Stop light not working

[Phil C]

I recently had the brake rod off my B40 to straighten it and replace the spring on the switch, etc.. Having put it back together I see the stop light isn't working. Checking with a multimeter, there are about 5.9v across the battery. Similar at the switch, and at the switch exit. Similar at one of the two lead contacts at the light fitting. Checking the bulb, I get a low ohms reading at both contacts, not the other. The other lights all work. What's wrong?   Phil

[Rog1]

Checking the bulb, I get a low ohms reading at both contacts, not the other.

I'm not sure what you mean here. Is it a tail and brake lamp, with 5 and 21 watt (or similar) filaments? If so, you should get a low Ohms reading between both base contacts, and different low Ohms readings between each base contact and the metal body of the bulb.

Was the stop light working before you took the rod off? Check the switch operation by putting the meter probes across it so that you get your 5.9 volts, then operate the brake lever. The voltage should fall to zero. If it doesn't, give the spring a tug and see if that does it.

[Phil C]

Sorry, should have explained more clearly. It's a two-filament (21/5w) bulb (tail light and stop light.) Using the multimeter, across the battery terminals I get about 5.9volts. Then keeping one meter lead on the battery earth, I check for readings along the circuit by putting the other lead at various points: at the switch (near the brake pedal) I get a similar reading (about 5.8-5.9.) At the other contact of the switch (only if operating the brake pedal) I get roughly the same. At the light fitting, having removed the bulb, I see two contact points. One reads 0, the other about 5.8-5.9v ( again, operating the pedal.)Is that what I should get?
Regarding the bulb, visually it looks okay. I get a low ohms reading across its two contacts, and when I connect each contact with the metal wall. 

The other lights all work, it's just the brake light that's not working.  Could anyone tell me where the problem is please?

Phil

[Phil C]

A bit more info: re the contact in the light fitting where I got a zero volt reading - if I turn on the tail light switch (!) I get a reading of about 5 point something volts. Phil

[Pete A10]

Couple of things to check - is one filament of the bulb blown? Also check that the switch (by the brake pedal) is pulling on completely - try operating it by hand. If you have got power everywhere then it must be a failure of one of the components.

[Phil C]

Filaments look good visually. Low ohms readings on the bulb. When I operate the pedal there are volts at the light fitting contact point. I don't get it. Maybe something very obvious I'm missing? Probably something to do with me having the rod and switch off I guess, but what?  Phil

[Calum]

If you're getting 6v at the terminal in the bulb holder with the switch pulled then the wiring up to there should check out... SHOULD.
Multimeters will show a voltage reading when the current is so so very low, so sometimes a dry/broken joint might still show as transmitting power but as soon as you actually load the circuit up (with a bulb or whatever) then sometimes the joint then 'fails' and there's 0v. In many cases a test light is mich better for fault finding (easily made with a bulb holder, some wire and a pair of crocodile clips or similar)

On the other hand I think your issue is probably an earth issue, or the bulb. You said the multimeter was connected to the battery earth. Try the meter across the bulb contact and the base of the lamp unit, or check for continuity between lamp base and battery earth.

[idie]

Remove both wires from the switch and touch them together. Does the light come on, which it should do. If the bulb lights then the fault is with the switch not carrying any amps. If the light does not come on then the fault is with either the bulb holder or the bulb.

[Blackthou]

This is quite a common occurence when moving the brake rod / switch. Try as Rog1 suggested, give the spring a tug and it will probably come on. It's then a question of sliding the switch anchor along the rod until the light comes on then back it off a bit.

Good luck

[ducati2242]

You say you are keeping multimeter on the earth on the battery . Are you sure the earth on the tail light is actually working you should be able to read 5.9 volts at the bulb holder earth being the bulb holder and live at the correct terminal from the brake light switch .

[Phil C]

Thanks everyone so far. Just been out in the garage. Continuity between the lamp housing and the battery earth terminal seems okay (beep on meter) but between the light unit bulb contact and the battery terminal there doesn't seem to be continuity. So it seems the problem is the connection between the light unit bulb contact and the lamp housing. Is that right? Not sure how to sort it yet - all advice much appreciated. I think (see photos) it's a non- standard fitting, maybe off a Honda?   Phil

[Phil C]

But it stilll seems a bit too much of a coincidence that I've noticed the brake light not working straight after I've had the brake rod off. Tomorrow I'll try as Blackthou suggests and if that doesn't work then look into that earth connection.   Phil.

[Phil C]

Stupid of me - of course the bulb is the link between the bulb connector contact and the light unit. So it's still s mystery. I'll check everything again.  Phil

[Phil C]

There will if course be a logical answer, but it's escaping me at present. The key factors I'm seeing are:
1. The bulb is good (tested with meter and made  both filaments light using spare wires on the battery.)
2. I have 5.8 volts at the bulb holder contact point (measured by multimeter with one wire on the contact point and the other on the battery earth terminal.)
3. There is continuity between the lamp unit and the battery earth terminal (beep on multimeter.)
The only remaining place where there could be a break in the circuit I think is where the bulb connects to earth, but the tail light works so that would seem unlikely.
What am I missing here? Something really obvious maybe?
Phil

[Phil C]

Further to my post just now, I guess there are actually two possible places where there could in theory be a break: both the bulb contacts. But as the tail light works that would seem unlikely. What am I missing here?  Phil