High/Low Voltage Alarm

[tmp1545]

After approximately 1 1/2 hours in flight yesterday (1979 M20K) my high/low voltage alarm started to "flicker" on and off sporadically for approximately 30 minutes.  The alarm never indicated a steady alarm.  The last 15 minutes of the flight there were no further alarm indications.  Any thoughts as to what may be the cause?  Thanks for any responses.

[ArtVandelay]

My 78J does the same thing on occasion, I verify its a phantom indication by checking my engine monitor. If it becomes a hard error you can recalibrate as per service manual.

[cnoe]

Rumor has it that cleaning the adjustment potentiometer with some spray electronics cleaner followed by turning the adjustment back and forth a few times followed by recalibration will make that problem go away. Here's a photo of the correct pot to clean/adjust (center of pic).

Sent from my iPad using Tapatalk

[NotarPilot]

Where is that circuit board located?

[cnoe]

Annunciator main PCB. It's an easy remedy, particularly if you have a variable DC power supply to do the calibration. My service manual calls for a set-point of 12.5V.

Sent from my iPad using Tapatalk

Also, note the two blue pots partially visible at the top of the pic; those are the low-fuel adjustments. I wouldn't mess with those.

[carusoam]

Nice photo CNOE.

Parts of a Mooney that aren't seen very often.

Best regards,

-a-

[Jeev]

Thank you for the pic CNOE!  My 78 J is doing the same thing while my engine monitor is steady at 14.1V.  I read the maintenance manual and it states to remove the glare shield, then the font panel of the annunciator.... Is it that self explanatory when you get a look at it under the glare shield or are there any tricks to it?

Thanks again,

SP

[N231BN]

It is easier to remove the annunciator from the aircraft to work on it. My '79 231 did the exact same thing. Cnoe's reply is spot on, that is exactly what I did to fix it.

[N231BN]

To add a little more to the subject, I think the issue comes from fretting corrosion in the pot due to the adjusting screw being a little loose. You can see where it was originally painted to somewhat secure it. I put a dab of super glue on the pot to hold it in place after adjusting it. It has flown about 50 hours now without the light blinking on me, hopefully it is good for another 2300.

[Jeev]

On February 17, 2016 at 9:39 PM, cnoe said:

Rumor has it that cleaning the adjustment potentiometer with some spray electronics cleaner followed by turning the adjustment back and forth a few times followed by recalibration will make that problem go away. Here's a photo of the correct pot to clean/adjust (center of pic).

Sent from my iPad using Tapatalk

This worked for me

[Jayhawk_aviator]

What type of electronics cleaner...the spray on screen cleaner or something else?  For cailbrating the pot on the bench, you apply regulated power to which pin then watch for the flashing?  Manual implies to do it on the aircraft while running and doesn't clarify either.

[carusoam]

We used to go to RadioShack to get electronics cleaner.  It is a spray can for cleaning and lubing plug parts.  This stuff does not conduct electricity.  It dissolves light corrosion issues.

Try not to get fooled by a screen cleaner.  That may conduct electricity with crummy results...  

letting the smoke out of an electronic device can get expensive quickly...

Best regards,

-a-

[cnoe]

CRC sells a product called QD Electronic Cleaner that I've had good results with. You can even buy it at some Walmart locations. I wouldn't use "contact cleaner" as it often has a lubricant residue. Long gone are the days of TF Degreaser which was an outstanding electronics cleaner (but very bad for the ozone layer).

I can't quote the pin number off the top of my head but it should show on the wiring diagram or board edge as simply 12+ or similar. If you can't find the info let me know and I'll look it up when I have a spare minute. As you stated just power up the board on the appropriate connections and set the voltage alarm as required using the pot shown.

Sorry for the brevity, headed out the door in moments.

[Jayhawk_aviator]

thanks all.  Used the QD cleaner and did the procedure, recalibrated per the mm.  didnt seem to fix it unfortunately.

[rcwilbur]

On 2/18/2016 at 7:31 AM, cnoe said:

 

Annunciator main PCB. It's an easy remedy, particularly if you have a variable DC power supply to do the calibration. My service manual calls for a set-point of 12.5V.

Sent from my iPad using Tapatalk

Also, note the two blue pots partially visible at the top of the pic; those are the low-fuel adjustments. I wouldn't mess with those.

I know this thread is a couple years old, but my annunciatior panel high/low voltage is flashing even though my Engine Monitor shows 14 volts.  I have 78 m20j and have read the service manual sections and your posts here.  Adjusting the pot seems straight forward, but do you have any information on the calibration procedure while adjusting the pot?

Thanks.

[carusoam]

Let’s see if Chuck answers... he was in the neighborhood just the other day.... but hasn’t posted much lately...

What part of the Maintenance procedure wasn’t usable?

I see chuck mentions using a variable voltage supply...

PP thoughts only, not a mechanic...

Best regards,

-a-

 

[cnoe]

Hi guys! True, I'm not on MS that often these days but I'm looking for help myself on another issue (may post a question shortly) and saw this inquiry.

So in order to earn my keep I'll attempt to help you out...

As I recall it's a pretty straight-forward process to tweak the potentiometer and eliminate the false alarms. For best results I'd recommend using a variable DC power supply such as this $60 unit on Amazon. https://www.amazon.com/RoMech-30V-Power-Supply-Variable/dp/B081SFKW2R/ref=sr_1_5?dchild=1&keywords=30V%2BDC%2Bpower%2Bsupply&qid=1600368169&sr=8-5&th=1

You can also use this power supply to exercise and evaluate your old autopilot servos and other such fun stuff!

