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Triumph TR6 - Head light low beam not working on one side

I have a '69 TR 6 with an issue that does not seem to be possible. The high beam works on both sides. The low beam works on the left (driver's) side, but not on the right side. It used to work OK, so I figured the lamp went out. I bought a new headlamp, but had the same results. I checked voltage on the connector pins. As viewed from the back side of the connector, the left pin is gnd; When foot switch is switched to low beam, the upper pin shows 10.95V and the right pin shows 0 volts. When the foot switch is on high beam, the upper pin shows 0 volts and the right pin shows 10.75 volts. This would seem to be correct. Now the part that baffles me: When I have the headlamp removed and ground the gnd pin and I have the foot switch on high beam (reading 10.75V on the right pin, when I run a wire from that pin and touch the high beam contact on the headlamp, it lights. When I touch the low beam contact, it also lights. Now when I push the foot switch and switch to low beam, I measure 10.95V on the upper contact. Keeping the gnd pin grounded, when I touch the high beam contact on the headlamp, nothing happens. When I touch the low beam contact on the lamp, nothing happens. I repeated it about 4 times with the same results. How can this be when there is 10.95V present? This totally baffles me and seems to defy the laws of physics (at least in my thinking). Does anyone have any ideas as to how this could be happening?

Thanks a lot,
DEH Hancock

Hi Ed

The Lucas wiring ghost is "alive" and well in your 69.
I suspect you are getting some voltage but no real current to light the light. You have a dirty connection somewhere. I would suspect the low beam connection from the left light to the right light located near the left light. Disconnect it and clean it and see what happens.
Michael Petryschuk

Thanks for the suggestion. I looked for the connection, but couldn't see it right near the left light. I think that it may be down at the horn, but the fan shroud is in the way so I'll have to take it off and look closer. I'll get back at that in a bit, but got side tracked with the Lucas ghost causing more havoc. Seems like I fix one thing and then something new pops up. My brake master cylinder went out, so replaced that. Then the day I finished that, I noticed oil on my floor mat and my clutch resevoir was low. Turns out the seals in my clutch master cylinder gave out as well. I bought a repair kit and did the master and slave rebuild at the same time. Thought while I was at it I would see if I could fix or replace the rheostat that never really worked since I bought the car 13 years ago. Took the rheostat out and cleaned it all out and got it working again. But since I got it working noticed that some of the gauge bulbs were out, so decided to replace them. My mistake was taking the fascia (dash) loose to get at the rheostat and bulbs. I was a little reluctant to do so knowing that Lucas electricals are pretty finichy, but decided to take the chance since it makes things easier to get at. Now I am fighting the Lucas ghost. All of the sudden my ammeter wasn't moving. I thought that I must have accidently disconnected contacts while pulling the fascia loose. Nope, they looked good. Checked voltage at the battery (with the engine running). Just under 12V. Are you kidding me? Could my alternator all of the sudden die? Disconnected the connectors to the alternator and cleaned them up. Then followed the alternator output wire back to the fuse box and cleaned that connection. Now I got my alternator working again.... So, you can see that I have been having a lot of fun lately. I had last week off from work as vacation, but back this week, so won't have as much time to put into it. The Lucas ghost will live to fight another day... Thanks again for your help. I'll let you know what I discover when I get back at that issue.
DEH Hancock

Sounds familiar Ed. One thing begets another with these cars. But we love 'em. It sounds like you have a few oxidized electrical connections which makes me hopeful your head light problem is the same.
Michael Petryschuk

This thread was discussed between 02/08/2013 and 08/08/2013

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