Dash voltmeter inconsistency

[imranino]

Hi there,

 

I wonder if anyone could help me figure this out, it's to do with the voltmeter on my dash board.

 

With the engine off, it looks like this:

 

 

When I turn the key to warm the glow plugs, it looks like this (starter battery is dead):

 

 

When the engine has just turned on (here in neutral, the dial goes up with more revs)

 

 

But soon thereafter it goes like this and stays there:

 

 

This despite my multi meter giving a reading of around 14.4v.

 

Any ideas?

 

Thanks in advance

[Jess--]

put your multimeter across the back of the meter, if that reads high (like the meter) then trace it's wiring back.

if everything looks ok then check the connections between the alternator and batteries (a poor connection here could cause all sorts of weird readings)

 

if your multimeter shows 14.4v at the back of the meter remove the meter from the panel and install it in the nearest bin

Edited December 1, 2016 by Jess--

[Iain_S]

On the other hand, although inaccurate, it is giving a useful indication, dipping to 10V (indicated) when the heater plugs are on, recovering to 12V or so when the battery is not loaded, and increasing when the battery is being charged.

 

When this changes, something is wrong ... (which might be the meter)

[alan_fincher]

The most important thing, I would say, is to be totally confident that your alternator is not delivering that very high indicated voltage in the last picture to your battery bank.

 

Unfortunately when our meter elected to show something not unlike your last picture, it was actually a genuine reading, needing urgent attention.

 

So, before scrapping the meter, make absolutely sure it is lying!

[NB Lola]

Moved to boat building and maintenance

Edited December 1, 2016 by NB Lola

[imranino]

Thanks for the replies!

 

Here's what I found: the readings taken by my multi meter behind the voltmeter corresponded to begin with. Both read around 12.8v when the engine just started. At the same time a reading on the battery read 14.1v

 

Perhaps even more confusingly, after about 10 minutes the dash voltmeter jumps up to 16+v, my multimeter reading behind reads 12.8v and the battery reads 14.1v

[Iain_S]

Thanks for the replies!

 

Here's what I found: the readings taken by my multi meter behind the voltmeter corresponded to begin with. Both read around 12.8v when the engine just started. At the same time a reading on the battery read 14.1v

 

Perhaps even more confusingly, after about 10 minutes the dash voltmeter jumps up to 16+v, my multimeter reading behind reads 12.8v and the battery reads 14.1v

 

That indicates the dash volt meter is faulty. Possibly an internal resistor that partially shorts out when it gets warm?

[imranino]

 

That indicates the dash volt meter is faulty. Possibly an internal resistor that partially shorts out when it gets warm?

 

Ok that makes sense. So is it normal that the voltage at the voltmeter (both on the display and on my multimeter) is different to that on the battery terminals (12.8v and 14.1v, respectively)?

[WotEver]

 

Ok that makes sense. So is it normal that the voltage at the voltmeter (both on the display and on my multimeter) is different to that on the battery terminals (12.8v and 14.1v, respectively)?

Nope. One of two possibilities (or both)

 

1. Poor connection between battery and meter.

2. Under-sized wiring giving large voltage drop.

 

Tony

[Sir Nibble]

Do remember that the thingy marked volts on your panel is not an instrument it's an indicator. The numerical reading is unimportant, behaviour is everything.

[jonathanA]

Not worth messing about you can get a new one off eBay for a fiver from the inscrutable orientals on eBay

 

Disconnect the gauge and check the voltage one wires, there's something else going on here.

[imranino]

EPILOGUE:

As suggested above, I replaced the volt gauge and indeed no longer having any trouble. Still showing a slightly different reading to what's on the battery, but I'll put that down to volt droppage?

[WotEver]

It could be voltage drop or it could simply be that as Snib points out above such indicators are hugely inaccurate.

[alan_fincher]

It could be voltage drop or it could simply be that as Snib points out above such indicators are hugely inaccurate.

 

You can always apply an accurate multimeter directly to the connections on the new gauge, which will establish if the difference is purely down to the limitations of that type of gauge.