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Headlamp Relays

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.

Headlamp wiring - click for larger size

Electrical Connections

The standard wiring for the headlamps is as shown in the diagram. A 12v supply from the main fuse connects to the main light switch, which has two 'on' positions for side lights and headlamps. The blue headlamp wire takes the current to the dipswitch section of the indicator switch and this in turn connects to either the blue/red wire (for dip beam) or blue/white (for main beam). These are connected to the headlamp units via the fuse box where each circuit is protected by a separate fuse. It can be seen that the whole working current for both headlamps (about 10 amps on main beam) and the side/tail lights (about another 3 amps) passes through the main light switch.

Fitting Relays

Relay position - click for larger size The two new relays can be fitted behind the dash beside the other relays.

Each of the two circuits (main & dip) is treated exactly the same. The wire from the dip switch 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.

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 only passes through the actual relay and the original fuse to the lamps.

This modification should, therefore, vastly reduce the likelihood of switch failure.


Click here for PDF version of Headlamp Wiring

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Indicator Stalk

Indicator connection modification - click for larger size When I replaced the indicator stalk, instead of modifying the connector on it to match the NG loom, I built an adaptor (see photo) which will make life easier if 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.


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