The Types & Uses of Ammeters
Ammeters used for measuring large currents that are more than a few hundred milliamps at most, require a supplementary low-resistance current-shunt, that conducts most of the supplied current in parallel with the low-current gauge-windings which operate the gauge. The gauge is then calibrated so that proportion of total current conducted by the gauge-windings, indicates on the scale, the total current supplied to the gauge, which is divided proportionately between the gauge-windings and the low-resistance current-shunt.
Fundamentally, there are essentially three ways as follows, in which an ammeter can be used to monitor a vehicle’s electrical system:
a) The total current supplied by the generator (i.e. dynamo or alternator via an external or internal voltage regulator) to the electrical system including the battery, but excluding the starter motor;
b) The total current supplied by both the battery and the generator (i.e. dynamo or alternator via an external or internal voltage regulator) to the electrical system, but excluding the starter motor;
c) The total rate of charging & discharging of the battery, apart from what is discharged to the starter motor when starting the engine.
The electrical circuit wiring diagram for the FWD Triumph 1300, illustrated on
Pages 134 & 136, in the Wiring Diagrams section of the following DIY workshop manual, shows the non-calibrated, internal-shunt ammeter (Item 3 on the circuit diagram | one of the gauges in the three-gauge cluster | the scale is simply marked C for charging and D for discharging) as being incorporated in a way, which indicates in accordance with none of categories (a), (b) or (c).
John Millward, Triumph 1300/1500 from 1965 ~ 1973, Car Repair Manual, Autodata, 1980, ACRM277, ISBN 0-85666-049-3.
By simply referring to the aforementioned electrical circuit wiring diagram, I have so far found it impossible to determine the intended purpose of the ammeter, as fitted to the FWD Triumph 1300!?! To have any hope of this, I would need to monitor the ammeter readings, whilst operating the various electrical systems of a FWD Triumph 1300.
Most ammeters used for automotive applications, are of the internal-shunt variety, whereby the low-resistance current-shunt is actually mounted inside the gauge-housing and external connection terminals to the gauge are of a heavy-duty, high-current type. However, there are remote-shunt (i.e. external-shunt) varieties of ammeter available (including some from Lucas, about which I learned many years ago), for which the low-resistance current-shunt is mounted remotely, outside the gauge-housing and external connection terminals to the gauge are of a light-duty, low-current type, because the gauge windings themselves take only a low current.
Although typically more expensive than internal-shunt ammeters, remote-shunt ammeters are the most versatile for the following reasons:
1) They enable one to monitor current flow without significantly modifying an existing wiring loom or needing to use very-long extra sections of heavy-duty, high-current cable.
2) They enable one to monitor current in circuits which are in a location remote from where the gauge itself is mounted
3) They enable one to use a single gauge to monitor current flow in different parts of an electrical system, by switching by means of a two-pole, multi-throw switch, the gauge’s electrical connections from a current-shunt in one location & circuit to another current-shunt in any other different location & circuit.
In principle, it should be possible to convert an internal-shunt ammeter into a remote-shunt ammeter, which is what I intend to do with one or more of the cheap new-old-stock, 52 mm VDO Cockpit style, -30~0~30A, -50~0~50A & -60~0~60A, internal-shunt ammeters that I bought for my rear-engined, 1973 VW 1600 Type 2 Westfalia Continental motor-caravan. I might do the same thing with the D~0~C ammeter of my FWD Triumph 1300 triple-gauge cluster.
Having a remote-shunt ammeter with multiple switched current-shunts, would enable one to monitor whether the battery was charging or discharging and whether the generator was approaching its maximum sustainable current load.
1968~79 VW Type 2, voltmeter & remote-shunt ammeter circuits
https://www.thesamba.com/vw/forum/album ... id=1982288
https://www.thesamba.com/vw/forum/album ... id=1982289
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Regards.
Nigel A. Skeet
Independent tutor of mathematics, physics, technology & engineering, for secondary, tertiary, further & higher education.
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Upgraded 1974 Triumph Toledo 1300 (Toledo / Dolomite HL / Sprint hybrid)
Onetime member + magazine editor & technical editor of Volkswagen Type 2 Owners' Club