THE MG TA ELECTRICAL SYSTEM

The electrics in the TA/TB are unique to other T series cars. Its Headlamps, Ignition switch, Dynamo and Voltage Regulator/Cutout being the main differences. (TA & TB electrical circuits are identical.)

The Headlamps, on early cars for sure, and perhaps the late ones as well the TB, had a solenoid operated reflector in the Near Side (Left Hand) headlamp which, when Low beam was selected, 'dipped' the reflector pointing the beam toward the ground while simultaneously dousing the Off Side (Right Hand) lamp so as not to 'dazzle' or blind oncoming traffic.

This single lamp dipping arrangement was used for headlamps on Home market and cars shipped to France but for TAs destined for other countries the use of a second dipping reflector was standard. Likewise, cars headed to the Continent for instance might also have speedometers which read in KPH instead of MPH.

Although the focus was 'fixed' on the headlamps as they left the Works if one replaced a bulb with other than Lucas there might be the need to re-focus them. This, according to the Owner's Manual, was accomplished by moving the bulb "backwards or forwards along the axis of the reflector until the best lighting is obtained".

Most TA owners today have changed to either sealed beam headlights or eliminated the dipping lens in favour of double filament replacement-type bulbs. Both methods give superior lighting than the original design. Use caution when replacing lamps which draw more current than was originally intended else the dynamo just won't be able to keep up. It should also be mentioned here that many owners have replaced the filament type bulbs in Tail and Stop Lights with LEDs which are far brighter, safer and offer much greater visibility. The use and conversion of LEDs should actually be another section that will be taken up later. (Be aware that LEDs are polarity sensitive. Most common LEDs are Negative earth and not suitable for the Positive earth TA.)


 

Solenoid Operated Dipping Reflector

The reflector pivots vertically within the headlamp bucket pointing (dipping) the beam lower, toward the ground. There's a nice article on the dipping solenoids at Austin 7 dot org's site located here. Included at the site are schematics on the solenoid (which physically appears to be the same used on the TA) with an interesting explanation of their workings and some good advise if you're troubleshooting or restoring one. Remember that the Austin 7 used a 6 volt electrical system, however, not 12 volts as is used on the TA!

THE BATTERY CHARGING SYSTEM

There are few components that make up the charging system on the TA. There's a dynamo (generator), a switch allowing the choice of three amperages passing from the dynamo to the battery. (Summer, Winter and full charge when the headlights are on) And between the dynamo and battery there's a cutout which 'cuts out' the circuit between the two when voltage drops below a set level.


 

Dynamo/Generator

The photo above shows the end plate of a TA dynamo with the three brush set up. The carrier for the smaller (3rd) brush is attached to a circular ring which is movable around the center of the plate. Movement of the plate, one way or the other, changes the amount of current (Amperage) the dynamo is putting out.


CJR3 Cutout

Usually with 'regulated voltage' charging systems the state of the battery determines the amount of voltage supplied by the dynamo/generator. This is not the case with the TA/TB charging system. Here the dynamo output is constant. It uses a CJR3 cutout with two additional resistances inserted in the circuit in conjunction with the PLC switch to select a voltage which is then sent to the battery.

CJR3 Terminus Guide

A cardboard/paper guide such as the one above is usually found underneath the wiring hook-ups on the back side of the CJR3 Cutout. It helps to simplify harness hookups from the dash and other points to the fused terminals on the front. I would presume these to have originally been placed on all TA CJR3 cutouts. I found mine quite oily after 68 years in the engine compartment but it cleaned up enough to read the printing.


Lucas PLC High/Low Charge and Lamps Switch

In the Low (Summer) position there are two additional resistances in the CJR3 cutout for the field circuit. When switched to High (Winter position) there is only one additional resistance in the field and selecting it sends a greater amount of charge (amps) to the battery.

The greatest amount of charge is needed when the headlamps are used and swinging the lever to Head sends the maximum amperage the dynamo is capable of creating into the charging circuit. That rate is determined by the placement of the adjustable third brush in the dynamo.

The diagram above illustrates how the switch works (looking at it from the back). The ignition On/Off contacts are in the center while the outer connections are for lamps and charging circuits.

And the illustration below might help understand the circuit using a CJR3 cutout and 3 brush dynamo.


Diagram of the Third Brush and Charging Circuit
The third brush can be moved to vary the output amperes.


THE THIRTY LIGHT

A green-lensed Thirty-Light is also incorporated in the electrical system being activated by a brass or copper contact inside the speedometer. These were also used in the TB and into early production of the TC. The reason for the Thirty-Light was to warn drivers that they were approaching the maximum speed limit enforced in many towns and cities throughout Britain in the pre war years. While it is interesting and amusing to watch it come on at about 20 mph and go off just over 30 a few modern-day TA owners have removed the wire leading to the speedometer and rewired the light for use as a Turn Signal indicator.



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B Davis.