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MG Car Repair/MGA 1500 brake lights/directionals wiring


QUESTION: Hi Howard,

I am currently wiring from scratch an MGA 1500 (with the single 2 filament bulb in the taillight); it is a hot rod with a Ford 302 V-8, hence the ground-up wiring harness.  I am not understanding the circuitry of the brake lights/directional lights, in conjunction with the direction indicator relay unit.  1.  Are the directional filaments and the brake light filaments the same?  Is it this relay unit that allows the brake lights/directionals to share the same filament?  If I wished to maintain a single two filament bulb in the taillight (hence maintaining the original functionality) am I obliged to use this relay, or is there a modern equivalent?

Thank you in advance for any and all information you can give me.



Yes the early MGA did use a 6 pin relay to operate the brake lights, turn signals and parking lights from a two filament bulb. My background was mainly British cars and I don't know of any later car that used that system.

Other then changng the front and rear parking, brake and turn signal lamps to the 1600 MGA which was like all newer cars.

I did find a wiring diagram of the internal workings of the relay which if you can't find a realy you may be able to make up some individual relays and resistors to make your own.

Is the old relay missing? If it is still there, you may be able to just remove the cover and clean all the contacts to get it working again.

If you don't have a diagram let me know and I will copy one and put it on my web site for you to copy. Let me know.


---------- FOLLOW-UP ----------

QUESTION: Hi Howard,
thank you for your prompt response.  In the interim I was able to locate a schematic for the directional indicator relay, so I am set.  The original piece in the car is gone; I see Moss sells a replacement.  Please correct me here if I am incorrect in my understanding of how those lights work.  When executing a right hand turn with the right turn signal on, and using the brakes to execute the turn, what I should see in the taillights is:  Left taillight is steady bright (from brake light switch), and right taillight is flashing bright, (from the relay and directional switch).


Hi Dan,

That is the way I read it too. I worked on many MGAs over the years but when a relay quit we just replaced it but I do reember on ocasions when the part was on order we would take relays and votage ruulators apart and clean the contacts to get a customer on the road.

But with the diagram of the internals of the relay I do believe you can Get a couple of double action relays and wire it up to operate. I have had to search for relays that had addedd functions and I had found on the Net a relay comapany that showed a lot of optional relay functions. Let me know if you can't find one and I will search too.


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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