Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - for the technocrats

To convert a pos. ground A to negetive ground when fitted with a Moss pos. ground electronic distributor, is there a diode in the circuit somewhere that can be reversed to allow a current flow reverse ? Or can the wireing just be reversed ? Or none of the above. Thanks Sean
S Sherry

I think you just re-magnetize the dynamo. There are posts in the archives on this I believe.
A Pearse

Sean is not asking how to change a positive ground MGA to negative ground. If he was asking that it is well covered in Barney's website. It is a simple procedure.
What Sean is asking is that if he converts his car from positive ground to negative ground, what does he need to do to a positive ground electronic Distributor.

M F Anderson

Not sure what electronic distributor you have, but usually they cannot be reversed. If it doesn't say anything about it in the instructions for the distributor, it would probably be much more than just reversing a diode.
Jeff Schultz

Electronic ignitions use a transistor to switch coil current. These only work one way (PNP Vs. NPN). Your best bet is to find someone still running positive ground points and work out a partial trade.

I suppose you could "mad scientist" your ignition- for instance switch the hot lead, not the ground. May work or, more probably, just fry something.

Have fun,

Bill Eastman

I cannot see that you can switch the hot lead. I assume that the Distributor only has one hot lead and it earths through the Distributor body. I can see what Bill is saying, have the ground lead going to the LT connector and then somehow insulate the Distributor body for a separate earth. A big call, I think.

M F Anderson

Bill, thanks for your comment. I understand PNP and NPN transistors. If they are not potted I should be able to swop them. But for now I'll put it in the too hard basket. For the record I wanted to fit the much brighter LED stop & tail light bulbs & need the current to flow the other way Sean
S Sherry

Sean, If your ultimate goals is brighter tailights, then I can recommend the Halogen lights that Little British Car Co sells. They are very nice and do not effect the flasher timing. I am running them in my 1500. I believe that LBC gets them from a supplier in Australia. That way you would not have to change your disty. Just another alternative.

C Schaefer

Thanks Chuck, Halagon tail light bulbs now on order from Vintage Bulbs in Adelaide in South Australia Sean
S Sherry

I perceive a problem with halogen tail lights in the MGA. The taillights are indeed quite bright, but there is not a lot of difference in brightness between the tail lights and brake lights (and turn signals on the 1500 model).

I was once following an MGA 1500 with halogen tail lights on a moderately overcast day. My first impression was that the brake light switch must be stuck on or the driver was riding the brake pedal, because it looked like the brake lights we on all the time. The next problem was that the brake lights were not much as attention getters because of a relatively small difference in brightness between tail light and brake light. That could lead to getting rear ended when you hit the brakes.

I also once saw an MGA 1500 with halogen bulbs in the front corner lamps. Here the plastic lens is close enough to the bulb (in a small sealed housing) that the extra heat of the halogen bulb was causing some degradation of the plastic in the lens, a noticeable amount to crazing of the plastic, close to melting on the inside surface.

Since I now have the later production style number plate lamp with dual bulbs, the number plate seems to be brighter than the tail lights, giving good visibility in the dark. This may be just a personal preference, but I don't like the currently available halogen bulbs, and I won't put them in my car. I think they could be better if the low filament was a little less bright while keeping the full brightness of the high filament for the brake light and turn signal.


Barney Gaylord

Barney, some of your comments are well taken. In the 1500, the taillights and brakelights share the common lens. I also have concern about the intensity difference between the two and whether a driver would react quickly enough when I applied the brakes. Having been rearended in my daily driver more than I can count on one hand and maybe two. I really don't want to count the number. I am VERY sensitive about this.

My solution is twofold: 1)although I would like to drive during the daylight with my lights on, I do not. I feel that difference between "off" and "brakelights" attracts the attention of a driver quicker and in my case, outweighs the other benefits of running with my lights on; 2)I have added a 3rd brakelight just above the license plate light with electronic logic controls of my own design which do not require additional wiring to run from the front of the car. It plugs into the existing harness at the right rear of the car.

I have also designed, but never built or implemented, a plugin DRL module which would run the bright headlights at a lower intensity while keeping the running lights off. This is what my '95 S-10 had and I never burned out a headlight bulb on it.

In 8 years of driving with the halogens, I have had no problems with the rear lens's discoloring from the heat. I do not run Halogen bulbs in the front. They are not necessary, I painted inside of my front housings white and they are BRIGHT!
C Schaefer

Thanks for the inputs. As I don't use the car at night, a high level brake light sorts the confusion problem with the 1500 set up. With more visable rear turn signals and side marker lights wired in [std. fitting for the Danish market ? ] I hope to keep rear enders at a minimum !! Sean
S Sherry

S. Schaefer, -- Re: third brake light on 1500 without running another wire in the side harness. Can you send me your circuit and logic used to do this? (See e-mail address above). I know how to do it with a few relays and a couple of diodes, or some other variations of components that would fit in the palm of your hand. I'm curious to see you might have a simpler circuit.

