Reversing Light Fault

[Sculthorpe]

Since purchasing the car the reversing lights have not functioned. I finally gained access to the switch on the side of the gearbox and replaced it. Now the lights function but the current draw is causing the fuse to blow. It is the proper 35 amp slow-blow fuse and everything is fine until putting the gear lever into reverse.

Finding the fault is where I need some advice. I've checked the harness and connectors where they are visible in the boot and everything is fine. Where do you suggest that I go from here?

According to the wiring diagram there is a body junction inline plug somewhere, and a gearbox in-line plug that I can't locate visually from under the car. Is it worth my time trying to find them? Where do I look?

This is a Federal car and branching off from the gearbox switch is a green wire to the seatbelt sensors. The sensors are unplugged and the wires remain under the seats. It seems to me that the green wires would be energized continuously and since there have been no problems previously there is not a short-to-gound issue. Is it possible that when the gearbox switch allows current to the lights and then to ground, that current also flows to the sensor plugs and then shorts to ground? I wouldn't think so but I'm grasping at straws.

Suggestions appreciated.

Mel

[trunt]

The gearbox plug is just around/under the starter, the body plug is under the carpet on the left sidewall of the footwell. It’s probably unrelated to the seatbelt circuit as the green wire powers both switches and if it only blows when you select reverse then it’s in the line from the switch to the lights.


Terry

[APS]

I would start by disconnecting the harness to the lights in the boot. See if you are still blowing the fuse. If you are then the fault is back towards the switch. If not then the fault lies in the boot area. Probably a short to earth somewhere.

If you have a multimeter test for earth from the switch back towards the lights. Have the lights disconnected or you may get a false reading.

[Sculthorpe]

Thank you Gents. I disconnected the green/brown wires from the reversing lights and the fuse is ok when the gear lever is in reverse. I want to use the car tomorrow for Drive Your Triumph Day so I will investigate further over the weekend. I'm not looking forward to the minimal access to the gearbox switch now that the prop shaft and exhaust are now reinstalled.

[trunt]

Do you mean you disconnected at the lights? If so then you don’t need to go near the gearbox, the short is in the light cluster.. was the green/brown wire connected to the correct terminal? (There is a ground terminal somewhere there maybe it was connected to that?) Or is there a short in one of the bulb holders or bulbs?

pull the holders, remove the bulbs and see if you see any issues..

[APS]

As Terry says you seem to have proved the fault is back towards the lights. There is no need now to go near the gearbox switch. You have a short to earth somewhere. Start with removing the bulb holders and make sure they are connected correctly.

[richardthestag]

Sounds like a bulb holder fault to me also. they come apart really easily. The prongs that secure the holder into the rear light are held tight to the bulb bayonet section by the black plastic surround. You can gently heat and lever the black plastic surround away and the prong section will drop away. I haven't yet worried about the bayonet bulb carrier. If however it is clear that the plastic insulator has failed and allowed the circuit to earth parallel to the bulb then you know what you need to replace.

If confused you might like to further isolate the fault by checking, at the expense of another couple of fuses, which side is causing the issue. Powerprobe is marvellous tool that save fuses, but you can buy a shedload of fuses cheaper that this diag tool

Please do NOT be tempted to raise the rating of the fuse in the fusebox! you have an issue and until it is resolved the fuse is protecting your loom

I suspect from what you have posted that either a bulb holder or one of the wires to the bulb holder is incorrectly routed

[Sculthorpe]

An update to my original post. As suggested I disconnected the wires at the bulb holders and no fuse issues while the car was at rest. The next day, feeling confident, I backed out of the garage and the fuse blew.

That made me think that the issue must have been further forward in the wiring, so back up on the jacks and I removed the connectors to the reversing switch, which was very difficult due to the limited space and because the previous owner had used a zip-tie to bundle the wires when the gearbox had been out for service. Again, no issue while at rest but the fuse blew again when backing out. What the hell is going on!

As a final test, I kept the car outside, replaced the fuse, and all was well until I pulled forward. So apparently, it was not the reversing wires causing the problem but rather movement of the drivetrain causing a wire to touch metal and to short out. Since the only thing that I had disturbed prior to this issue was to replace the reversing switch, I now believe that I must have tugged too hard on the zip-tied wires when connecting them to the new switch and caused a crack in the insulation of one of them.

As a last resort, I purchased a small wire nipper that *might* fit in the reduced space near the top cover so that I could try to clip the zip-tie without clipping a wire as well. After 15 minutes of contortions I was successful and separated the wires from one another. A short drive today gave no blown fuse so it appears that I am closing in on the issue although I still have to make a repair. Everything on that fuse is functioning again except for the reversing lights.

It looks like the only way to confirm where the fault lies is to remove the gearbox wire harness in order to examine things closely. I'm not looking forward to that while the gearbox is still in place.

Why can't leaking electrons leave a trail like leaking oil does?

Mel