M20K 252 alternator(?) issues?

[Jlocke]

Any help or advice most appreciated!

A month ago I was flying to Dallas and when about 18 miles out noticed that the "DC %" indicator read zero.  Yikes!  I slowed, turned off any electricals I didn't need, lowered the gear and landed.  While taxiing everything went dark.  The maintenance folks at the FBO traced the problem to a bad alternator and broker coupler.  They replaced those and my partner in the plane flew out commerically and flew the Mooney home.  He then flew it on a couple of trips averaging maybe 1.5 hours each.  He reported that all was fine.

Today, I went to fly a little and upon start-up noticed the "DC Volts" indicator reading zero, the "Load" reading very low and the "Alt %" at 100%.  While I guess that looks fine, the POH says it indicates a low-battery condition with the alternator not contributing any juice.  So, I shut it down.

The POH also refers to a button that can be pushed on the loadmeter to check the load.  Pardon my ignorance...but I don't see a button on the instrument!  I've attached a pic of the panel showing the instrument I'm reading; it's on the far left.

Any thoughts at all?  Ideas?  Surely the dang alternator hasn't gone out again...right?

[jackn]

My 252 is about the same vintage. Looks like you have a rare single alternator version, thats why you don't have a push button(see my picture). Do you have a picture with the enine running and avionics master on?

[Jlocke]

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I just got back from the airport where I gathered some more information.  Below are two pictures.  The one on the left shows the indications with only the master switch on.  The picture on the right shows the indications with the engine running at idle.

Also, at idle, the annunciator panel indicated a flashing "High Low Volts" red warning light.  The POH says this means the alternator is producing no or low voltage.  If I ease the power up to 1400 RPM the light goes out.  Still, no voltage is indicated on the "DC Volts" indicator.

As I was leaving, a guy working on a Seneca in the hangar suggested recycling the alternator field by turning the switch off and on while the engine is running.  Sounds reasonable, I guess.  But, even if this works, this still indicates a problem somewhere, right?

All this is starting to look like what I saw descending into Dallas; no "DC Volts" indicated and the "High Low Volts" red warning light coming on in the pattern...with lower RPM's.

Again, any thoughts are most welcomed!

[kortopates]

Jim,

Looks like Jack explained why you don't have the button. Notice your meter is already showing volts at the bottom. On our dual alternator meters, the lower scale reads provides two outputs, load without pushing the button and Volts with the button pushed. I haven't seen a POH for the single alternator verison, but if it doesn't also discuss your meters layout, I would be suspicous that your POH may have been replaced with the wong version and would suggest containing Stacey Ellis at Mooney (you could use technicalsupport@mooney.com).

A very high alternator output is indicative of a low battery, but you should not be seeing 0 volts. Something is wrong -   You would not have been able to start up with 0 volts. There are some fuses on the back of the meter itself (at least for the dual alternator version) and maybe one is popped, or a bad econnection etc. But besides the meter not registering battery voltage, if the high load reading is accurate I'd check the battery too. It may be very weak.

[Jlocke]

Paul,

My run-up procedure calls for checking the voltage output (on the bottom part of the meter).  It indicates voltage (normally) when the engine is started.  Then, during run-up, you advance the throttle to 1200 RPM and turn off the alternator switch.  The indicated voltage should drop to 24 volts.  Turning the alternator switch back on should return it's output reading to 28 volts.

Well, obviously, none of this can be done/checked when the indicator shows nothing.

And, as you pointed out, the 100% indication for the alternator percentage is indicative of a low battery.  But, the battery has started the engine several times and has provided the panel with electricity for about twenty minutes of tinkering with things.

Thanks for the email address; I will drop him a note.  I appreciate the reply and your help!  Since the alternator was just recently replaced maybe (hopefully) we're looking at some minor problem.

[kortopates]

Jim, What your neighbor suggested is the first thing you should have tried the first - but see your POH emergency procedure page 3-19 Alternator Low Voltage.

I re-looked at page of 7-48 of the POH is describes both meters for single and dual alternators - so I am not concerned about your POH being the wrong version - just your understanding of what you have. Notice the warning of not starting the engine with an external power source with a depleted battry. Have your A&P check our the lack of voltage reading and potential low battery issues before next flight. Good luck!

