Brake Lights

[BigCol]

My brake lights have stopped working...

the switch is OK (but I also have a replacement) - closed circuit when plunger out, open circuit when pushed in

bypassing the switch with 12V lights the rear lamps

putting 12V on the switch and closing it (ie letting the plunger out; either switch) lights the rear lamps

however, the feed to the switch has 12V on it

but when I connect that to the switch and close it, the rear lamps DON'T light!

similarly, if I short the feed to the switch to the output, the rear lamps DON'T light.

it is almost as though there is 12V there but insufficient current to light the rear lamps

is that possible?

if so, what would've caused that to happen (ie what do I need to do to fix it?)

 

[Jonathan Kay]

1. All other circuits and lamps working normally?
2. Incandescent rather than LEDs?
3. Any work done recently?
4. Are you sure that the lamps are coming on fully in those tests?
5. Do you have a wiring manual?

Jonathan

[Jonathan Kay]

it is almost as though there is 12V there but insufficient current to light the rear lamps

is that possible?

Yes, that suggests a high resistance somewhere in the circuit. When there's no current the feed side stays at 12 V but when the current flows some of that potential difference is lost across the causative resistance and isn't available to the lamps.

In general you can test for that in several ways:

1. Temporary jumping: replacing bits of the circuit with good bits, and you've done some of that already.
2. Measuring the voltage (to a good earth) at various points while the circuit is closed.
3. Measuring the resistance directly. But if I understand the BC experts correctly 2 above is usually better than this.

putting 12V on the switch and closing it (ie letting the plunger out; either switch) lights the rear lamps

however, the feed to the switch has 12V on it

but when I connect that to the switch and close it, the rear lamps DON'T light!

The most obvious cause of that would be a high resistance in the feed upstream of the switch. That's not as common as in the switch or in a connector or in the earth from the lampholder. But it might be worth inspecting and wiggling the wiring and contacts at the fuse and the wiring to the switch. Then try Method 2 (ignition on and switch closed) starting with both sides of the switch and then those upstream bits as well as everywhere you can get a probe at the rear end.

Jonathan

[BigCol]

1.  yes
2.  yes
3.  none; other than cleaning the switch as that was where I thought the initial issue was (and it seemed to be as the lights worked temporarily after doing so)
4.  yes
5.  yes

[Roadsport06]

Have you switched your hazards on and off a few times?

[BigCol]

I also posted on facebook and 7-DNA.  The consensus seems to be, as Jonathan suggests, to be a high resistance somewhere.

My bet, as the lights look fine downstream, to be in the feed upstream of the switch.

I am just back from a ten-day French blat where we encountered lots of different weather much of it wet!  I suspect the 6-way connector that links the 12V feed in the fusebox and the cable that goes to the switch to be a good bet.  Will disconnect and bypass various bits to identify where the issue is...

But's that's a job for tomorrow!

[BigCol]

Roadsport06 said Have you switched your hazards on and off a few times?

Nope...  happy to try anything...  what is that likely to do (other than miraculously fix it)?

[Jonathan Kay]

There are several reports of intermittent faults inside the hazard light switch causing problems with the indicators. And of them being temporarily being cured by switching it a few times.

I don't think I've seen any affecting the brake lights, and that would be harder to explain. But it only takes a few seconds. so it could be cost-effective...

Jonathan

[BigCol]

Am happy to try anything - as you say it could be a low-cost solution but I wont be holding my breath!   

[Roadsport06]

Ignore me, I misread it as indicators. I can't see brakes being linked. I'll read it properly later and have a think.

[BigCol]

Not to worry...  but not surprisingly, it didn't help!  

Have performed various tests that have made me more confused (will post details later) and run a live feed to the brake light switch as a temporary solution so I am mobile/legal.