Garmin MFD voltage swings?

[Marco Flamingo]

My Garmin had more trouble than usual getting GPS sattelites last time out. I'm in covered (metal) moorage, so I often don't see Lat Lon until minutes after leaving the marina. But last time out, it was more than half an hour and then I lost coverage several times for +20 minutes at a time. Unacceptable. And, maybe related, the depth sounder wasn't registering most of the time.

I thought it might be voltage related. For some reason, when I turn the MFD to the "fishing" screens, it shows voltage. At the dock without the engine running, I got a low voltage alarm several times (it is set at 11.8V). So I removed the power connection from the back of the MFD and put my multimeter on it. 12.60V. My battery monitor, connected directly at the batteries, showed 12.61V, for a drop of .01V. Turning on the battery charger increased voltage, but the power connector at the MFD was always only .01V less than the battery monitor.

But watching the voltage numbers given on the MFD showed that it waivered down below 11.8 (causing the alarm to go off) and then back up to 12.2 or 12.3 volts. Radar was off. And the sounder didn't find the bottom when sitting in 15-20 feet of water.

The MFD is on a nav instrument circuit breaker with the VHF and another depth sounder. Turning both of those off made no difference. Before I get too far into troubleshooting, I was hoping somebody might have an explanation why the MFD was registering voltage swings when the multimeter on the power connection (and the battery monitor) wasn't.

 

[High Wire]

What you didn’t address is what is the voltage at the MFD leads with the MFD turned on. Testing with the leads removed also removes the load. My Garmin 5212 draws 38 watts/3.2amps. That could drop the voltage as you are seeing if a bad connection in the circuit.

 

[C lectric]

HW +1

Testing of this type for this reason always needs the load connected.

Remove the load and almost any lousy connection can pass enough current to give a DMM enough to read a good voltage.
DMMs only need on the order of a few microamps in many/most cases to give a good reading. Good as they are, DMMS can easily be fooled for this reason.


I sometime have used a very fine wire strand inserted into a plug so the connection can be completed , plugging in, and allow the DMM to be connected.
That should give a decent idea of whether or not you have excessive Vdrop in the supply circuit. Don't for get the ground if this shows high Vdrop

If that still shows good then it could be the MFD wiring or plug itself.

Sometimes oxidation, very light corrosion, can cause this meaning the plug pins need cleaning. Carefull as a bent pin is trouble. Use a good electronic/electrical corrosion removal spray and a soft toothbrush. Replugging once or twice will clean the sockets, We also used to use a pencil eraser, soft one on the pins.



If that does it then get some silicone GREASE or dielectric grease and smear
the pins with a light coat so when you replug it the grease will seal the air out
giving , should give, a longer life.

 

[High Wire]

My #1 culprit for problems like this has been corrosion of those inline fuse holders that come on electronics. I'll put it at 9 out of 10. #2 has been PO's wiring.

 

[Marco Flamingo]

Thanks. More things to check out.

 

[Turtle Blues]

Suggest when checking;
Is the MFD fed from the main 12v circuit panel. I have seen temp load fluctuations due to other devices drawing current, fridge being a primary culprit.
Daisy chaining alternator charging circuit can allow voltage drops.
Recently saw setup where primary charging went to start battery 1st. then house battery 2nd. Breaker panel fed by house battery. The MFD voltage display showed the dips in voltage as the alternator ramped up and down feeding this loop.
As HW says accurate voltage reading must be with circuits under load.
Also verify wire size is adequate for load and length of run. Undersized wire will add to the problem even before it gets warm.

 

[twistedtree]

All good suggestions, especially to pay just as much attention to the ground circuit as the power circuit. Weakness in either will give you the same symptoms.