quite a bit lower than that.

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