Remove the annunciator unit from the panel and open up the box to expose the PCB. The correct pot to adjust is shown in the middle of the attached pic; it's the one with the red blob of sealant on it. With the power supply preset at perhaps 10V and a nominal amperage (1A?) turn the power supply "OFF". Then connect the positive lead of the power supply to the appropriate connector pin on the bottom of the PCB and the power supply's ground lead to the PCB's ground pin. I don't remember exactly which pin numbers these are so if you don't know and don't have the schematic let me know and I'll try to find that info.

From there you turn the power supply "ON" and confirm an output of <12V and confirm that the "LOW VOLTAGE" annunciation light is ON.  Now slowly increase the voltage output until the "LOW VOLTAGE" annunciation light goes OFF and note the voltage (I wouldn't go any higher than 14.5V or so out of caution). NOTE: The previous step is just to give you an idea how far out of adjustment it is and isn't critical or even necessary.

To make the adjustment set your voltage back to ~12.5V output on the power supply and then use a small screwdrived to break the potentiometer's adjustment screw loose and turn left or right until the annunciation light just extinguishes. After making the adjustment fine tune things by again reducing the power supply output voltage from ~14V very slowly until the annunciation light comes on (note the output voltage at that point). Make minor adjustments until the light comes on when voltage is at 12.5V. Once you've got it set put a little dab of torque-seal or fingernail polish on the pot to keep it from vibrating back out of adjustment.

That's pretty much all there is to it. Feel free to inquire further if needed.

CNoe

[carusoam]

Hey!

Nice to see you CNOE!

Best regards,

-a-

[cnoe]

4 minutes ago, carusoam said:

Hey!

Nice to see you CNOE!

Best regards,

-a-

A pleasure as well, as always. I indeed just posted my own inquiry on the Modern Mooney section. No big problem, in fact I just got out of Annual a week ago and am doing well. Hope the same for you and all!

[tls pilot]

In most folks opinions who use electrical contact cleaners, a superb product is

Caig Labs DeOxit D5 or D100L ( is 100% deoxidizer/cleaner) a little goes a long way

It is available on Amazon https://www.amazon.com/CAIG-LABORATORIES-D100L-25C-Contact-Cleaner/dp/B0000YH6F8/ref=pd_sbs_267_4/132-3351496-1120008?_encoding=UTF8&pd_rd_i=B0000YH6F8&pd_rd_r=7bac1973-efde-4008-9101-87dd6463c0e7&pd_rd_w=mOsrM&pd_rd_wg=jXtKz&pf_rd_p=703f3758-d945-4136-8df6-a43d19d750d1&pf_rd_r=1JT1VEHK1TR3Z079DEQN&psc=1&refRID=1JT1VEHK1TR3Z079DEQN

Edited September 17, 2020 by tls pilot

[rcwilbur]

4 hours ago, cnoe said:

Hi guys! True, I'm not on MS that often these days but I'm looking for help myself on another issue (may post a question shortly) and saw this inquiry.

So in order to earn my keep I'll attempt to help you out...

As I recall it's a pretty straight-forward process to tweak the potentiometer and eliminate the false alarms. For best results I'd recommend using a variable DC power supply such as this $60 unit on Amazon. https://www.amazon.com/RoMech-30V-Power-Supply-Variable/dp/B081SFKW2R/ref=sr_1_5?dchild=1&keywords=30V%2BDC%2Bpower%2Bsupply&qid=1600368169&sr=8-5&th=1

You can also use this power supply to exercise and evaluate your old autopilot servos and other such fun stuff!

Remove the annunciator unit from the panel and open up the box to expose the PCB. The correct pot to adjust is shown in the middle of the attached pic; it's the one with the red blob of sealant on it. With the power supply preset at perhaps 10V and a nominal amperage (1A?) turn the power supply "OFF". Then connect the positive lead of the power supply to the appropriate connector pin on the bottom of the PCB and the power supply's ground lead to the PCB's ground pin. I don't remember exactly which pin numbers these are so if you don't know and don't have the schematic let me know and I'll try to find that info.

From there you turn the power supply "ON" and confirm an output of <12V and confirm that the "LOW VOLTAGE" annunciation light is ON.  Now slowly increase the voltage output until the "LOW VOLTAGE" annunciation light goes OFF and note the voltage (I wouldn't go any higher than 14.5V or so out of caution). NOTE: The previous step is just to give you an idea how far out of adjustment it is and isn't critical or even necessary.

To make the adjustment set your voltage back to ~12.5V output on the power supply and then use a small screwdrived to break the potentiometer's adjustment screw loose and turn left or right until the annunciation light just extinguishes. After making the adjustment fine tune things by again reducing the power supply output voltage from ~14V very slowly until the annunciation light comes on (note the output voltage at that point). Make minor adjustments until the light comes on when voltage is at 12.5V. Once you've got it set put a little dab of torque-seal or fingernail polish on the pot to keep it from vibrating back out of adjustment.

That's pretty much all there is to it. Feel free to inquire further if needed.

CNoe

Thanks very much for the detailed post. If you have the schematic with to pin out that would be very helpful. The Mooney maintenance manual does not give a pin out. Thanks very much for your help.

[cnoe]

My schematic .pdf files are pretty crappy but this shows the pin assignments for serial #s 24-0378 thru 24-0757.

I don't remember for sure but you can try applying + voltage to pin #11 and ground to pin #15. If that's not right then you might use pin #1 for the + voltage.

As I recall they may be fairly well marked on the board.

Hope this helps. I'll check back to see how it goes.

CNoe

P.S. Having trouble with attachment. Will try in a second post.

 

 

[cnoe]

Sorry for the delay. MS seems to be down a bit lately???

Here you go.

CNoe

[rcwilbur]

Thanks very much.  I will let you know how it goes.