Ammeter Question

[ta240]

isn't it a good idea to switch to a volt meter at that point? That way the wiring can be updated so that the full amperage doesn't run all the way into the passenger compartment and through the dash.

-Trevor

[V8-240Z]

Since the output of the alternator loops through the ammeter before going to the fuse box I would say that something needs to be changed. Any addon accessory past the ammeter/fuse box will push the ammeter to its limit or over. The 75 280Z used a shunt under the hood to drive the ammeter with its millivolt output going to the ammeter that is actually a millivolt meter with a amp scale on it. Then in about 76 the ammeter was changed to a volt meter.

[Nissanman]

Essentially, the ammeter measures what is going into or out of the battery, NOT the load on the electrical system.

[if it did measure the total load, then, yes, it would have to be ranged to match the capacity of the alternator].

The ONLY time an ammeter DOES measure the total load, is when the engine is OFF and the ignition is ON.

In this case, it will read "-" always, since the storage battery is DISCHARGING.

So, in a normal system, the battery is a storage device which provides a ready current source to turn the starter.

Also, it provides enough reserve to power up any electrical circuit/accessory, including the alternator exciter windings, until the alternator starts to function at a certain RPM.

Then the alternator can take over the load, which may only be 20A or so, right up to its' rated capacity under extreme conditions.

What you then see on the ammeter, is not that total load, but the charge current that the alternator feeds into the battery, the trickle current.

This current is determined by the potential difference of the battery voltage to the alternator voltage, the potential difference, PD.

The storage battery is nominally 12V, the alternator normally supplies a voltage of 13.6 - 13.8V.

Hence the flow of current INTO the battery or ammeter reading slightly on the "+" side of "0'.

In a normal system , a simple test to demonstrate the workings, is to have the engine idling with a "+" indication on the ammeter.

Then, switch on the headlamps.

You will observe a quick but definite flick of the ammeter back to "0", maybe even go "-" but it will recover and settle back to the small "+" value.

This is the alternator not supplying the extra load, the battery supplying the shortfall current, the regulator sensing the increased load and then compensating for it by increasing the alternators current output.

Quid pro quo.

If the ammeter doesn't settle back to the "+" reading, there is a regulatory/charging issue.

If you don't already have one, the installation of a maxi-fuse/fuseable link on the output of the alternator, to match the cabling rating is alwas a good idea.

A 85A alternator will make short work of a cable system with a rating of 30A.

Any fuse is there to protect the wiring, period.