Modern Turn Signal Relays for Your MG TC
by David Edgar, TC 5108

These instructions are for making your own turn signal relays from modern components that are cheaper and more available than older Lucas turn signal relay boxes.  These modern components are in no way original looking but they are small enough where you could mount them inside the old Lucas relay boxes to at least appear original if that is your bent.

What is the purpose of a turn signal relay box?

You only need relay boxes if any turn signal filament shares it's duties with a brake lamp filament.  Relay boxes simply take a left or right brake lamp out of the brake lamp circuit and connects the turn signal circuit. This is the case with a TD and also with a TC EXU where those lamps on the petrol tank are used.  A true EXU has no "D" lamps but many owners have installed two of them on both EXU and normal TCs.  If you have two brake lights and are braking, both (hopefully) brake lamp filaments are lit up.  If you also have turn signals and lets say you are signaling to the right, then the left brake lamp continues to show the steady brake signal, but the right lamp filament now starts to flash.  The steady brake signal on the right side is interrupted by the relay and is fed the pulsing turn signal input until you turn off the turn signal.  The relay boxes act the same way even if you are not braking at the time you signal.  The brake lamp circuit is interrupted if there is a brake signal or not.

If you are adding turn signals to the rear of a TC and you mount stand alone turn signal lamps then you don't have the conflict and you do not need relay boxes.  You just wire the brake circuit to the brake lamps and the turn signal circuits to the turn signal lamps.

One quirk of the TC EXU model is that the front turn signal filament comes on with the brakes as they are wired together with the rear brake lamp.  The Lucas relay boxes used on the EXU couldn't separate the two.  Later TD and TF models eliminated this quirk with a different relay box and electrical circuits.  Making up your own relay boxes can eliminate this fluke.

Modern Electronic Relay Units

Modern relay units can be found at a number of electrical supply houses but I used the ones at Radio Shack for convenience only.  The Radio Shack unit I used was the mini 12VDC/5A DPDT #275-249 priced at $5.29 in 2004. You need a DPDT (double pole, double throw) in order to separate the front signal lamp from the rear otherwise you get the quirk as described above.  Dimension wise it is about and inch long, half inch wide and 3/4 inch high. You would need two of them, one for each side (left and right).  I soldered TC wires directly to the pins on the relays.  You could also solder the relays onto a circuit board and install screw terminals if you wish.

The 5 amp rating is fine for one front and one rear lamp per side. If you wanted multiple lamps for some reason and you exceed 5 amps then Radio Shack does carry larger 10 amp versions as well (#275-218) for a few dollars more.  And for these 10 amp units you can buy a base socket that you wire up to and just plug the relay in.  You still need to solder wires to the base or connect the base to a circuit board and screw terminals.  My only concern here is with road vibration and that the relays do not vibrate loose from the base socket.  I have seen other relays available that have base sockets with screw terminals and wire bales to keep the relay in position.  Look around and you can find the best units for your TC.  Just remember that you need a DPDT relay in an amp rating high enough for the lamps you are running and that the voltage rating of 12 VDC is met.


I glued both of my small relays together just so I had one unit flopping around under the dash rather than two separate ones and then wired EACH relay up as follows.

N.O. (normally open) terminals - wire both relay N.O. terminals to wire leading to flasher unit "L" terminal.

N.C. (normally closed) terminals – connect wire from brake lamp switch to one of the two N.C. terminals (the second N.C. terminal will not be needed and will remain unconnected)

Common terminals - Identify which half of the relay you connected the brake switch to above and then connect the common terminal, on the same half of the relay, to wire going to rear stop bulb filament. Second common terminal goes to wire feeding front turn bulb filament (remember that one relay is for right side and one relay for left side)

Coil terminals - wire one to ground and one to to turn indicator switch (one relay to left switch terminal and second relay to right switch terminal)

What happens is when you step on the brake, your stop lamp switch feeds current to one N.C. terminal on each relay. Current then flows to common terminal and to your left and right brake lamp bulbs.

If you have your right turn indicator switch on, then current flows to the right relay (coil terminal) and then to ground (green path in diagram below).  That relay is now activated and switches N.O. terminals to common (red path in diagram). When this happens two things occur. Your brake lamp is disconnected from the brake lamp switch (the right side in this case) and your bulbs (front and rear) are now controlled by the current flowing from the flasher unit. The double pole double throw (DPDT) relay is necessary so that the front turn signal bulbs are isolated from the rear turn signal bulbs and do not activate with your brake switch (the reason one N.C. terminal is not connected).  The left relay in this example is not activated and the brake switch continues to feed the left brake lamp with a steady brake signal (blue path in diagram).

Many people have been confused when wiring up their relays.  Please note that pin locations will vary between relay manufacturers and even vary over the years if bought from Radio Shack since suppliers to them may change on items.

The schematic above was drawn to make it somewhat simple visually and terminals on the relays you buy may or MAY NOT be laid out the same.  In other words, the N.O. terminals are not automatically the upper most terminals.  They could be further down.  

You MUST make connections based on terminal type (N.O., N.C., COM, Coil) and NOT location on above schematic (this is fairly common practice if you have ever worked with electronic items).  Relays you buy should come with a diagram showing terminal function (N.O., N.C., etc).


  • 125VDC/5A DPDT Mini Relay
    Coil Ratings: 12VDC, 60mA, 200§Ù
    Contacts and Rating: DPDT 5A at 125VAC
    Size: 7/16x3/4x1/2"
    Radio Shack part number 275-249
    (two blue relays shown above glued together)
  • 12VDC/10A DPDT Plug-in Relay
    Coil Rating: 12VDC, 130mAh, 160§Ù
    Contacts and Ratings: DPDT 10A at 125VDC
    Size: 1-5/8x1-3/4"
    Radio Shack part number 275-218
  • Base socket for 10 amp relay
    Radio Shack part number 275-220