Stop light not working

[DAVE BRADY]

Hi Phil,

The tail light filament can work even if the stop filament is blown. If it is not obvious try another bulb.  I think you have established that there is power to the switch so as has been said disconnect the two wires from the switch and touch them together.  The stop filament should light.  If it does then the problem is in the switch.  Give the switch a bit of a work out by repeatedly operating it over its full movement to re-establish the internal contact and perhaps a bit of contact cleaner. 
A bit of a simple one but is the movement of the brake rod actually operating the switch?

Dave.   

[Phil C]

Hi Dave, I've tried touching the two switch wires together: the bulb didn't light. I've tried moving the switch on and off but that made no difference. With the brake pedal depressed I have volts at the bulb holder contact, so it seems to me the switch is okay. I can't think of anything other than the connection between the stop light filament and the lamp body. I guess I could try another bulb, but I'm a bit loath to as I know both filaments work.  Any other ideas anyone please?  Phil

[DAVE BRADY]

Hi,

If you have a spare bulb you have nothing to lose giving it try as a process of elimination.

Dave.

[Phil C]

True. I'll buy one this afternoon. Fingers crossed.  Phil

[Rog1]

If no joy with a new bulb, use a test light (as Calum suggested).

First connect the test light between the battery side of the switch and earth. If it doesn't light, the problem's in the wiring to the switch.

If it lights, connect it between the brake light side of the switch and earth, and operate the switch. If it doesn't light, it's the switch.

If it lights, connect it between the brake light side of the switch and the earthed part of the tail/brake light bulb holder. Operate the switch. If it doesn't light, it's the earth.

If it does light, the problem is between the switch and the brake light. Then, you might have a broken wire that was undetected until it was disturbed.

[Phil C]

Thanks Rog1. I haven't yet got a new bulb (going to get one later today), but have just tried what you advise. The test light comes on each time, so if the new bulb doesn't work then I need to look at the wire between the switch and the light unit. Thanks again. Ph

[MGI]

Have you checked the actual spring unit is making contact?

[Phil C]

Er...how do I do that? I'm suspecting the problem is within the light unit.

[DAVE BRADY]

Hi,

I think MGI means the spring loaded part that has the contacts on.  If you get your little finger and push the contacts in where the bulb goes it should move quite easily and spring back up.  Also, some times if there has been a poor connection a bit of corrosion can occur on the contacts so make sure they are clean. A bit of a scape with a small blade will do it but switch off the power or you may blow the fuse. 

Dave.

[Phil C]

Oh yes, the spring operation seems fine.

[Phil C]

Amazing. It was the bulb. I checked the duff one three ways: visually, with a multimeter for continuity( gave a healthy beep), and with the battery on the bench and a couple of bits of old wire( both filaments lit up.) The two contacts on the bottom of it look fine. I assume the earth connection on the bulb is the metal housing which fits into the female bayonet fitting, in which case it too looks fine. I wonder what's wrong with it. But anyway, I put in a new bulb and it worked.  Thanks again everyone. Good practice for me!  Phil

[Rog1]

Good news! One take away from this is before taking something apart, check if it's working first; I've been caught out like this in the past.

[Phil C]

I wonder what was wrong with the old bulb.

[Stubaker58]

Could it be that the contact on the base of the bulb is not actually touching the bulb holder contact when the bulb is inserted?

[courtney]

What an illuminating topic.