ammeter wiring

[jeff-d]

hi, i have a alternator on my car is the wiring for the ammeter ,( solonoid to ammeter then ammeter back to soloniod as on a dynamo or is it different on the alternator, thanks

[bmcecosse]

Yes that's it, but you must use heavy gauge cable, and make sure it is very very well insulated and protected where it passes through the bulkhead - because it is unfused and has the potential (!) to cause a fire......

[autolycus]

To add to Roy's advice: use either 44 or 65 strand x 0.3mm cable and properly crimped-on connectors (really needs a ratchet crimping tool). Make sure the connections are clean, tight and sleeved against accidental shorting to earth.

Kevin

[jeff-d]

thanks bmcecosse & autolycus for your advice, much appreciated

[IslipMinor]

Cable size depends on the alternator output. Most important is from the alternator to the ammeter, which must be a minimum of 65/0.3mm for a 35A alternator, 84/0.3mm for 42A, 97/0.3mm for 50A and 120/0.3mm for 60A. If you are using a Lucas ACR alternator, they have two main output connections, so 2 cables half the size is probably easier to route through.

[bmcecosse]

They have two output terminals - but I have only ever used one.....

[autolycus]

A car ammeter is normally wired so that it shows whether their is net current flowing into or out of the battery. This does not include the current to the starter motor.

If the battery is fully charged, there should only be a very small current flowing into it - a couple of amps at most. With the engine stopped, there will be a discharge related to the current demand of whatever is turned on. A standard setup might draw 25 amps or so, so with the engine stopped, that's what the ammeter will show - a discharge of 25A.

Now start the engine, and if the revs are high enough the generator - whether alternator or dynamo - will fulfil that demand, plus a few amps left over to charge the battery. The ammeter will now show just that couple of amps, not the 27 amps that it's putting out. The only time it will show a massive charge is if the battery is completely flat, or very faulty. The alternator may be rated at 90A, but it will only put this out if it is faced with the combination of non-standard equipment drawing very high currents and a flat battery. But the ammeter still won't show 90A - just the net current flowing to the battery.

If you the ammeter in series with the alternator output, it will only show a discharge if the diode pack fails in the alternator. The full output of the alternator or dynamo should never flow through the ammeter.

The three advantages of alternators over dynamos are:

reach full output at lower engine revs
less maintenance required
electromechanical device (control box) replaced with solid state electronics.

What's the highest reading anyone here has seen on their ammeter, and under what conditions did it happen? (Ammeter straight across the battery, treating it like a voltmeter, doesn't count, as it will have let all the smoke out.

I'd just fit a voltmeter.

Kevin

[bmcecosse]

The ammeter is MUCH more fun - and more useful -and 'period' for the car. But the voltmeter is certainly easier to install.......

[autolycus]

You're quite right Roy, what I really meant, but was (unusually) too polite to write, was: "If someone needs to ask how to wire one up, I'd suggest they stick with a voltmeter." Connect a voltmeter wrongly, and the worst that's likely to happen is it doesn't work, though using a good thick cable to a voltmeter ("to be safe"), and connecting it to somewhere unfused at the other end, could still melt quite a bit of copper if the insulation is damaged.

I spouted the conventional line on alternator advantages: you could argue that they fail less predictably and less roadside fixably than dynamos, and are more likely to leave you with a completely flat battery that they've drained for you through a duff diode. And you wouldn't know it was happening straight away with a voltmeter, but you would with an ammeter. A big alternator could also take rather a high proportion of the output power of a poor little sidevalve.

Kevin

[bmcecosse]

I have no great feeling either way re dynamo/alternator - for a standard car the dynamo output is more than satisfactory. And repair is almost always limited to new brushes. And, it can work +ve or -ve earth as required. I happen to have a small stock of ACR alternators, all bought for £1 each at autojumbles, so - I use one, and carry a spare!

[IslipMinor]

A car ammeter is normally wired so that it shows whether their is net current flowing into or out of the battery.

The reading on the ammeter is the net result, but the cable from the alternator/dynamo to the ammeter is taking the total of the load + battery charge, which can be a lot more than just the amount needed to top the battery up, which is what the ammeter shows during normal running. I have a 55A ACR alternator so I use both alternator terminals and 65/0.3 cables to the 60-0-60 ammeter, but from the ammeter to the battery I use a single 120/0.3 cable.

This does not include the current to the starter motor.

Definitely not! Would need a very BIG ammeter and don't fancy running starter motor sized cables around the car!!