[kortopates]

Jim, The run-up practice of turning off a alternator is only necessary with two alternators so that you can see each alternator is actually functioning at the proper 28.1+ voltage. With two alternators the #2 takes almost the whole load and you can't really see that the #1 is putting out till you fail #2 by turning it off. That said, keep folllowing your POH which still has you following the same procedure. But recognize you'll see the Annunciator voltage light blinking when you turn on the master, as well as 24volts on the meter (when working) and then see it the light go off and voltage go up to 28V after start up to verify the system - thus you can make the checks without failing/turning off the alternator after startup. If you by chance have a JPI or the like engine monitor that also reads system volatge you can verify proper volatge there. I beleive your annunciator is already verifying this but would expect the light to come off well before 1400 RPM  and more in the 1100-1200 RPM range (unless you have everything turned on in your electrical system like landing lights and pitot heat!). This is further evidence that your battery is too week. Its also possible you have a regulator issue but right now the symptoms suggest an issue with the meter or wiring to it - which hopefully is very simple (meters themselves are very expensive) and  likely a weak bettery.

Incidentally if you don't yet have a engine monitor - seriously consider getting one installed soon. Your engine is much to big of an investment to operate it without one.

Good luck! 

[jackn]

I don't know if you have a bad alternator or not. Alternators have a notorious reputation for infant mortality(no other polite way of saying that). I feel confident you have a short somewhere in your electrical system. Good luck.

[Geoff]

Jim,

I offer this as an alternative (no pun intended) line of inquiry...

Although I fly a 231, I had a similar situation occur.  You mentioned that the alternator was recently replaced, but did they replace the alternator coupling at the same time?  If not was the coupling properly installed on the new alternator?  The coupling has a 500hr inspection cycle I believe.  Check your logbooks to see when it was last inspected/replaced. 

[Jlocke]

Many thanks for all replies and ideas--and wishes of good luck!

Paul, I do have a JPI and will check the voltage reading there.  Good idea.  I didn't think to do that.

Geoff, yes, the coupler was replaced at the same time as the alternator.

[Jlocke]

Quick update:

We have a Davtron instrument in the plane that is apparently connected to the alternator output.  We think this is the case because it shows (digitally) around 24 volts with the engine running and the Alt Field switch off and then a shade over 28 volts with the Alt Field switch on.  In other words, it shows readings like you would expect the standard instrument to show.

My partner flew the plane yesterday, three hops, all with the voltage showing as you would expect on the Davtron digital readout and with no degradation of the battery.

I'm guessing we're dealing with a loose wire or a bad "normal" instrument (the one in the pictures I posted above).

Still, any thoughts are most welcomed.

[jackn]

If I believed my ship's gauges, I would be flying around with 0 oil pressure(a bad thing). That is why I installed an edm-830, and am upgrading that to a certified 930. Just be careful, the gauge may be trying to tell you something.

[kortopates]

I'd suggest pulling off the left subpanel enough to  connect a multi-meter to the ammeter connections and see if you are getting voltage there or not. If you aren't trace the wiring back till you find the problem and if you are gettting voltage, besides the meter it self, there are fuses right on the meter. Electrical part numbers for fueses and shunts (and your meter) are  listed in the back of the Mooney Maint. manual just in front of the electrcal schematics in the rear. 

However, I wouldn't be concerned with the lack of voltage given you have the another way to get voltage off your (uncertified) davtron,  but I would be very concerned about the 100% load on the alternator and wouldn't fly it till the problem is resolved.

[kortopates]

I thought I'd leave an update to this thread, because I had a very similar issue this past weekend. My load meter went to 100% on both alternators and the third scale, total load and voltage went to 0. Yet my JPI indicated normal buss voltage of 28+. With engine off, both alts loads dropped down to zero, but volatge needle still nothing, with only the master on needles pegged again. I thought for sure I was going to need a new load meter (2 to 3 amu!) but upon further investigating, I had a couple blown fuses and that's all it was. I was lucky once again - 10 years ago I had a dead gauge that also turned out to be blown fuses. Unfortunately the fuses are hard to get at but probably worth checking early on if you have a load meter issue.