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MG MGB Technical - Tail light problem

Met a nice policeman last night coming home from a meeting...seems my tail lights were out...brake lights work, headlights work, both hi & low beams, turn signals work, 4 way flashers work. He was very nice and let me proceed with my promise to keep my four ways on for the last 20 miles of my trip. Car is a 69 B roadster. All connections in the boot look solid...
Bob Dougherty

To follow up, no lights when I put light switch in park, parking lights & license plate light inop. Cursory check of the fuses looked good but on closer inspection the bottom fuse (two fuse block) was burned out...solved...NOT! Replaced with a known good fuse and still no tail lights, parking lights or license plate lights...switch bad in the park position?
Bob Dougherty

Bob,
have look in your Driver's Handbook or Bentley for a wiring diagram, you may have an in-line fuse(s) for all those lights after the fuse box and light switch.

Generally it's not a good idea to replace blown (well actually melted) fuse(s) until you know the cause of the fuse going as the fault could continue before the new fuse goes. Fuses that melt slowly are often from overload/overheating, if they go instantly it's often a short of some kind, e.g. exposed wire touching.

Check all your wires and connections, the problem might be at the switch but not necessarily the switch. If your gauge lights work then it's probably not the switch but after it. Check for frayed wires or exposed connectors, for both sides to go it could be before or at the double bullet connector where the sides split into two lines.

You could check all the bulbs work in themselves but it would seem great odds and low possibility that all/one side/many should go at once - but you also need to know they all work for problem solving. Change any that have gone black or grey, change them in their pairs to keep lights even and avoid cheap bulbs, get heavy-duty or fast-response.

Whilst there clean inside the lenses, the bulbs and reflectors to get as much light out as possible (see and be seen) - don't use any polish on plastic reflectors I made that mistake, it rubs the reflector shiny surface off.
Nigel Atkins

What about the front parking lights? If they are not working either it's probably the switch.

The later 2-fuse box cars had two in-line fuses for the parking lights - one for the rear lights and one for the fronts. These will probably be where the rear harness meets the main harness with all the other connectors for the rear and gearbox harnesses. If this is what you have, and the fronts work, then it will probably be the fuse going to the rear harness, or its connections that has failed.

I thought for the 2-fuse boxes that the white and green wires were on the bottom fuse, and if that is blown you wouldn't have any tach, indicators, brake lights and a number of other things so should notice that before being pulled over.

If it's in the brown and purple wires and had blown you wouldn't have any horn, courtesy lights or headlamp flasher.

I'd be grateful for my own info if you could confirm which colour wires are on the top fuse and which on the bottom.
paulh4

Connector in the trunk that connects all the lights that are out. I would pull the connector and see if you have power there.
Bruce Cunha

Just thought, what Bob, being in USA, calls a 69 could perhaps be a car made in 1968(?) (if that makes any difference).
Nigel Atkins

A good number of them were, but both 68 models (made in 67 and 68) and 69 models (made in 68 and 69) all had the in-lines that fused the front and rear parking lights separately. It could also have been a 70 model made in 1969, but they have the 4-fuse fusebox which Bob indicates his doesn't have.
paulh4

Problem solved! Paul you were correct of course, the brown wire does go to the top fuse, I misspoke when I said bottom fuse, I meant top. The parking lights in front were also not working so I thought that confirmed a bad switch...however, thought I'd double check my work before ordering a new switch and tearing things apart! First I checked what I thought was the blown fuse from the top of the fuse box...it was not blown after all, the discoloration was on the outside of the glass, the filament was fine. I then checked the two fuses on the fender well on the red wire circuit...there it was, the fuse toward the firewall was most definitely blown! Replaced it with a new 35A fuse and all lights now work!


Bob Dougherty

Curious - the two separate fuses should be one powering the front and one the back. So if only one of those was blown the other end should still be working, unless there was another fault somewhere else. But if both ends are working now, that's what matters.
paulh4

This thread was discussed between 07/10/2018 and 12/10/2018

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