Volt Meter/Ammeter

[MarkyC]

Help!!
I'm a little confused, should i have an ammeter or a volt meter in the dash? Or more to the point what's the difference and which is most useful?

Thanks
Mark.

[craggle]

An ammeter shows the amount of current flowing through the gauge. The gauge is fitted in between the main power cable and the vehicle wiring loom. It is not however connected between the battery and the starter.

I have an ammeter in my car and I had to modify the Dax loom to use it. There is a cable that runs from the starter terminal to the fuse box and I had to cut this cable and wire the gauge in the gap.

An ammeter shows whether you are in a charge or discharge state. Without the engine running the needle will move into the -ve side to show you are using power. Once the engine is started the gauge shows +ve charge until the battery is sufficiently re-charged. As you turn electrical items on the needle shows a discharge and then returns to the 0 position once the alternator takes over again. The needle should hardly ever move from 0 when the engine is running.

A volt meter simply shows the volts in the battery.

I find an ammeter more useful (that's why I fitted it) but I am used to old Land Rovers that had dynamo systems on them that would show discharge if the lights were on at tick over.

Maybe it's not needed on modern alternator systems?

Craig.

[MarkyC]

Thanks for that explanation Craig,
So both will let me know if the battery is doing ok?
Mark

[craggle]

Both will give a good indication. I think an ammeter might give more away if you have alternator problems but either should do you fine.

A volt meter is a lot easier to wire in as well!

Craig.

[dbsmith]

Depends what you want with the reading.
An ammeter will show you the current being drained at any point in time, voltage (my opinion) is a better indicator of battery and alternator charging (and at the moment I am finding this more useful with a duff alternator to figure out when voltage is dipping below a critical level). Alternators typically need to charge above 13.8v to be effective, batteries after a full charge are typically above 14v and anything below 12v means it will be struggling. This would be harder to tell with an ammeter.
With an ammeter, you are recording at any point in time how much current is being drained and supplied, this I think is more useful in more modern cars with loads of electronics, keeping an eye on current drain particularly if you have a problem where battery is being flattened (or in the case of my old range rover, understanding how much current was being drained when it was switched off to figure out how many hours it would take to flatten my battery typically overnight !) or if you are used to reading & understanding current drain.

Personally I'd go with the Voltmeter if I had to choose one, particularly with a car that's likely to be standing around in a garage for a long time. However either is fine & you'll quickly get used to one or the other.

You could of course put both in...then the problem is solved

[mylesdw]

The ammeter is great for noticing electrical problems; if you sit in the car with the engine off and switch things on and off it is very easy to get to know what movement on the ammeter to expect; it is quite easy to see that a brakelight bulb is out or that the fans are not running. I think an ammeter gives an earlier indication of a failing battery; the voltmeter might show 13.8V but what it doesn't show is that it is the continual 20A that is being pushed through a dying battery.

[dbsmith]

Originally Posted by mylesdw

the voltmeter might show 13.8V but what it doesn't show is that it is the continual 20A that is being pushed through a dying battery.

Good point.

[wilf]

Car manufacturers have taken ampmeters out of cars for two reasons - one, most dozy blighters have no idea what they mean, and two, it eliminates some heavy (and hence costly) cabling from the looms.

So blame the accountants.

Personally, I think eliminating joints in cables carrying significant currents is a good idea in home built cars, and so I vote for a volt meter.

Dash gauges are so inaccurate that they only give an "indicative reading" anyway. Long live the AVO!

[MarkyC]

Thanks for the replies,
I think I'll go with the volt meter. Out of interest what size cable would the ammeter need?

Mark

[craggle]

On the Dax loom they run 3 separate cables from the starter motor, battery connection to the ignition switch and fuse box where they are distributed to various circuits. These 3 cables are about 4mm diameter including the insulation.
To replace all of them I used a cable that was about 8mm diameter including the insulation. Those may be a bit too big but better safe than sorry.

The wires to a volt meter can be your normal size loom cables.

Craig.