Is your MGA 1500 turn signal relay giving you problems? Failed coil? Can't make the contacts connect? Have you checked the price of a new one recently? (Hope you were sitting down at the time). If you think it's repairable, maybe you should try fixing it. But if you're convinced that it is truly shot and beyond salvage, then you might spend an hour to replace the internal works with a pair of $5 double pole double throw 5 amp relays from Radio Shack. This little project was completed in December 2004.

The black relay shown here is just over 1" long and only 1/2" wide. It is Radio Shack number 275-249. Two of these can serve the same function as the original turn signal relay. See the MGA 1500 turn signal circuit here: circ_f2.htm. You can also assemble two of these new relays inside the same case as the original relay unit, and no one would ever know that you have modernized your electrical system.

Starting with the standard turn signal relay unit (shown at top right), remove the cover and gut the contents. Cut the wires connecting the relay coils to the bottom terminals (4 & 8) and the grounding point on the metal base. Turn it over and grind off the riveted tips holding screw terminals 3 and 7. Twist and pull the relay coils to remove them from the base. Then re-attach the loose screw terminal plates using a pair of #8 screws with nuts. Also install ring lug wire terminals on the top side. If you put the nuts on the bottom, grind the screw tips flush with the nuts so they will not short out on car body when installed. Cut the four steel posts off about 3/8" from the base. Cut a short slot in the top center of each of the four posts suitable for wire connection. De-burr the posts and throw out the old contact parts and relay coils.

There is a thick phenolic plate surrounding the top four terminals. Cut this plate flush with the inside of the four posts and remove the center piece to leave a flat floor on the thinner phenolic plate. Sand the floor a bit in this area to clean and scuff it in preparation to accept epoxy.

Cut nine pieces of wire at least 3 inches long. Use 16 AWG stranded and insulated PVC wire. Strip one end of each wire 1/4" long. Use acid core solder (or solid solder and soldering acid) to tin the tops of the four steel posts. Use resin core solder (or solid solder an resin flux) to solder the wires to the nine contact points as shown above left. The picture above right shows the bottom side of the base with the two terminal plates secured with screws.

Position the two new relays on the base as shown at left. Trim wire ends at the four posts if necessary to allow the relays to fit in between. The relays will be epoxied to the base, and also (preferably) to each other. Wipe the joining surfaces of the relays and the floor plate with isopropyl alcohol (rubbing alcohol) to clean and prime to promote good adhesion. Allow to dry. Glue the relays in place with your favorite five minute epoxy. (JB Quick Weld has good heat tolerance). Position relays with the coil terminals toward the grounding end of the base, and keep them at least 1/4 inch away from the opposite end of the base. Allow epoxy to set.

Refer to the diagram at left. Cut each wire to suitable length leaving 1/4" bare for single terminal connections and more length bare for multiple terminal connections. Twist the stranded wire snug, skewer it onto the small pins and solder it in place. For terminal 5 it is only necessary to connect the wire to one NC pin on each relay, but it is convenient to connect to all four NC pins, which will also share the electrical load for brake lights over the dual contacts.

Okay, that's it. Test the new relay, put the cover back on, install it in your MGA, and it should be good for another 40 years (maybe).

There are a few additional notes about these relays, such as the operating spec's.

Expected life, Electrical: 100,000 cycles (at rated voltage and current)
Expected life, Mechanical: 10,000,000 cycles (with no load)
Absolute limits for ambient operating temperature: -22 to 131 dF (-30 to +55 dC)
Coil voltage: nominal 12VDC, maximum 15.6VDC
Coil current: 60mA (nominal)
Coil resistance: 200 ohms +/- 10%
Pickup voltage: 9.6 VDC
Dropout voltage: 0.6 VDC

The one point that stands out here is the operating temperature range. I don't suppose many people will be driving their MGA when temperature is lower than -22dF. But this relay lives in the engine compartment where I am SURE that air temperature can and does go higher than 131dF. I haven't had a problem with this yet, but if it ever fails I would post a note here. The appropriate cure for that would be to use relays with higher temperature rating, which are likely to cost considerably more. If anyone else has experience with this application I would like to hear about it.

Addendum September 12, 2006
After 14 months and 10,000 miles I am happy to report that these relays are doing just fine in real world service. I have been seriously autocrossing, then parked with temperature gauge pegged and the Dutch oven going around the exhaust manifold under the carbs. I have driven a number of rough roads. I have driven the car in 100dF weather towing the little trailer at speed, and have been stop and go in heavy traffic with the temperature gauge near the high end peg, sweltering heat, scorching seats, sheet metal hot enough to burn you hand, and the carburetors suffering serious vapor lock symptoms. Through all that the new relays are soldering on, so I now have lots of confidence in this modification. I will definitely post here if anything happens to the contrary.

Addendum July 12, 2009
Now 4-1/2 years and 30,000 miles in service, and it's still working fine.

Addendum January, 2010
It has been brought to my attention that Moss Motors is now supplying an "upgraded" turn signal relay. If you would rather spend money than fiddle, this one uses the same sealed relays and has a circuit board rather than hand wiring. Kudos to Moss Motors for making it a commercially available item.

Addendum July, 2010
However, there may be a problem. Remember the 131dF maximum operating temperature rating? See next page.

Addendum October 13, 2015
Today I have one reported failure of the Moss turn signal relay unit. James Keith in Greensboro, North Carolia, USA reports a connection failure between terminal #5 (brake light input and terminals #3 & #7 (brake light outputs). The turn signals work okay, so the switched output contacts (turn signal outputs) are good. This may be a cold solder joint between terminal #5 and the relay input pin on the circuit board. Not yet determined.