Hybrid wiring scheme for an MG TA

This scheme was developed for my 1939 MG TA for two reasons – firstly to incorporate key MG TC elements (dynamo, control box and distributor) which were already fitted to the car, and secondly to bring the car up to acceptable modern standards of practicality and reliability while as far as possible staying true to the original design.

Hopefully many of the suggestions and modifications described here will be useful to TB & TC owners as well, although it is unlikely that another TA exists with exactly this combination of TA/TC key parts. Purists should look away now!

Please note this article is intended as a follow-up to Bob Butson’s excellent article on wiring details in TTT2 of December 2012.

Non-TA MPJG parts

The original 3-brush TA dynamo set fire to its wiring early in my ownership in the 1960s on a long journey and the only dynamo available at that moment en route was from a TF. This worked fine thereafter but I had to change the CJR3 control box (which has no regulated charging function) for the post-war RF95 (which does). This removes the need to alter High/Low charging via the ignition switch, so I fitted a TC ignition switch to avoid confusion in the future.

The TC-style DKY4A distributor was already fitted to my TA MPJG engine on purchase so proved easy to fit transistor ignition (I’ve had my fill of adjusting points over the years – no more!).

Figure 1 – Wiring Diagram. Click to View Larger Version

The Wiring Diagram is based primarily on the TA original wiring diagram, but modified to accept the post-war RF95 Control Box (Figure 5 shown at the end of this article), and other changes to make the car more reliable under modern driving conditions. It uses the two TA wiring looms (Main and Dashboard), but with the addition of indicator wires to the former. It proved possible with these looms to reuse most of the TA cores and follow the original Lucas colour scheme.

Early on I decided to use Negative Earth as this is compatible with modern car accessories. I also added several features to the wiring, primarily an accessory circuit, and a dedicated earthing system (see later paragraph under “earthing”). The accessory circuit used plain brown wire as this colour did not appear in the Lucas list.

Some of the unusual features of the wiring are:

• Multi-connector terminal block used between the dashboard wiring and the car wiring. This enabled all dash wiring to be installed off the car (using a purpose-made angled wooden frame). See Figure 2.

Figure 2

• Solenoid starting as I never liked the pull wire. This removed possible clearance issues at the switch on my TC-style starter. The small starter button was chosen so I did not have to alter the existing hole in the chromed centre dash panel.

• Extra 25A in-line fuse added to the main power input to the dash, as suggested by Brian Rainbow as a useful safety precaution. See Figure 2, larger yellow/black wire.

• Single 12V battery, for economic and simplicity reasons. This freed up one 6V battery location for a spares box.

• Battery cut-out switch installed on the rear axle ramp.

• Bullet connectors were used wherever necessary. These were crimped rather than soldered as I found this quicker and more effective.

• The Pertronics transistor ignition fits inside the distributor cap. It uses a wire to each side of the coil, both wires being passed through a thin black tube to make them less obvious. See Figure 4. (I carry the original contact plate already fitted with points and condenser in case the electronics ever fail). Note that the circular plastic cover over the cam lobes has to match the cam profile. The Blower manual Supplement No.1 – Page 528 in my copy – provides more information on these profiles. My TC distributor has an asymmetric cam and uses Pertronics Part no. 142 (see Figure 4 below).

• A separate klaxon horn is fitted on a small angle bracket and hidden under the front valence (great novelty value!).

Figure 3
Figure 4
Figure 5


As the chassis was powder-coated, and everything else had new paint, I decided to earth each electrical item by black wire. I ended up with an independent system fore and aft, the front terminating at the 3-way brake connector bracket on the chassis, and the rear at the battery to chassis earthing point. Both points were scraped to bare metal and used serrated washers for a good ground, and greased to prevent corrosion.

These systems do mean extra wires but being black are well hidden along the chassis. It was well worth the effort as all electrical components worked first time and continue to do so.

Accessory Circuit

This is identified by brown wires only, has a separate fusebox and essentially runs along the inner wooden bulkhead and, apart from the extra panel showing below the dash (Figure 3 above), is out of sight.

The circuitry provides indicator and hazard light power, switches and indicators for same, two standard cigar-lighter power outlets and the klaxon horn. It allows use of satellite-navigation equipment, charging mobile phones, and anything else considered essential en-route by inserting a single or double USB adapter plug.

Indicators – These work in conjunction with the Hazard circuit, but are fused separately. There is a buzzer wired in series with the indicator lamp as the indicator switch is not self-cancelling and it’s easy to overlook the indicator warning lamp in bright sunlight.


To improve the feeble glow-worm bulbs on the dashboard and inspired by Bob Butson’s suggestions in an earlier article (ref. TTT2 April 2012), I converted the dash bulbs to LEDs, which are much better. I also use halogen headlight bulbs, but retain standard bulbs elsewhere for now, although this may change as LED technology improves.

Note re. Ignition Warning Lamp – MG designed this to take a 2.5V 0.2A MES (Miniature Edison Screw) bulb, but after blowing several of these I discovered that the instrument restorer had rewired it to take a 12V bulb – this now works fine.

I prepared a spreadsheet some time ago to record the technical details of the lighting changes (included in Bob’s latest article and repeated here for completeness, with a couple of tiny changes). Also listed in a table are the Lucas part numbers for the main electrical components for TA-TF.

Spreadsheet – click for bigger version

Suppliers (usual disclaimers)
Wiring Looms: Autosparks
Electrical components: AES
Side/tail lamps & internal conversions: SVC

My thanks go to Bob Butson, Stewart Penfound and Brian Rainbow for reviewing this article. Brian has added some comments which I thought useful to pass on as is:

(1) Re – fitting a two brush dynamo and regulator/cut-out to suit: You could stay with the CJR3 but get it modified to have a solid state cut-out/regulator, several UK firms can do this. Alternatively fit an RJF50 cut-out/regulator (as fitted to MG VA etc) that looks externally the same as a CJR3.

(2) I am not sure that the Lucas DKY4A distributor has the same advance curve as the DK4A 405494 distributor fitted to the TA as standard. Accuspark now produce an electronic conversion mounted on the correct base for the DK4A, available in positive or negative earth models.

(3) The 25 Amp inline fuse added to the main power input to the dash is not really necessary. I only advocate using the in-line 25 amp fuse connected to the main battery feed whilst testing all electrical circuits after fitting a new loom. It is easy to get a wire inadvertently earthed and end up burning out a new loom. [I still find this fuse useful as it provides a quick disconnect to the dash, along with the multi-connector mentioned in the article, see Figure 2. Perhaps I should downgrade the 35A fuses in my RF95 Control Box to 25A, but the Box came with 35A so I will see how they go long term first – Ian].

Ian Linton

2 thoughts on “Hybrid wiring scheme for an MG TA

  1. Michael Bath says:

    Excellent write up on updates to the TA electrics. Interesting to note the little point I mentioned about the difference in the voltage of ignition warning bulb. Well done Ian, funny how after all these years we can still improve, change or modify our lovely old motors. Best regards,

  2. Tony Slattery says:

    You have wonderful timing – just rewiring TA0355 which has this regulator, BUT still has the original Ignition and Lighting/Charging Switch – another challenge !!!!.

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