|The replacement brake
light switches sold today are extremely light duty and don't hold up to
the 3 amps drawn by the brake lights for very long before the contacts
burn again (I have had them burn in just two weeks). Adding a relay to
the brake light circuit will remove the heavy current from the brake
The relay to use is
a general purpose 30 Amp automotive relay, sometimes known as a Bosch
relay. They are available from Radio Shack and most auto parts stores.
The relay can be mounted anywhere that is convenient. I mounted mine
close to the original brake light switch, but it can just as well be
mounted in the trunk (for the MGAs or MGBs), close to one of the tail
light/brake light assemblies (if you do this, you will need to bring a
source of 12 volts, other than the 12 volts that is switched by the
brake light switch, back to where the relay is mounted). In the
accompanying diagrams, I show a 0.47 microfarad capacitor across the
contacts of the brake light switch and a diode across the coil of the
relay. These are optional parts and can be left out if you wish. I added
them as additional protection for the brake light switch. The capacitor
is just soldered across the switch terminals. The diode can be soldered
across terminals 85 and 86 of the relay. The advantage of the capacitor
is that the capacitor acts as an arc suppressor and the diode collapses
the field of the relay coil, eliminating any inductive surge across the
brake light switch when it opens.
If you feel that you are electrically
challenged, or just donít want to fuss around putting the circuit
together, I can supply you with a completely assembled circuit to fit
you car. If you would like to purchase one of these relays and a
capacitor, the price is $10.00USD (in July 2003). Send me an email to
confirm current price, availability and shipping costs to your location.
I also need to know the polarity of your car and if it has a
hydraulically actuated switch or one that operates off of the brake
pedal. Send a check or money order to:
1913 South Marine Dr.
Bremerton, WA 98312
I will ship the parts upon receipt of
your check, or your money order..
Note: In the following two
diagrams the only difference is the orientation of the diode.
Positive Ground Vehicle
Negative Ground Vehicle