Fitting an ammeter

Fit an ammeter into the dashboard of your car to warn of faults in the electrical system. It can be easily connected in cars with either a dynamo or an alternator and will help avoid more serious problems.

While the small warning light on the dashboard tells you something is wrong with the car’s charging circuit, an ammeter will give you advance notice of trouble so you can trace the fault and put it right before any major problems arise. An ammeter indicates the rate of charge and discharge of the battery and will indicate how much current is going into, or being taken from, the battery at any one time. For example, if the fan belt is failing, it will slip before it finally breaks and cause the charge rate to fall. This fall-off will show up on the ammeter, but it will not light up the warning light, which glows only when the charge current falls to almost zero.

A slipping or broken fan belt is only one of the faults which will show up on the ammeter; others include a defective dynamo or alternator and short circuits, overloading and other faults in the electrical system.

SELECTION The type of ammeter you buy will depend on whether your car has an alternator or a dynamo. If the car is less than two years old, it will be fitted with an alternator, which gives a high charging capacity and, when several accessories are fitted, an equally high discharge capacity. In this case you should choose an ammeter capable of coping with the maximum load; one with a 50-60amp capacity in either direction is the most suitable. If your car is more than two years old, it will probably be fitted with a dynamo and in this case an ammeter with a 30amp capacity will usually suffice.


The ammeter should be mounted in a convenient place on the dashboard in the same way as any other electrical accessory and the wire from the built-in lamp should be connected to the existing lighting circuit. Before you start wiring in the ammeter itself, disconnect the leads to the battery to guard against the possibility of touching a live wire against the bodywork which might spark off a fire in the car.

Not everything served by the battery goes through the ammeter; extremely heavy current equipment bypasses the instrument to prevent overloading. For example the starter motor is not fed through the ammeter and, in many cases, neither are the horns – although depending on the amount of power they consume, the horns could go through a 50amp ammeter without overloading.

The point to which you connect the ammeter depends basically on whether the car has a dynamo or alternator.

With dynamo Cars fitted with a dynamo also have a regulator. These fall into two types which are distinguishable, when the regulator cover is removed. by the number of ‘bobbins. On a two bobbin regulator, disconnect the wire from the A terminal and connect this to one of the main terminals on the ammeter, extending the lead if necessary using 44/0.3sq mm wire and suitable connectors. Connect the other main ammeter terminal back to the A terminal on the voltage regulator. If the regulator has three bobbins, connect the ammeter to the B terminal on the regulator. In both cases the lead removed from the regulator will probably be brown.

With alternator Alternators are of two types: machine-sensed and battery-sensed. Manufacturers do not recommend the use of an ammeter with a machine-sensed alternator because of the additional resistance the ammeter imposes on the circuit; so always check which type of alternator is fitted to your car.

With a battery-sensed alternator you can connect the ammeter to the starter solenoid. This will have four wires attached to it: two thick ones about 8mm diameter which connect to the battery and starter motor, one thin wire which connects to the starter switch and a fourth wire about 4mm diameter which supplies all the electrical accessories. It is this fourth wire which you should connect to the ammeter and the second wire from the ammeter should be connected to the terminal on the solenoid from which this was removed. Direct connection to battery If the live terminal on the battery has two wires – one 8mm and one 4mm diameter – attached to it, you can cut the 4mm kfjrin lead and connect the ammeter between the cut ends with suitable connectors. Again, check if the car is fitted with an alternator and, if so, whether it is machine or battery-sensed.

Wherever you connect the ammeter, once the installation is complete check the connections are the right way round by reconnecting the battery terminals and starting the engine. When you rev the engine, the ammeter should show a charge rate; if it does not, simply reverse the wires.

Reading the ammeter

When you turn on the ignition, make a habit of checking how much discharge the ammeter registers

– it should be only a few amps. When the engine is running at fast tickover the needle should be roughly in the middle of the scale and, as the engine speed rises, it should swing over to the positive side

– the amount depends on the state of charge of the battery. After a few minutes the needle should fall back to a charge of a few amps, indicating the battery is charging properly. Note the general pattern and. if it changes, investigate the problem.

Even if several pieces of equipment – such as the headlamps, windscreen wipers and healer – are all in use. the ammeter should still show a slight charge rate when driving at normal speed. If it shows a discharge, something is wrong and again you should investigate. For example, a sudden swing to discharge of about 10 to I5amps may mean the fan belt has broken; but if the fall is gradual, showing a small discharge even when the engine is running fast, the belt may be slack and need adjustment.

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