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 almost 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. Interestingly, the Blower Workshop Manual shows a wiring diagram for the TB in which the Near Side light is completely extinguished when Dipping is selected with no indication of a solenoid dipping mechanism.

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 favor 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 in both day and nighttime driving. 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.


THE STARTER MOTOR

The correct starter motor for the TA was a Lucas M418A A84 with the cable operated solenoid mounted on the side as shown in the examples above.

A MODIFICATION
Although not original to the T-ABC some owners have replaced the pull start knob and wire with a push button switch connected to a remote electric solenoid. I've heard two reasons for doing so; No longer are you pulling up against the wood dashboard as we do for the Choke and; Because they were period-correct for some automobiles and were exchanged for sheer ease and convenience of starting up.


ORIGINAL WIRING DIAGRAM

From the Blower Workshop Manual

This diagram follows the correct color code for Home models with one dipping headlight. Click the image to view it larger.

For a hybrid TA wiring diagram using an RF95 Control Box and two-brush dynamo, double filament headlights, turn indicators and other accessories see Ian Linton's rendition here. Interesting reading and a comprehensive article on a number of electrical modifications he made when he restored TA3120. Bob Butson has also written a very informative article on his use of LEDs when restoring TA0844. Once again, John James' online magazine, Totally T-Type 2 comes to our rescue. Read Bob's article here.

Here's the circuit diagram from the TA Owners Manual showing the route taken when wiring double filament headlamps. The connections are made on either side of the Headlight Fuse. Click the image to view larger.


THE TA DISTRIBUTOR

A Lucas DK4A distributor came as original equipment on our cars. It uses a micro-adjustor underneath to fine tune the timing.

The distributor with cover removed

- Instructions From The Workshop Manual -
The following parts of the distributor require lubrication:
1. Distributor Shaft: Add a few drops of thin machine oil through oiler provided about every 1000 miles.
2. Cam: After the first 500 miles, give the cam a smear of engine oil every 3000 miles.
3. Cam Bearing: About every 3000 miles, withdraw the moulded rotating arm from the top of the spindle by pulling it off , and add a few drops of thin machine oil. Do not remove the screw exposed to view, as there is a clearance between the screw and the inner face of the spindle through which the oil passes to lubricate the cam bearing. Take care to refit the arm correctly and to push it on to the shaftas far as possible, otherwise there is a risk of tracking and burning of the moulding.
4. Contact Breaker Pivot: Every 5000 miles place a spot of oil on the pivot on which the contact breaker rocker-arm works.
5. Automatic Advance Mechanism: About once every year the moving parts of the automatic timing control must be lubricated with a good grade engine oil. To render the control accessible, remove the distributor moulding and lift off the rotating distributor arm, then remove the contact breaker moulding by withdrawing its two securing screws.

Using The Micro-Adjuster
Sitting under the distributor and often misunderstood is the Micrometer Timing Adjustment. The Workshop Manual says that; 'With a clean engine, and using first grade fuel, the micrometer scale should be at "O" before timing. The final setting can be made by use of the micrometer adjustment after running the engine. If the firing is found to be slightly too early or too late, adjust the knurled knob until the best engine performance is obtained. The adjustment should not be altered by more than one distributor degree at a time (one division on the scale is equivalent to two distributor degrees or 4 degrees on the crankshaft). I believe it is marked + on one side and - on the other so you know whether you are Advancing or Retarding the spark. And you should be able to feel the clicks.

Electronic Ignition
Kits are available online for DK4A distributors with Positive Earth. However most everyone agrees that if you do convert to electronic ignition you best keep a plate with points and condenser and rotor button in the toolbox because when these do go out there's no amount of fettling that's going to get you going again.


Capacitor/Condenser Exchange

A number of T-ABC owners have reported capacitor failure in recent years not only of the original old style but new ones as well. Good reason to keep a spare in the tool box. Here's an example of a newer Lucas capacitor (originally intended for an MGB or Midget) which has been hidden inside the distributor cap replacing the long original-type.

