February 2009 (Improvement)
When the main (Lucas) light switch failed for the second time (apparently overheated), I felt it was time to consider adding a pair of relays for the headlamps. I was somewhat surprised as the switch in question normally supplies the current direct to the headlamps (via the dipswitch/indicator stalk) on the MGB and does not have a habit of failing as far as I am aware.
Also, separately, the indicator stalk started to play up by losing contact to the dipped headlamps whenever I pushed it upwards (clockwise). This could also be the result of overheating but I suspect it is more likely due to the constant twisting action when operating it because of the modification made to the standard MG stalk.
The two new relays were fitted behind the dash (photo 1) beside the other relays. Image 2 shows the old and new wiring for the headlamps. For full wiring details see the downloadable PDF.
Each of the two circuits (main & dip) is treated exactly the same. The relevant wire from the dip switch (blue/white or blue/red) is interrupted just before the fuse box and re-routed to relay terminal 86. Terminal 85 is then connected to earth completing the low current section of the circuit.
For the high current section a 12v supply is connected to relay terminal 30. Terminal 87 is then connected to the fuse box where the original wire from the dip switch went thus feeding the respective lights.
The principle is that the light switch & dip switch now only supply current to the relay (a very small current indeed, about 30mA) and the main current (about 10A) only passes through the actual relay and the original fuse to the lamps. The light switch does still pass the side lights current, of course, but that is only about 3 amps.
This modification should, therefore, vastly reduce the likelihood of switch failure.
When I replaced the indicator stalk, instead of modifying the connector on it to match the NG loom, I built an adaptor (photo 3) which will make life easier when I have to replace it again. However, the original Ripaults connectors (as used on the NG loom) no longer seem to be available so I changed it to a standard spade type multi-connector. I shall now use this type in future when I do further adapting or replacing.
Click this icon for a PDF version of the Headlamp Wiring which you are free to download.