Stock ammeter and 12V conversion - query

[fortyonerag]

I recently had a charging system problem on my 41. Car is stock except for a 12V negative earth conversion.


The stock ammeter has the inductive loops on the back of the gauge that the main power line feeds through.


The ammeter failed to show any deflection even though I was drawing on the battery with a dead generator (Armature is open circuit).



Do I have a defective ammeter, or are the 'loop' style ammeters marginal with the reduced current (compared to 6v) of the 12V system?


Pretty sure the ammeter is dodgy, but all advice appreciated.

[38 coupe]

If you didn't disconnect the regulator and generator you should have pulled enough current to show on the ammeter. Do you have another ammeter to verify with? I use an old Model T ammeter to verify operation since it is sensitive enough to read something while holding it next to a wire, and I don't have a clamp type multi-meter.

[fortyonerag]

I haven't had a chance to get my ammeter out yet. I use the guage as a general indication that the system is charging.

Now that the battery (Optima) is charged I pulled on the headlights. The meter 'wiggled' a little, but no real deflection to speak of. This is what fooled me before I think.

I think I'm in the market for a standalone ammeter guage.Does anyone make a really small one?

[Jack E/NJ]

I'd use an analog or digital voltmeter instead. Some simply plug into a cigaret lighter socket. Most draw only microamps or less so easy to wire to just about any hot source. The analog voltmeter gauges are usually about 2" in diameter.

[Juergen]

The inductive ammeter is failure proof in the sense if it goes bad, nothing bad will happen. Most (all?) add on ammeters are directly in the circuit and if they open, the generator/alternator can max out as it sees no voltage at the battery. I would replace with another similar model induction type which are fairly cheap and available

Most manufacturers replaced the direct measure ammeters with a voltmeter which is almost as good as they indicate battery voltage but not current.

[terry]

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I converted my 36 to 12v neg and did what Jack suggested. I plug a digital 12v cellphone recharger into the cigar lighter lighter and it gives a continuous reading of the battery charge. This is letting you know if your regulator is working properly. On the highway, the reading never goes over 13.7 v for me. For show, just replace the "voltmeter" with the cigar lighter. terry

[tubman]

I used one of these to find an overcharging problem with my Corvette.

[drolston]

The ammeter will work correctly with the 12 volt system, except that with negative ground instead of the original positive ground, it will read backwards. Unless you reverse the direction of the wire through the loop on the back of the ammeter, which most people will do during 12 volt conversions.





The headlights are probably about 60 watts each, which would pull ten amps. That is more than a wiggle on the gauge. Did the headlights actually turn on?

[fortyonerag]

I reversed the wire in the ammeter when I did the conversion. When I checked the other day the headlamps came on fine brakes too, with only a small wiggle in the needle.

Thinking perhaps a replacement ammeter and perhaps even an idiot light out of view to all but the driver.

I have seen bluetooth units that send an alarm to your phone. Anyone have any experience with these? Phone apps seem a bit gimmicky to me.

I like the idea of a digital voltmeter, but not the look. Maybe I can conceal it in the ashtray, but if its hidden I might not notice a problem until too late (Like the other night, but that's another story my darling bride can relate. She was a good sport about it tho').

[big job]

remove amp meter you may have a surprise. I found micro spider web in there believe it or not in my 46. And that web is so small I used a spy glass to remove it. another nature marvel...

[terry]

Are your headlights the newer LED type? They wouldn't show nearly the draw on the ammeter as the original lights.

[fortyonerag]

Thanks for the leads gents, much appreciated.

[Charlie Stephens]

Your updated charging system will probably put out more amps than your ammeter was designed for. Run a shunt to take some of the current and remember the ammeter is reading low. I like the look of original instruments and would have mine rebuilt if faced with the problem. That is how I wired my '32 with increased amperage (see picture).

Charlie Stephens

[petehoovie]

Originally Posted by Charlie Stephens

Your updated charging system will probably put out more amps than your ammeter was designed for. Run a shunt to take some of the current and remember the ammeter is reading low. I like the look of original instruments and would have mine rebuilt if faced with the problem. That is how I wired my '32 with increased amperage (see picture).

Charlie Stephens

[Jack E/NJ]

fortyonerag> > The stock ammeter has the inductive loops on the back of the gauge that the main power line feeds through.>>

[fortyonerag]

Thanks gents. Good idea, but I don't have the option of a shunt with the hall effect/inductive meter movement.

I will replace the stock ammeter. The reduced current of a 12V system might be a help.
What kind of draw would result in a full scale deflection on one of these meters?

Nice to keep an eye on the voltage too. Considering one of these:

https://www.ebay.com.au/itm/314536379014

A rough indication of charge system performance only - tucked out of sight so only the driver can see. Thoughts?

[petehoovie]

Originally Posted by fortyonerag

\Nice to keep an eye on the voltage too. Considering one of these:

https://www.ebay.com.au/itm/314536379014

A rough indication of charge system performance only - tucked out of sight so only the driver can see. Thoughts?