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Add-On Ammeter

06-02-2008, 11:16 PM
I would like to install an ammeter in my 99 Ranger 4.0L. I would like to use an ammeter which uses shunt so that I can use a lower AWG wiring from the engine compartment to the gauge. Does anyone know where I can get an ammeter equipped with a shunt? Most ammeters available in auto parts store need to be connected in series and thus must use the same size wire as the accessory wire and that's too heavy a wire to run through the firewall.

06-03-2008, 05:41 AM
The only place I know of where they're readily (and cheaply) available is eBay. You can buy the ammeter and shunt as a pair, or purchase separately. Digital display modules with either LED or LCD readouts are available, in different current ranges. If you were going with a digital module then it would be handy if the LED display could be dimmed for night-time driving, or for the LCD, a backlit display would be good, preferably one with an illumination wire which could be hooked into the lighting circuit for automatic illumination - I don't know if either of those options exist though.

I'm in the UK, and our high street electronics store, Maplins, doesn't stock them and I had a quick look on your Radio Shack website and they don't appear to stock them either.

An eBay search for ammeter and shunt will give you an idea of what's available. The supply voltage for the ammeter module is typically 5 to 15V, so that can be wired directly into a vehicle's electrical system without requiring any modification, but you'd need to give a little thought as to the best way to physically insert the shunt into circuit.

I fitted a centre-zero, 60-0-60, moving coil (analogue) ammeter to one of my cars some years ago but never found it to be particularly useful. If I was going to make the effort again, I'd fit a good quality voltmeter instead but that's just personal preference - if you want an ammeter there are some neat little units available.

06-03-2008, 09:11 AM

Thanks for the input. I have the following gauges already installed in my vehicle: oil pressure (which reads actual pressure in psi), temperature gauge (which indicates actual temperature in degrees Farenheit) and a vacuum gauge (which I use to maintain efficient speed). I prefer installing an ammeter as opposed to a voltmeter because the gauges allows me to monitor my charging circuit. I already have an OE voltmeter (not really a meter because it does not indicate actual battery voltage) more like an "idiot" light like so many of the gauges used in vehicles nowadays. I believe I can just get the standard ammeter sold in most auto stores and use it with the correct shunt. Shunt rating will be max load x .6666.


06-03-2008, 02:16 PM
You're fond of gauges eh. The instrumentation in modern vehicles does leave a lot to be desired by comparison with the elegant, calibrated gauges which were fitted as standard many years ago.

It sounds like you want an analogue meter, and if you're thinking of using a regular ammeter connected to an external shunt then that raises some issues which you may or may not be aware of, so excuse me if I'm only stating the obvious here.

First thing to be aware of is that a regular ammeter will already incorporate an internal shunt resistor. The bulk of the current will flow through the shunt, with the remainder flowing through the meter. That works just fine but as you pointed out earlier, it requires a suitably heavy gauge of wiring to carry the anticipated current, and that isn't always convenient.

The way to avoid the heavy wiring is to use an external shunt, which connects to a high input resistance voltmeter, the display of which is calibrated in amps. That avoids the need for heavy wiring, as the current flow through the wires which connect the two items is negligible. That's the principle of operation of modern digital ammeters.

A problem arises however if you add an external shunt to a regular analogue ammeter, because of course the regular ammeter already contains its own internal shunt, and if you add another shunt in parallel then you dramatically affect the meter's accuracy. Actually, you end up with three resistors in parallel - the relatively high resistance of the meter itself, the low resistance of the meter's internal shunt, and the similarly low resistance of the external shunt. The bulk of the current will flow through the two low-resistance shunts, but of course the whole idea of adding an external shunt was to avoid heavy wiring, so the connecting wire would not be a suitably heavy gauge for the current which would be diverted through the meter's own internal shunt, so that route is fraught with problems.

You will occasionally see analogue meters in which the shunt resistor is a separate item, designed to be located remotely, but they are the exception rather than the rule - that is what you would require for the installation which you are proposing and that will indeed probably prove difficult to source.

There might be the possibility of dismantling a regular ammeter, removing the shunt resistor, installing it remotely and then using small-gauge connecting wire, but analogue ammeters are precision instruments and so that would require some care, plus it would still leave you with the problem that the resistor was not designed for remote location so it would not be as physically strong and robust as a shunt which was intended by design for remote location. That's not really something which I would care to attempt.

It's possible that I'm not understanding what you propose though, so your solution might indeed be workable. At first glance, I don't see where the max. load x 0.6666 is derived from.

06-03-2008, 02:32 PM
You are correct about the existing analogue ammeter having internal shunt. what I'm talking about is more in commercial application whereby the ammters were designed for external shunts. Of course I'll have to get one that is applicable to automobiles. I'll be checking electrical/electronic instrument stores. The value .666 is a ball park figure of what size shunt to use based on the max load of the circuit. These gauges may not mean anything to ordinary drivers since all they are concerned about are their speed, cellular phones and navigation. Thus "idiot" gauges were born. For example, what is the purpose of the tach in most cars with auto transmission except that it added to the cost.

