OldBob Posted July 4, 2021 Report Share Posted July 4, 2021 The brake lights on my '55 TR2 have suddenly stopped working - everything else still working OK though. So I suspect the brake light pressure switch. But, checked the wiring to the switch and it doesn't match the standard wiring diagram I have. Mine is a 2 terminal switch which has 1 black wire and 1 Green with red stripe going to it. The odd thing is 12v is present in both wires when running. Anyone else have this arrangement? - or understand how the switch works with 2 live wires? The wiring harness was replaced 30 years ago BTW and everything was working fine until now. Many thanks Bob Quote Link to post Share on other sites
RobH Posted July 4, 2021 Report Share Posted July 4, 2021 Well that colour scheme is indeed odd but the wires may not actually both be live - your meter could be lying to you. Most digital meters take so little current that a tenuous connection can give spurious voltage readings, particularly where an earth has been lost. I suggest you find out which wire is actually live using a 12v lamp with wires attached. The other wire will be the lamp feed and if that is then connected to live the lamps should light providing the earth is OK - which it may not be. Quote Link to post Share on other sites
David Owen Posted July 4, 2021 Report Share Posted July 4, 2021 45 minutes ago, RobH said: Well that colour scheme is indeed odd but the wires may not actually both be live - your meter could be lying to you. Most digital meters take so little current that a tenuous connection can give spurious voltage readings, particularly where an earth has been lost. I suggest you find out which wire is actually live using a 12v lamp with wires attached. The other wire will be the lamp feed and if that is then connected to live the lamps should light providing the earth is OK - which it may not be. I just had the same problem and Rob's solution was the solution. I cleaned the ground connection in the boot with some sandpaper and it cured it. Quote Link to post Share on other sites
OldBob Posted July 4, 2021 Author Report Share Posted July 4, 2021 Many thanks Rob, I'll try out your suggestion tomorrow and let you know. Regards Bob Quote Link to post Share on other sites
OldBob Posted July 5, 2021 Author Report Share Posted July 5, 2021 21 hours ago, RobH said: Well that colour scheme is indeed odd but the wires may not actually both be live - your meter could be lying to you. Most digital meters take so little current that a tenuous connection can give spurious voltage readings, particularly where an earth has been lost. I suggest you find out which wire is actually live using a 12v lamp with wires attached. The other wire will be the lamp feed and if that is then connected to live the lamps should light providing the earth is OK - which it may not be. Test bulb tried Rob and both wires that go to the brake switch are definitely live - although one wire gives a brighter bulb than the other. By connecting both the wires together the brake lights work. Again I suspect the brake switch but until the replacement arrives I can't confirm. Still don't understand how two 12v wires touched together can complete the brake light circuit? Bob Quote Link to post Share on other sites
RogerH Posted July 5, 2021 Report Share Posted July 5, 2021 6 minutes ago, OldBob said: Still don't understand how two 12v wires touched together can complete the brake light circuit? Bob Hi Bob, remove the green wire and check the volts on it -it should be Ov. - leave disconnected Now check the volts on the black wire. It should be 12V Positive earth Whilst you are deving into the unknown why not put the colour coding back to standard - Green = Ov Green/Purple = 12v (positive earth) Roger Quote Link to post Share on other sites
RobH Posted July 5, 2021 Report Share Posted July 5, 2021 (edited) 1 hour ago, OldBob said: Test bulb tried Rob and both wires that go to the brake switch are definitely live - although one wire gives a brighter bulb than the other. By connecting both the wires together the brake lights work. That is a bit of a mystery and you are right that a replacement switch will obviously fix it. Just what is going on with those connections is difficult to fathom though. I wonder whether a relay has been used? The dimmer bulb connection must be coming through some resistance. I believe that early TR2s only had a single central brake light. Perhaps this is to do with altering the circuit to light the two others as well? Edited July 5, 2021 by RobH Quote Link to post Share on other sites
OldBob Posted July 5, 2021 Author Report Share Posted July 5, 2021 1 hour ago, RogerH said: Hi Bob, remove the green wire and check the volts on it -it should be Ov. - leave disconnected Now check the volts on the black wire. It should be 12V Positive earth Whilst you are deving into the unknown why not put the colour coding back to standard - Green = Ov Green/Purple = 12v (positive earth) Roger The wiring colours to the switch are Black & Green+Red stripe Roger. -12v is on both wires (its a pos earth car) and the wiring is part of the replacement harness, making me a bit reluctant to unwire it. I guess back in the 80's this is how the replacement harness was supplied. Bob Quote Link to post Share on other sites
OldBob Posted July 5, 2021 Author Report Share Posted July 5, 2021 31 minutes ago, RobH said: That is a bit of a mystery and you are right that a replacement switch will obviously fix it. Just what is going on with those connections is difficult to fathom though. I wonder whether a relay has been used? The dimmer bulb connection must be coming through some resistance. I believe that early TR2s only had a single central brake light. Perhaps this is to do with altering the circuit to light the two others as well? Yes Rob, been trying to fathom it out today without sucess! And likewise, a dimmer lit bulb by the green wire made me think of something in the circuit but there isn't any relay (except the overdrive one). Maybe coming through more dash wiring is the reason - the engine wasn't running, I just had ign on. The car has had the mod you refer to btw where the centre and wing brake bulbs all illuminate making 3 in total. Never been an issue in years though. Waiting for new switch to arrive with fingers crossed! Bob Quote Link to post Share on other sites
OldBob Posted July 11, 2021 Author Report Share Posted July 11, 2021 New switch fitted and 100 miles done today with working brake lights Quote Link to post Share on other sites
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