MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Headlights Switch

I have a 1971 MGB with what appears to be a headlight switch from a later model. It has three prongs in the back. How do you check if the switch is good in all positions?

Also, are the headlight switches interchangeable between years? Is there a difference between headlight switches for MGBs made for UK and those made for US? There is a new Lucas headlight switch for sale on eBay, listed as "UK and may not fit USA model", with the Lucas number of 39787 and BL number BHA5111. Does anyone have an idea of whether it would fit my car? Am I better off just buying a new one from Moss? Thanks all.

Curt
Curt

Use continuity tester (the one's with the battery and light inside) and test to see how current flows at each switch position. In the off position no current should flow between any terminals. In the middle on position current should flow between two terminals...the hot wire and the parking lights and in the full on position current should flow from the hot wire terminal to both other terminals as both the parking lights and headlights are on at that point. Went through this recently testing a hazard switch that had different terminal/pin positions than the original. Someone correct me if I am wrong. I had never hear of the UK vs US stuff not fitting. Sounds odd. Perhaps someone else can enlighten.

JTB
J.T. Bamford

didn't complete that....and to figure out which is the hot terminal...I would think the two switches that current would NOT flow between with switch in full on position would be the headlight and parking light terminals and the other one would be the hot terminal but you would have to confirm that testing the continuity from that terminal to the other two.
J.T. Bamford

You could start with a continuity tester or ohm meter, but the switch may test OK then not work under load. Sometimes the internal connections have "arced" and are damaged. This results in an intermittent operation and the switch working if "wiggled" Nothing more fun than having the lights go out when driving down a country road at night.

Dave
dave

Ohms or continuity isn't good enough, you need to measure the volt-drop in the switch, if any, on full working load. Resistance can be low at the microscopic currents used my ohmeter and continuity meters, increasing as the current through the switch increases. Resistance in the switch will cause heating and low output of the lights. All headlight switches are the same electically but the physical arrangements vary between years and markets. All three connections should be isolated in the off position, two connected in the parking light position (brown and red or red/white) and the third still isolated, and the third (blue) connected to the other two in the headlight position.
Paul Hunt

This thread was discussed between 29/07/2004 and 30/07/2004

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.