06-03-2008, 10:51 PM
Selectron: Got an idea. If I opt to monitor only by charging system, that is, how my alternator/regulator is functioning, I can use the standard ammeter with built in shunt correct? which is readily and inexpensively available in auto parts store. As such I don't need to have a large size wire running between the engine compartment and the cockpit. I already have a built in voltmeter but it does not indicate voltage, only normal range and the extremes. BTW, these built in voltmeters, what do they exactly measure. It seems like the needle is stuck in the midrange and does not fluctuate. If the battery is no good obviously the needle goes on the LOW indicating what? On the HI end, what battery voltage does that indicate since battery voltage ordinarily won't go beyond 14-15 volts unless the alternator-regulator is malfunctioning.

06-04-2008, 11:33 AM
The voltmeter question has a straightforward answer, so I'll reply to that one first. If you took a regular voltmeter which indicated from 0 to 16V and connected it to the car's electrical system, it would indicate the system voltage and prove to be quite useful. It isn't ideal though, because we aren't really interested in voltages below 10V. I've shown that meter on the left, below, and you'll see that 3/8ths of the scale contains the voltages that we're interested in, but the other 5/8ths serves no useful purpose.

What would be much more useful would be a meter with a range of 10 to 16V, and that can easily be achieved just by adding a 10V zener diode in series with the circuit. At voltages below 10V, the zener does not conduct and so there will be no movement of the meter pointer for all voltages below 10V. However, when the applied voltage is equal to, or exceeds, 10V, the zener will conduct and the meter will indicate voltages above that level, and thus the full scale is now of practical value. This is known as either a suppressed-zero voltmeter, or an expanded-scale voltmeter.

At its most basic it really is as simple as adding a zener and altering the resistor value, but the circuit in use in a modern car's voltmeter would likely be more complex - the principle remains exactly the same though - just clip the unwanted voltage so that the meter only indicates above a certain threshold.

There are probably two reasons why the pointer rarely appears to move on a modern car's voltmeter. First is that the modern, solid-state voltage regulator is a complex, efficient device, capable of maintaining a near-constant system voltage regardless of load, and the modern alternator has a sufficiently high current capability to make that possible. The second reason is that the meter movement will be highly-damped, which is to say that it responds slowly to a change in voltage, and thus brief fluctuations go unnoticed. I think they use such heavily-damped meters just to avoid the perceived annoyance factor of a less-damped, flickering needle. There may well be other reasons but they're likely to be the two main factors.

Click to enlarge: (

06-04-2008, 11:49 AM
I beg to differ with you regarding the value (to me anyway) of the voltage read out for a standard volmeter. There are two data that I need regarding the state of my battery - the state of charge which normally obtained using a hydrometer, and it's capacity to take the load. Obviously with the no-maintenance betteries, it's impossible to read the specific gravity of the cells. One way I know of testing capacity however is that a fully charged good battery voltage will not go below 9 volts when cranking (no start) the engine for twelve seconds. Further, if not to cut down in cost of the voltmeter, why does it matter if the voltmeter reads 0-15 volts? Most drivers don't pay any attention to it however there are those like me who may have use of the information.

On the subject of the ammeter, looking at the schematic diagram of the charging circuit, which wire (between the regualtor output and the battery) do I need to interrupt to install the ammeter?

06-04-2008, 01:01 PM
Regarding the ammeter - in order to monitor the charging system, you'd need monitor all current to-and-from the battery except that which flows to the starter motor. That would mean you'd need to interrupt the heavy auxiliary cable leaving the battery positive terminal, after the main fuse but before it arrives at the alternator's B+ terminal and before any feeds have branched off from it. That would require some pretty heavy wiring to be routed through the firewall to the dashboard - at least as heavy as the original and preferably even heavier because you would be increasing the length and therefore increasing the resistance. It's difficult to physically route and accommodate such heavy wiring - it's also a fire risk because it can carry such a high current, and if it should go open-circuit then you're left stranded with no electrical supply to the vehicle. It's a lot of disruption for what might be considered as minimal benefit. The remote shunt method wins every time because it adds no extra length to the existing wiring, it adds only minimal extra resistance, and you can use the thinnest of wire to connect from the shunt to the dashboard gauge.

I have installed the internal shunt type ammeter in the past, with its associated heavy wiring, but it isn't something I'd do again. I mounted my analogue ammeter in the car's centre console, forward of the gearshift lever. During normal driving, deflection of the pointer was minimal so it always sat somewhere very close to centre-scale, but because it was located to the side of me it was difficult to see just exactly which side of zero it was on. It would have taken a pretty catastrophic failure in the charging system (broken belt, etc.) before it would give me any truly meaningful indication that I had a fault condition, and that could be indicated just as effectively (and implemented much more easily and safely) by a good quality voltmeter.

If I was to install an ammeter again, I'd fit a digital module - high resolution, easy-to-read, easy-to-interpret, easy to install, safe wiring - the old analogue ammeter simply can't compete if you leave the aesthetics aside and look purely at the practicalities. There's a lot of personal preference and bias in that though and I'm sure that if there were more people in the conversation then somebody else would be proclaiming the benefits of the analogue ammeter but I just don't see any.

This is what I think you have in mind:

Click to enlarge. (

06-06-2008, 01:03 PM
The Digital Module concept sounds interesting. Can you give provide a bit more technical description of the device, how to connect, and above all where to buy? BTW, although I was an Electricians Mate while I was in the Navy, I'm eager to learn new things. One subject that I have been closely following is the application of lead-acid cell battery (off-the-shelf) in electric automobiles.

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