Plotter dies when starting engine

[Sandibar]

I have a problem with my Garmin GPSMap 720 plotter. When I start the engine, it dies and needs to be re-booted. Last weekend I noticed a new, even more annoying development: While motoring and dropping the revs, putting the engine on neutral it also died.
The plotter is wired normally through the switch panel and no fuses pop when this happens. I have a separate 90AH starter battery and a 210AH housebank with a off-1--both-2 switch. Never use "both" but often charge also the house bank from the alternator. The plotter problem is not dependent which battery is used.
Any ideas what could cause this and how to fix it? I guess the starter motor and alternator current fluctuations are the root cause. Is there some way to buffer the electronics (thought the battery itself would do this), or is there some magic wiring rules that I have failed to observe?

[Astrid]

You seem to have the preferred system. I would hazard that perhaps your 1-2-both switch might be bad. You could replace it with a simple parallel switch to use in emergencies to start your engine. The 1-2-both switch is really the weak link in the system.

[Stu Jackson]

Before you question your switch consider the wiring, voltage drop and connections to your plotter.

[mrm]

Originally Posted by Sandibar

I have a problem with my Garmin GPSMap 720 plotter. When I start the engine, it dies and needs to be re-booted. Last weekend I noticed a new, even more annoying development: While motoring and dropping the revs, putting the engine on neutral it also died.
The plotter is wired normally through the switch panel and no fuses pop when this happens. I have a separate 90AH starter battery and a 210AH housebank with a off-1--both-2 switch. Never use "both" but often charge also the house bank from the alternator. The plotter problem is not dependent which battery is used.
Any ideas what could cause this and how to fix it? I guess the starter motor and alternator current fluctuations are the root cause. Is there some way to buffer the electronics (thought the battery itself would do this), or is there some magic wiring rules that I have failed to observe?

Symptoms look to me like a brownout type scenario (btw, if it is indeed the case, shame on Garmin for not including necessary reset circuitry in their design).

Since it happens on either bank, I would suspect oxidated/corroded wiring/contacts somewhere in the common path, but first check if batteries are fully charged and not sulphated. If batteries don't test OK, address that first.

From your description It looks like situation is getting progressively worse, so perhaps charging is inadequate (partially failed diodes in alternator??) . Did you measure voltages? Specifically:
- voltage on each bank after it had been unused for a while
- voltage on each bank while using starter
- voltage on each bank while motoring at high RPM
- voltage on each bank when idling
- how fast voltages drop after engine shutdown when normal loads are present

[jonnypaget]

My guess it's a neutral fault , try running a neutral straight from battery to plotter direct, tap into plotters neutral as close to unit as possible, I'd be interested to know if this cures the problem.

[Sandibar]

Thanks for the quick replies! A couple of additions to the story:

- The batteries are ok, both are 1-2 years old, have been maintained properly take charge and start the motor without problems. The house bank is wired to a Nasa battery monitor.
- Can't vouch for all the connections, will go over these when I get back to the boat
- The main switch is old, probably as old as the boat, 29 years.
- The problem has been there (except the revving issue) since I got the boat and added the plotter. Since then I've replaced all batteries and the panel. Cabling and connections to panel and from panel to plotter are original.

Would measuring the voltage at different points of the path from battery to plotter provide any clues? I.e. would a differing voltage while cranking the engine make it possible to narrow down the location of possible bad connections?

Edit: Thanks also to Jonny. Will try that, but would still need to find where the bad connection is.

[mrm]

Originally Posted by Sandibar

Would measuring the voltage at different points of the path from battery to plotter provide any clues? I.e. would a differing voltage while cranking the engine make it possible to narrow down the location of possible bad connections?

Certainly. Given a circuit with a charge source, a battery and a load, including multiple connections as illustrated, measuring abnormal voltage drop in a part of the circuit narrows down the search. For the pictured example, say:

voltage 'e:m' is 12V (battery voltage under load)
voltage 'g : o' is 10V (voltage at load)

so 2V are lost somewhere. Now you measure, for example:

'e:d ' = 0.1 V
'd:f'' = 0.2 V
'f:g' = 0.4 V
'm:l' = 0.1 V
'l:n' = 0.9 V
'n : o' = 0.3 V

Such measurements would prompt for closer examination of the l:n part of the circuit including wiring and connectors. Charging side can be tested using the same method.

[Sandero]

Wire your electronics "through" a small motorcyle 12 battery... it will maintain 12v and prevent spikes.

[Sandibar]

mrm - thanks a lot. Will grab the multimeter and start digging.
Sandero- thanks for the suggestion. Will try to tackle the root cause first, but will keep this in mind.

[boatpoker]

Originally Posted by jonnypaget

My guess it's a neutral fault , try running a neutral straight from battery to plotter direct, tap into plotters neutral as close to unit as possible, I'd be interested to know if this cures the problem.

"neutral" in a DC system !!! ?

[jonnypaget]

Originally Posted by boatpoker

"neutral" in a DC system !!! ?

Common/negative I'm sure you know what I meant!

[RaymondR]

Try putting a 2,000mfd capacitor across the power cable connections as close to the plotter as possible.

[Sandibar]

Originally Posted by RaymondR

Try putting a 2,000mfd capacitor across the power cable connections as close to the plotter as possible.

Interesting, this would counter any short time voltage fluctuations, I assume? I remember from my past manufacturing ship luminaires that a capacitor was specified in all the fluorescent lights we delivered.

[Cheechako]

A couple of my boats , at least did that. The gps etc restarts when you start the engine. One thing that fixed it was a battery isolated from the cranking/charging circuit.

[Nightcrawler]

First, no expert here. My Garmin and Simrad Autopilot were acting up, dim screen, rebooting. All brand new installations, new hardware 2016. Finally, after checking every single connection, I ran a voltage check across the 12V breaker. Simrad was getting 6V, Garmin 8V. Checked several more and found other inconsistencies as well. Since all these devices are fused in-line, tried tying them directly to the 12V bus in the panel. Problem solved.
I'd check the voltage across the breaker(s) first.