My circuit logic has up to about 1/2 second time delay (half-flash-cycle) in both switch on and switch off for the third brake light, and I'd like to know if you have some logic that could eliminate the delay for switch-on time. This on-delay would be when brakes are applied while one of the turn signals is in the off-portion of the flash cycle.
Barney Gaylord

The circuit was built on a SMT PCB and potted. The operation of the third brake light is virtually instantaneous. The circuit operation is as follows:

Both power and signal are taken from the two turn signal lamps. Both inputs are fused for protection. IF only one lamp is on, then nothing happens. When both lamps are on then power return is provided for the LED lamp lighting the LEDs. When both lamps are off then the LED lamp is immediately terminated. When both brake and turn signal are active, C1 holds the gate of the power FET up for a period greater than the repetition rate of the turn signal thus keping the lamp on.

It is limited to about 2 Amps load. I don't know if I would do it the same next time. But it is what I did some years back. It still works. I'll also send a few photos of the finished product.

All rights reserved

C Schaefer


C Schaefer


C Schaefer

And finally, installed under the boot, tie-wrapped to the rear crossmember. You can see how small it is.

C Schaefer

Love the subject line. We technocrats are doing great (whether others may understand or not). Before getting any deeper into discussions of electronics vs relays, or cost comparisons, or why all this might be better than adding a single wire from brake switch to rear of car, I feel the need to push a point about the logic problem.

Regardless of how the circuitry works, it is dependent on two input signals from the left and right brake lamps, which also serve as turn signals (on the 1500 car). The third lamp can indeed be instant on when both rear lamps are energized at the same time, and can also be instant off when both signals go off. But consider that the turn signal has a cycle time of about 1 second while it is flashing, 1/2 second on and 1/2 second off.

When one turn signal is flashing continuously, if you step on the brake immediately after the flashing lamp goes off the other rear lamp will light up as a brake light, but the third lamp will not come on until the flashing turn signal regains power for the next on period of the flashing cycle. This delay before light for the third lamp could be up to 1/2 second.

On the flip side, when one signal is flashing and you have the brake on, if you release the brake immediately after the flasher comes on, the third lamp will stay on until the flashing lamp goes off again. That is, the output circuit stays latched on until both signals go away. This means the third lamp could have up to 1/2 second delay before switching off.

I would not be too concerned about the delay before off, but a 1/2 second delay before on could be critical when trying to prevent a rear-ender.
Barney Gaylord

Yes, you are right. My module cannot tell the difference between a turn signal vs a brake and light-off condition of a turn signal. It MUST see two lights on (left and right) at the same time in order to power the 3rd light. Actually, light on is not true. It must see power to the circuit. It is independent of a burned out bulb or a poor bulb connection or bad bulb housing ground.

Consider the condition of driving down the road and making a LH turn. Turn on the signal to indicate your intent. The LH turn signal comes on (then off - then on - etc.) This is the 1st indication to the driver behind. Then you downshift and apply the brakes. ~50% of the time, the turn signal light will be on and the 3rd brakelight comes on immediately. The other 50% the turn signal light will be off. Apply the brakes and immediately the RH brakelight comes on. Up to 1/2 second later the 3rd brakelight comes on. This is still better than no 3rd brakelight at all. 75 % of the applications are less than 250 ms. It takes approximately a quarter of a second for incandescent brakelights to reach 90% brightness.

In this synopsis, I think that the usual MG driver will downshift once or twice before applying the brakes. This delay is significantly more of a delay than the zero to 1/2 second delay of the circuit.

My main goal at the time was to provide a 3rd brakelight without running a wire from the brakelight switch. I have said, I don't think that I would build it the same way again and it would be a bit more complicated to address your concern.

4 of my rear enders were sitting still in stopped traffic. This would not have helped in those situations at all.
C Schaefer

Have you seen the Signal Dynamics brake light modulators?
I don't know if these would be compatible with the MGA lighting system but they are certainly worth looking at to create a more visible brake light operation

Rich McKIe

This MGA owner does not down shift at all ! Ilearned after a life time in the trade that brakes are cheaper than gear boxes ! I never ever want to have to pull the motor and gear box on my A again. I used to be able to drop the Auto box in Lincon Continentals in half an hour.In the early sixties I was much younger. The Yanks knew something that the Brits didn't.
And the third brake light,just run a twin cable from the control box to a LED strip light mounted on the panel behind the seats. Simple and effective and colour code it. Sean
P.S. I can send a pretty picture if I can work out how to down load one. Has the system changed ? I've done it before.
S Sherry

This thread was discussed between 25/10/2008 and 31/10/2008

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.