As can be seen in the photo a metal 'bridge' had to be added for making the contact at the rear of the newer capacitor with the base of the distributor. The pig-tail at the front required another screw and nut. When mounting a modern capacitor/condenser inside the distributor using extra screws and nuts as shown here make sure they're tight! Although the engine might stop before damage inside took place (if a screw came loose) still, you don't want that gamble. And remember, they do not have to be mounted inside the distributor to work.

Another Approach
Capacitors similar to original are available and can be sourced from Graham Brown of JB Vintage Spare of Croydon via E-Bay. Brian Rainbow may have the best method of all. He carries a spare distributor base with new Graham Brown capacitor that as been pre-fitted, gapped and checked rather than carrying spare points and condenser. This can be replaced road-side much easier than fiddling with the individual items.

You can read more detail about this and his description of a quick and safe way to remove/replace your distributor in Issue 14 of the Totally T-Type2 found here.


A VACUUM ADVANCE FOR THE TA

There's a very interesting article here written by David Heath in Issue 34 of TTT2 explaining how he modified a Lucas 59D4 distributor for use in his TA.

The 59D4 is a vacuum advance type distributor as found on a number of Metro and Minis and once reconfigured for a TA requires a considerable amount testing before turning loose on an MPJG engine.

I corresponded with David looking for more information about modifying a 59D4 distributor for my own use. He generously offered help and advice with various issues I had as the project progressed.

After acquiring a distributor I modified the mechanical advance and also replaced the drive gear on the shaft. These gears are now available new from the Distributor Doctor in UK. He can also supply (or they can be sourced online) a set of assorted Lucas springs needed for tweaking the advance curve in the final stages of development.

59D4 with DK4A Drive Gear Installed (vacuum unit removed)

I made all the mods to the distributor, made a bracket to hold it securely in the block and even modified a spare intake manifold to accept the vacuum line. I ran it for a couple of hundred miles in mechanical-advance-only configuration and at that point my curves were good enough to try out the vacuum advancing but would still require a considerable amount of time selecting the correct set of springs so as not to burn holes in pistons.

This is best done, as David explains, on a 'rolling road' where the car is stationary and mechanical adjustments such as springs on the weights, carburettor needles and even octane ratings can be tested easily without starts, stops and working along the roadside. Without a proper rolling road I got cold feet and went no further than an initial test to see that the vacuum advance was indeed working.

But I can say this for the 59D4, it is a very good distributor!
Perhaps because my DK4A is old and maybe a little loose in the bearings and the 59D4 is newer and possibly tighter it fires more accurately than the original and I'll certainly be keeping it.

It was a very interesting project that I'd recommend for anyone wishing to learn more about timing advance curves and the workings of a distributor whether you follow through to a fully working vacuum advance distributor or not. And because a good used 59D4 is far less expensive than replacing a 'proper' DK4A with a little work you'll have a dependable and very serviceable spare.


BETTER TIMING MARKS

One of the essential needs in setting up the vacuum advance distributor is a reliable means of viewing Top Dead Center during dynamic timing. The factory markings on the crank pulley and timing chain cover are about worthless unless you have the radiator off. I'm sure this has been a sore point for lots of us wanting to use a strobe light for dynamic timing. And in fact it's impossible without adding marks of some kind.

One owner I spoke with told me that he had used a wire marker attached to one of the engine strap bolts then painted degree marks on the pulley. Another suggested putting marks to represent TDC on the pulley and timing case housing over on the left side of the engine.

I used a heavy wire (clothes hanger!) and attached it underneath one of the bolts securing the timing chain cover. My greatest concern was that it might not be stable, would vibrate, and not allow an accurate reading with the strobe but was pleased when it remained steady with varying engine speeds.


Timing Marks

The photo above was taken before adding two more white marks (at 5 and 10 degrees BTDC) on the pulley near the wire marker.
π x D will give you the Circumference of your pulley. Divide that by 365 to find the length of each degree.

Notice the white mark at the back of the pulley. This photo is a little confusing because that large nut is holding the dynamo to the upper mounting bracket. In the photo it looks like there are 3 lines. (That short vertical portion is actually on the front of the seal housing.) If it could be viewed from directly overhead that line at the back of the pulley would line up perfectly with the white line on the top of the crankshaft seal housing. This is a good example of what you are faced with when trying to view TDC from the side of an MPJG engine. And the reason for creating better marks for dynamic timing.




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