Electropneumatic Trafficator Timer


It's Easier to Fix Than to Remember its Name

Cruising the Net

Paul Kile

MGA turn signals have driven many people insane. Unlike most 'normal' cars, the MGA uses one or two devices that were placed in the system to confuse the amateur. Lucas did this on purpose to try to get you to take your car to an 'authorized Lucas service representative' for repair. Well, I am about to divulge the sacred knowledge that will allow YOU, the owner, to enter the inner sanctum!

The first of the devices, the one that I will discuss today, is common to all MGAs. This is the turn signal switch, the part that lies behind the turn signal knob on the dash. If we are to use its full name, the one that accurately describes its function, we would call it the Electropneumatic Trafficator Timer.

How is it supposed to work? Well, if everything is copasetic, the turn signals will come on when you twist the knob on the dash and will stay on for about 20 seconds. During that 20 seconds, the knob will slowly return to the centre position and click off. When you move the switch to activate the turn signals, you close an electrical contact and force air out of a small chamber with a piston sealed with a leather cup. When you let go of the switch, the spring loaded piston draws air slowly back into the chamber through a small hole. This intake of air takes time, hence the 20-second delay before the switch clicks off. The piston turns a small cam that reopens the electrical contact. If you take the switch apart you will see that the parts are simpler than the explanation. More on this later.

There are only a few things that can go wrong. The most common is that the switch clicks off right after you turn it on. There is no delay action. The reason for this is that the air is coming back too quickly into the chamber. If you stick your head under the dash and look at the back of the switch, you will see three wire connections arranged around the edge of the switch. Right in the centre of the back of the switch, there is a small recessed hole. There is a small setscrew inside this hole that controls the amount of air coming into the chamber. Take a small slotted screwdriver and turn the setscrew in slightly, then try the switch again. Keep doing this until you get the right amount of delay action.

What if you turn the setscrew all the way in, and there is still no delay, or what if the screw is already turned all the way in? There is a cotton plug behind the setscrew that acts as a bleed seal for the air, this gets hard and non-pliable with age. The next step is to take the setscrew all the way out (careful, it's small) and then take a small needle and tease out the cotton seal. It may be easier at this stage to take the switch out of the dash first. When you get the cotton plug out, knead it between your fingers until it is soft and pliable. Stuff it back in the hole, put in the setscrew, and readjust the screw as described above.

What if there is still no delay action, or if the switch doesn't click off? Then you'll have to take the switch out of the dash and take it apart. When you remove the switch, note the colour codes on the wires, the switch connections are clearly marked B(attery) R(ight) and L(eft). When you get the switch out, take the three screws out of the front of the switch and slide the plastic cover off. You will see the metal switch contacts and the white plastic cam that activates them. To further dismantle the switch, you will see three long screws that hold the switch together. Loosen these carefully in stages, while holding the switch together, there may be a slight spring tension to work against. Carefully let the switch open up, you will then see a metal crossbar attached to the shaft, and the contoured back of the piston. The piston will either pop out freely or will be stuck in its housing. If the piston is stuck, you may have to pull it out with some needle nose pliers, or use air pressure through the setscrew hole. There is a spring behind the piston, don't lose it.

Once the piston is out, remove all the 30-year old grease with some alcohol or Trichloroethane based brake cleaner. DO NOT use carb cleaner, or you will damage the plastic. Fold out the edge of the leather seal and re-lubricate everything with Vaseline or white lithium grease, then reassemble the switch. If the switch still will not stay on for 20 seconds, try wrapping a turn of masking tape around the barrel, under the leather seal. This will effectively increase the pressure of the leather against the wall of the switch.

Before reinstalling the plastic cover, you may want to clean up the switch contacts with fine sandpaper. Don't forget to readjust the setscrew at the back for the proper delay time. Now you are ready to reinstall the switch in the car and you have saved yourself 150 bucks for a new switch.

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