Interference in GPS

[Dieselsnplanes]

Recently had a GTN 750 installed with some other instrument upgrades and it now has this odd quirk. It’s a 67 M20F model and previously it had nothing for avionics other than a storm scope and LORAN from 1993. 
The issue is when the 750 is running ONLY IN THE AIR, with turning the landing light ON. It gets a power interruption and causes the screen to go out and static interference in the headset. The installer has checked all the grounds and power and it refuses to do it on the ground. The landing light is a LED along with the rest of the lights, they do not interfere with it. I come from an automotive electrical background and it seems to be a back feed of some sort but my A&P says the main bus and avionics are completely separated. I don’t know enough about the electrical systems on an aircraft and I’m reaching out to see if anyone has had a similar issue after avionics upgrade. All grounds and power have been checked and it only does it when in the air. According to Garmin the screen goes out after a power interruption not more than 50ms and the unit resets after 500ms of lost power. I’m only loosing the screen and getting electrical interference in the 750 radio via the headset. Any pointers would be great or where to look.

[carusoam]

Welcome aboard DNP...

Sounds like you are onto the fix already...

What do you have for a landing light?

The original GE bulb is known for using lots of power...

It is really odd that your landing light could rob enough power from your radio...

You may want to review your wiring, compare it against the drawing, and find out why the radio and the landing light are the least bit interconnected...

Often, new owners find that the instrument panel can be missing the grounds that are expected to be there...

Cleaning connections on some things is often helpful...

Mooney electrical schematic can easily be found in the POH.

With your background, problem solving is going to be pretty easy...

Does your ship have a JPI?  
You can download it’s data, port it to Savvy.com, enable sharing, post link here

Is the GTN the only thing getting affected, or does everything else go dim or dark momentarily?

PP. thoughts only, not a mechanic...

Best regards,

-a-

[whiskytango]

I would start to think about what changes when you are in the air:

1. Gear is in retracted position

2. Engine thrust, drag and aerodynamic loading causes minor deflections in the airframe that are not present when it is static on the ground

3. Thermal expansion / contraction

These are just a few that come to mind.

Could it be a pinched wire / scuffed insulation?  Marginally loose connection?

Intermittent problems can be challenging.  

 

 

 

[Browncbr1]

Sounds like you need a different avionics tech working on it.

Apart from that, make sure no wires are chaffing out on airframe intermittently grounding.  I had that happen with shielding inside one of the nacelles affecting a jpi.  

Edited August 20, 2020 by Browncbr1

[Dieselsnplanes]

Seems to only do it when coming down from cruise or when the landing light is turned on, I’m not sure if it’s the unit since everything’s been checked out or something more simple like low voltage 

[takair]

I would think the voltage on the Garmin would need to drop under 10 volts to see what you describe.  I would clip a meter to the GNS breaker and see what the meter tells you (while flying). While technically a different bus, electrically there is really not much isolation between the main bus and the avionics bus.  It is really only a switch or relay junction apart.  Do you have a voltmeter in the plane?  What is it reading.  My guess is, as others described, that you have a short or partial short.  My guess is it within the engine compartment, possibly adjacent the landing light.  Lots of sharp edges and not a great way to route wires in that area.  Has that been looked at?

[Shiny moose]

I would look at the GPS power, and ground, check pins, CB connection and ground connection check these for proper crimping. With what you offer for information this sounds like a loose connection. No other unit on the avionics bus flickering or reseting? 

Please post what you find. 

 

[FloridaMan]

Try putting a ferrite bead around the power leads to the landing lights. I had reception issues with my 430W after a lot of work was done and isolated it down to the landing light. 

[Dieselsnplanes]

I ended up finding the issue myself. I took my volt meter and found that with the landing light on and wiggling the wire from under the panel the light would flicker. Upon removal of the cowling the old 1966 wiring had been previously repaired with a butt connector and that connector had been rubbing on the alternator which rubbed through the plastic sheathing and the crimp was shorting out on the alternator.

Surprisingly this didn't trip the breaker for the landing light the entire time but made quite the spark show when wiggled.

I had my A&P repair the wire and have had no more issues. This was quite frustrating as I'm in light duty diesel repair and it took me 2.5hrs to understand the whole feed from B+ to each component and find the short. Something an A&P and an avionics shop didn't find. Nether are bad by any means but just the fact I simply didn't want to spend time looking for something I'm technically not allowed to be looking for in the first place. I had both telling me it was the unit and I was convinced it wasn't. 

I thank you all for your input and support!

[EricJ]

10 minutes ago, Dieselsnplanes said:

I ended up finding the issue myself. I took my volt meter and found that with the landing light on and wiggling the wire from under the panel the light would flicker. Upon removal of the cowling the old 1966 wiring had been previously repaired with a butt connector and that connector had been rubbing on the alternator which rubbed through the plastic sheathing and the crimp was shorting out on the alternator.

Surprisingly this didn't trip the breaker for the landing light the entire time but made quite the spark show when wiggled.

I had my A&P repair the wire and have had no more issues. This was quite frustrating as I'm in light duty diesel repair and it took me 2.5hrs to understand the whole feed from B+ to each component and find the short. Something an A&P and an avionics shop didn't find. Nether are bad by any means but just the fact I simply didn't want to spend time looking for something I'm technically not allowed to be looking for in the first place. I had both telling me it was the unit and I was convinced it wasn't. 

I thank you all for your input and support!

First, kudos for sorting it out.   I'll also point out that nothing prohibits an owner from troubleshooting a problem, if you're not disassembling or assembling complex structures that don't fall under Preventive Maintenance.   The good news is that removing inspection panels, cowlings, etc., and even repairing landing light wiring, all falls under Preventive Maintenance and can be done by an owner with a maintenance logbook entry.   The FAR 43 Appendix A list of Preventive Maintenance items specifically includes landing light wiring as an item that can be repaired by an owner.   I know it's kind of late now, but if it does happen again there's nothing stopping you from addressing it yourself as the owner.

Just FYI.   Glad you got it sorted out!

[RobertGary1]

On 8/20/2020 at 6:24 AM, Browncbr1 said:

Sounds like you need a different avionics tech working on it.

 

Yes, this! I would expect any good shop would just take the plane back and fix it without charge. Shouldn't make the customer fiddle around with it. That's how they earn their $10,000 in labor to install these things.

 

-Robert

[carusoam]

Congrats DNP!

+1 on Eric’s detail above...

We are allowed to find all of the challenges in our ancient systems....

A few we are allowed to fix ourselves...

With your knowledge... approaching your mechanic with “hey look what I found...” is a great way to get the fix started....

Great follow-up! Thanks for sharing the details...

Best regards,

-a-