[bmcecosse]

" Would need a very BIG ammeter " - one would of course use a shunt - it would be interesting though! For those who like to plaster dials all over the place - a 'starter motor ammeter'! Definitely would be one up !!

[autolycus]

I think that IslipMinor and I are talking about different physical manifestations of electrically equivalent circuits: he is (I think) running four cables to the ammeter: two from the alternator, plus another one connected to the same terminal of the ammeter; then one more from the ammeter to the battery. Seems a rather trickier way of doing it than running the alternator cable(s) to the solenoid, and then being able to use lighter gauge cables to and from the ammeter. What's the maximum the ammeter ever reads - in either direction? And does this current need a 120 strand cable?

Kevin

[bmcecosse]

Yes - I'm a bit lost with the other ideas - ammeter simply goes in between solenoid and battery. Thus it reads the current In and OUT of the battery. It needs to be heavy enough cable to handle everything on the car - in case it ALL gets switched on at the same time (maybe at MOT) without the engine running/charging the battery. This size of cable should be adequate to handle the max power the alternator may push into the battery - rarely more than 15 amps, and even then only for a few seconds in my experience. Otherwise - the alternator/dynamo just balances the power requirements of the car - and hardly anything flows through the ammeter.

[IslipMinor]

ammeter simply goes in between solenoid and battery

Roy,

No it doesn't, that is the supply to the starter motor!

The ammeter goes in between the alternator/dynamo and the solenoid, and the supply to the car is moved from the solenoid to the alternator side of the ammeter.

Kevin,

An ammeter, or at least the shunt, which all ammeters use, has to be in line with the wiring from the alternator/dynamo to the starter solenoid or pull switch. The actual gauge that displays amps is a sensitive voltmeter that measures the directional volt drop across the shunt.

The first cable goes from the alternator to the ammeter and the second from the other side of the ammeter to the solenoid/battery.

The supply to the car is connected to the same side of the ammeter as the alternator, which then means that the ammeter shows negative if the battery is supplying more of the car load than the alternator, and positive if the alternator is covering the car load and charging the battery.

If you connect the alternator directly to the solenoid, how do you propose to connect the ammeter? Wiring in an ammeter is a bit complicated, which is why the move was made to voltmeters, which are very simple and need only very light gauge wiring.

As in my earlier post, the cable size is related to the alternator output - 120/0.3 is only required for a 55/60A alternator. What output is yours?

The maximum reading is totally dependent on the battery state and the installed load in the car.

After a cold start I will see around 40A charge for a few seconds with nothing else but the ignition turned on, and equally if I turn on the headlights, wipers and heater before starting the engine I will see around 30A discharge until the engine is running (headlights are 100/80W).

[bmcecosse]

Obviously NOT the starter load But everything else - the BROWN cable. This is a very simple thing being made complicated!

[jeff-d]

sorry but a bit confused, some say i wire the ammeter to the solenoid and some say to the alternator, which is the correct way, thanks.

[bmcecosse]

The solenoid - it's got nothing to do with the alternator.......

[IslipMinor]

Hopefully the diagrams below show clearly what is required to fit an ammeter into a car with an alternator. The gauge obviously must have a scale that at least matches the output of the alternator, and the wires from the alternator to the ammeter and from the ammeter to the solenoid are related to the alternator output, and must be a minimum of the sizes posted earlier.

Just for clarification, the original Smiths diagram included the ammeter, which I removed to be able to show the steps required to install it.

For comparison, the same diagram shows the simplicity of fitting a voltmeter, just 2 small wires, one from a switched and fused ignition supply and the other to earth or other side of the battery - works for either -ve or +ve earth systems.

One very interesting finding is that although the temperature and fuel gauges all use a stabilised 10v supply, but the electric oil pressure gauge runs directly from the 12v. Didn't know that! I wonder how many retrofitted electric oil pressure gauges have been connected to a voltage stabiliser?

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[autolycus]

Just to wrap up the ammeter wiring question, IslipMinor and I agree on the principles, but we have different preferences about where to make the physical connection shown at the top of the last diagram he posted - under the bonnet, or on the back of the ammeter.

On the question of stabilised supplies to electrical oil pressure gauges, I'd guess that oil pressure gauges are not "hot wire" instruments, like some temperature and fuel gauges, because the parameter they're measuring (oil pressure) changes much more rapidly than temperature or, you hope, fuel level. Because those voltage stabilisers are thermal, they need highly damped instruments. Does this sound reasonable? I've never owned a car with an electrical oil pressure gauge - what sort of transducer do they use?

Kevin