convert mopar ammeter to voltmeter - how to

[UKcuda]

OK, so we've been discussing the fitting of aftermarket voltmeters and retaining the original appearance of the gauges.

In a previous thread I said  had looked at the stock ammeter and there seemed to be no way to convert it to a voltmeter, because it is not a coil/shunt design.

I got to thinking about this again - so what if it doesn't have a coil, I can add one.

Here is my progress so far:

Here is the stock ammeter -

[UKcuda]

OK, so on the back of it you can see the brass conductor and in front of that a little brown sausage shaped thing attached to the pointer axis.  On the next picture you can maybe see it a bit better.

The little sausage is a permanent magnet.  When current flows across the brass bar it induces a magnetic field at a right angle to the flow of current.  The magnet "sees" that field and swivels left or right depending on which way the current is flowing.

In this picture you can't see it but there is also an iron horseshoe shaped piece wrapped round the ends of the brass bar.  This attracts the ends of the magnet out to the sides and pulls it back to center position when there is no current in the brass bar.

What I need to do is to introduce, via a 12 volt coil, a magnetic field at a right angle to the brass bar.

Here is what I will use:

[UKcuda]

This is the bit I need:

[UKcuda]

So now here is the gauge with the coil inserted in the back.  The first picture is with the voltage off, the second is with a 12v supply to the coil.  You can see the sausage has moved.  In the second picture the pointer is showing close to full scale deflection - it works -  :  :

[UKcuda]

This has taken me about 20 minutes so far, but now I have to figure out a way of fixing the coil in place.  Also I need to see if I can bend the pointer so that it shows over to the "D" side when it is at rest.

If I can do that I should be able to adjust the position of the coil so that the gauge reads a little below center at 12v and tips over the center towards "C" at about 14v.

Since the coil is rated at 12v it might not be a good idea to expose it to 14 volts all time the car is running, so I think I will also need to put a suitable resistor in series with it.

I'll update when I've had chance to try out these ideas.

[UKcuda]

Houston we have a problem !

Actually two problems.

1st problem - as the sausage magnet swivels, the end which is being attracted to the coil gets closer to it and the pull therefore increases.  When it reaches a certain point the attraction increases too fast and the gauge pegs all the way.

2nd problem - although it is easy to adjust the pointer position (by a special process known as "bending it"), this alters the position of the sausage magnet when the gauge is reading in the center and therefore messes up the sensitivity of the magnet right in the range where we need it.

Turns out both these problems can be solved with a couple of tiny magnets robbed from some fridge magnets.  In the next picture you can see these magnets glued onto the brass strip which holds the pointer bearing.

By positioning them carefully (using superglue) it's possible to create a permanent magnetic field sufficient to pull the pointer over to the "D" side of the gauge.  They are the two little grey discs near the top.

You can also see the coil fixed in place with some quick setting rubber compound and the earth from the coil which is wrappen round onto one of the mounting studs.  This can now earth straight to the dash housing as there will be no live feed to the old system.  The live (red) wire goes to ignition circuit.

[UKcuda]

Time to test it - this is with a 150 ohm resistor on the coil.  The coil measured at 200 ohms.

First photo is with no power to the gauge (ie. ignition off).  Second photo is 12v power to gauge (ignition on).  Third is engine running and system in "charge" state.

[UKcuda]

As you can see, that extra 2 volts or so from the alternator doesn't move the pointer very much, but when revving the engine from idle it can clearly be seen to move. 

I need to increase the sensitivity a little and try to get 12v dead center, then it should be fine.  I think I need to go with about a 100 ohm resistor in place of the 150, but I can't find one.  The maximum movement in any case will be about one sixth of the swing from 0 - 12 volts.

Also I'm going to feed it from closer to the alternator and see if that helps - these tests were done with it running straight off the battery posts.

[UKcuda]

Final report:

few more adjustments and a 75 ohm resistor - this is as good as I think I can get it, I think this is not a bad result actually.

Photos as follows:

1     off
2     on (12v not running)
3     on with ign and headlamps on (not running)
4     engine running  (lamps off)
5     fast idle

[ShelbyDogg]

You also have to allow full deflection if the system is over-voltage, i.e 18V.

That way you know it is overcharging the battery.

[UKcuda]

You also have to allow full deflection if the system is over-voltage, i.e 18V

It will do that, I put a little rubber bump stop on the "D" end so the pointer stops on D, zero volts is actually further to the left than that. [what I mean to say is that it stops on the lowest scale mark near to "D"]

The first 4 volts or so don't move it at all, the travel across from D to the center is from 4 to 12 volts, that's how I got it to show more deflection from 12 to 14 volts on charge.

As you can see, it sits noticeably over the half way mark at 14 volts +/-.  If the regulator is working it shouldn't ever go beyond that.

If the regulator went to 18v it would move well across the scale towards C.

All I need to do now is fit it in the dash - LOL

[72rtchallenger]

that looks good , nice idea

[wally426ci]

 

[Bullitt-]

       Great work...I've dreamed on this a bit but could not find info, even my EE brother-in-law could not give a clue... 

[UKcuda]

I've dreamed on this a bit but could not find info,

I couldn't find anything either, the only thing I found out that I didn't already know is that this design of ammeter is also used in the De Tomaso Panteras (gauges made by Veglia).  It seems the Pantera chaps are keen to change to voltmeters as well, but they do it by swapping out the whole gauge.

It just came into my head yesterday morning that maybe I could add a coil to make the guage move and I remembered I had that little relay lying around in my electrical junk.

When I found that the coil easily moved the gauge I knew it had to be do-able.

The thing with the fridge magnets was just lucky (I guess maybe you say "refrigerator") - I needed something small and strong.  First I went off to my daughter's room because I knew she had some magnetic earrings, but she had thrown them away (do'h !) so I was pacing round the kitchen thinking "magnets, magnets, magnets" and suddenly I saw all the dumb stuff stuck on the fridge door - eureka!

In the end I used the two little magnets stacked up one on the other (not like in my photo) - that gave a strong enough field to pull the pointer right off the end of the gauge, then I glued a little rubber stop in the back so it didn't go more than the end of the markings.  Theres an aluminium quadrant on the back end of the pointer round where the axle pin is and it was easy to glue the little stop on the brass conductor so it blocks the quadrant from rotating any further. 

With the magnets like that I cranked up the coil (lower resitor value) to neutralise the stronger field and get the pointer back to center - that gave it more movement per volt across the scale.

One funny thing I noticed was that holding the gauge face up and turning it around the pointer was moving slightly, I guess due to the earth's magnetic field.

When I get time to fit it in the dash we shall see how stable it is in service - I think it will be OK.

I think getting it to work right depends a lot on what coil and magnets you have to play with.  I wouldn't recommend it for everyone, but it's good fun if you are a bit mental and enjoy goofy projects  like me.