Confused

[cliff.b]

Bought some LED tail/brake lights and they didn't work in the car, although works fine when 12v applied on the workbench. 

Then I realised that the original filament tail light illuminated even when the bulb holder wasn't seated, so no path to ground. Only possible circuit is through the brake light wire and sure enough, when that was removed the tail light went out.

So fitted the new LED bulb with the brake wire disconnected and it lit when I touched the bulb holder to ground, but when I connected the brake light wire it went out, although holder still grounded.

With the original bulb everything works fine.

I'm sure there is a logical explanation but it is eluding me at the moment 😒

[cliff.b]

Thinking about this further, I can see that with one lamp holder removed, there is still a path to ground through the filaments of both brake lights. With the tail light on, it became slightly brighter when the bulb holder was grounded, but IF the circuit was through 2 other filaments I would assume it would be far dimmer.

[NonMember]

18 minutes ago, cliff.b said:

With the tail light on, it became slightly brighter when the bulb holder was grounded, but IF the circuit was through 2 other filaments I would assume it would be far dimmer.

It's actually quite hard to see the difference made to a 5W bulb by the presence of two 21W bulbs in series. The 21W filaments don't get enough current to make them hot, so their resistance stays really low. So I think your observed behaviour is exactly as expected.

However, this does leave a question over:

1 hour ago, cliff.b said:

when I connected the brake light wire it went out, although holder still grounded.

My best guess for this is that the LED bulb you have is simply the wrong type. If you fit a "single filament" bulb in a stop/tail fitting, you normally get the front side lights coming on with the brakes, as well as the other stop/tail light being the same brightness as either stop or tail.

Were you doing this test with the engine running? I wasted many hours chasing down a problem with my Vitesse's front side lights not working with the headlights on (but fine with them off) until I realised the battery was a bit low on charge and the LED bulbs just don't work below 9V.

[cliff.b]

Yes, been doing some calculations and reckon the voltage drop through the brake light filaments would be fairly low, so that explains that bit.

I have been testing this with the engine off but my meter is showing well over 12v at the lamp holder. What I can't understand is why the LED bulb lit with the brake light wire disconnected but holder grounded, then went out when I re-attached the wire. That wire should only go to the other brake bulb and one side of the brake pedal switch.

What I don't know, of course, is the internal circuitry of the LED bulb. Are there 2 separate sets of LEDs for tail & brake, or is it say 50% of them for tail and 100% for brake? If the later, then the 2 wires may be connected within the bulb in a way that I can't see 🤔

[NonMember]

You said it works on the bench but did you try all combinations? In particular, try powering the tail light pin while grounding the brake light one, as that's what it should be seeing in the condition where it doesn't work on the car.

[cliff.b]

14 minutes ago, NonMember said:

You said it works on the bench but did you try all combinations? In particular, try powering the tail light pin while grounding the brake light one, as that's what it should be seeing in the condition where it doesn't work on the car.

I will try that and also what happens on the car when the brake is applied. One thing that has occurred to me is that I have only replaced one bulb, so the brake light wire which I have been talking about is still connected to the filament bulb on the other side, although I don't see why that would make a difference. However, I will try disconnecting it at which point that wire shouldn't have a circuit anywhere, unless there is a fault of some sort. But if there was a fault, it doesn't affect the old bulbs. I will meter that cabling out to check, I think 

[JohnD]

Don't forget that the D of  "LED" stands for 'diode'!   They are one-way items, and will only work if the polarity is correct.  Never a problem with incandescent bulbs!

John

[cliff.b]

38 minutes ago, JohnD said:

Don't forget that the D of  "LED" stands for 'diode'!   They are one-way items, and will only work if the polarity is correct.  Never a problem with incandescent bulbs!

John

Yes, checked that. Both parts of bulb work if positive connected to the relevant terminal & negative to the metal body. And as stated earlier, the tail light illuminated if the brake wire is disconnected but goes out as soon as you reconnect 🥴

[Wagger]

Check when fitted in the car that voltage is present at the bulb and that it is the correct polarity.

[cliff.b]

1 hour ago, Wagger said:

Check when fitted in the car that voltage is present at the bulb and that it is the correct polarity.

Yes, checked with my meter earlier and all good.

[Wagger]

14 hours ago, cliff.b said:

Yes, checked that. Both parts of bulb work if positive connected to the relevant terminal & negative to the metal body. And as stated earlier, the tail light illuminated if the brake wire is disconnected but goes out as soon as you reconnect 🥴

Does the voltage across the bulb drop or disappear when you connect the brake light wire?

[Steve P]

Don't know it it helps but when I fitted Led stop and tails to my Vitesse the sidelights and indicators came on when the pedal was pressed.Turned out to be just the earth loop on the back of the rear light was not getting a good earth through the body,all good after a cleanup with Emery paper.

S

[cliff.b]

43 minutes ago, Wagger said:

Does the voltage across the bulb drop or disappear when you connect the brake light wire?

I will add that to the list of things to check when I get to look at it again. Probably not until next week now 😒

[cliff.b]

6 minutes ago, Steve P said:

Don't know it it helps but when I fitted Led stop and tails to my Vitesse the sidelights and indicators came on when the pedal was pressed.Turned out to be just the earth loop on the back of the rear light was not getting a good earth through the body,all good after a cleanup with Emery paper.

S

Yes, that does help in so much as there is clearly more stuff I need to check out. Need to go round everything with my meter, measuring resistance etc.

[cliff.b]

I had forgotten about this issue and looked at it again this afternoon.

I found that connecting the tail light side of the LED bulb to a 12v battery on the workbench it lit as expected, but if I touched the brake light connection to -ve it went out. So clearly the 2 parts of the bulb are not isolated and there must be some sort of electrical connection between them. 

Anyway, observing this everything fell into place. I had originally only changed one bulb as I wanted to compare brightness and as discussed previously that meant the brake light wire now had a path through the other filament bulb to ground. Removed the filament bulb, tried fitting LED again and all working, although not as bright as I hoped and disappointing difference in brightness between tail & brake lights. Should probably have bought better ones 🙄

[Stratton Jimmer]

My Sixfire has all LED lights which are bright and effective. They were fitted by the PO but came from a company who's website is at: https://www.classiccarleds.co.uk/ (Thank heavens for having a history file!)

[cliff.b]

1 hour ago, Stratton Jimmer said:

My Sixfire has all LED lights which are bright and effective. They were fitted by the PO but came from a company who's website is at: https://www.classiccarleds.co.uk/ (Thank heavens for having a history file!)

Many thanks 👍