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Confusing voltage reads: Davtron = 28v, JPI 730 = 12v


pwnel

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Hi folks,

Became the owner of a '85 231 last week.  As is normally the case a couple of things got missed in the prebuy and I'm going through those now.  My Davtron M303 reads voltage at 28.3V while my JPI indicates it at 12.1V.  This is a 231 so it's a 12V system and battery so a read of 28.3V just shouldn't happen.  So I figure:

1) The voltage read circuit is INOP (OAT looks fine), or

2)  It is wired incorrectly somehow.

As an electronics engineer I've learned that even though this might be minor, squawks like these often lead to bigger faults somewhere, so I'm keen to figure this out. I'm also not trusting the OAT (which is critical), if the unit is reading voltage that far off.

I'm replacing my INOP clock with a Davtron M800, so if that panel is going to come off we might as well fix everything.

Any input would be welcome!

IMG_0256.JPG.21f8efb20ba57a5a06952f855b6e8956.JPGIMG_0257.jpg.569c4c528902d6ace5f74ade7540f3e2.jpg

Edited by pwnel
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1 hour ago, milotron said:

I am not that famiiar with later Ks; are you sure it is 12V? serial number -1000 and higher is 28V.  Not much to mess up on the Davtron. 2 wires, a sensor and no settings.

 

iain

Well, to be honest I didn't open up the avionics bay (yet) to put a meter on the battery. Just assumed that the 28V versions start with the 252 as I've read all over here.  (i.e, serial 25-1000 and up).  This is one of the last 231's built (25-0883), and I believe the 28V switch happened only with the 252.  (I would go and check, but it's 1h15min drive!)  Yes, not much to mess up with the Davtron, but mightly confused that the 730 would read 12V and this one 28V.

 

 

Edited by pwnel
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Mooney 28 volt systems started in 1986 with the 252 (Serial #25-1000). The 201 didn't switch to 28v until 1987 (Serial #24-3000).

The JPI is reading correctly, but the 28 volts showing on the Davtron is puzzling since all it is doing is just displaying the voltage it is measuring.

http://www.davtron.com/cmsAdmin/uploads/m303_brochure.pdf

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Just now, LANCECASPER said:

Mooney 28 volt systems started in 1986 with the 252 (Serial #25-1000). The 201 didn't switch to 28v until 1987 (Serial #24-3000).

Yip. Still no idea then why this shows 28V.  The M303 is OAT covers all voltages from 10V - 32V so should work in both a 231 and 252.  (Davtron's M800 clock has 14V and 28V versions - and that's how this started - I need to order the correct clock version).

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On 4/14/2018 at 10:21 PM, pwnel said:

Yip. Still no idea then why this shows 28V.  The M303 is OAT covers all voltages from 10V - 32V so should work in both a 231 and 252.  (Davtron's M800 clock has 14V and 28V versions - and that's how this started - I need to order the correct clock version).

A lot of people are going with this clock now since it has usb charging ports. I get 7% off on these plus free shipping through Spruce if you decide you want one.

http://www.aircraftspruce.com/catalog/inpages/mcicharger10-05774.php?clickkey=3849#

 

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Probably want to check for a transformer in the panel or back shelf there somewhere...

You may have a radio or two installed that is 28V...  and the Davtron is connected with it...

How does that sound, got a GPS that has been added to the panel?

PP thoughts only.  Some Mooneys have the opposite challenge.  28v with a step down transformer for the 12V cigarette lighter plug...

Best regards,

-a-

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8 hours ago, carusoam said:

Probably want to check for a transformer in the panel or back shelf there somewhere...

You may have a radio or two installed that is 28V...  and the Davtron is connected with it...

How does that sound, got a GPS that has been added to the panel?

PP thoughts only.  Some Mooneys have the opposite challenge.  28v with a step down transformer for the 12V cigarette lighter plug...

Best regards,

-a-

I suspect you're right. The OAT guage sits on the avionics bus - only active when avionics master is on.  Most likely a step-up converter was installed for the GNS 430 installed in 2000 (later upgraded to WAAS).  Which makes this most likely a 28V-only 430, which Garmin no longer supports either.

A step-up converter is a poor idea - it's noisy and probably will cause issues when I want to do a drop-in IFD440 replacement.  One more thing I would want to have cleaned up when I do a panel upgrade. 

12 hours ago, LANCECASPER said:

A lot of people are going with this clock now since it has usb charging ports. I get 7% off on these plus free shipping through Spruce if you decide you want one.

http://www.aircraftspruce.com/catalog/inpages/mcicharger10-05774.php?clickkey=3849#

 

Thanks, great suggestion!   Been flying with power bricks stuck into the side pockets to keep the iPad and Foreflight going on long x-countries.

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