Completely weird

[Unclefester]

OK, I've given up on this one.

 

I've been doing some work on the crossflow today, fitted the new seats and french dipping headlamps.

 

I was going to test the headlamp dip levels when I noticed the rear right sidelight wasn't working.

 

Expecting to find a dead bulb, I opened the lamp unit, removed the bulb, and it looked fine. I put another bulb in, that didn't work either. I then swapped the bulb to the left unit, and it worked.

 

I checked the right brake light...fine. Indicator, fine. So the earth is obviously OK.

 

Then, mystified, I used a tester on the contacts in the bulb holder, which looked absolutely perfect, new, no corrosion, no water ingress.

 

Just under 12 volts, same as the left rear light which works.

 

So....I have a light unit which is providing 11.6 volts to the brass contact, which is in contact with the lead bit on a bulb which is perfect (I tested both filaments with a battery).

 

But it refuses to light up.

 

 

It's as if the power gets to the brass contact but refuses to enter the bulb.

 

I don't know what else I can do!

 

I'm going to have to take it to a garage, but what else can THEY do?

 

It's completely baffling....any bright ideas?

 

 

 

Lucy in the Sky with Diamonds..ooooh hooo hooo!!...

😬 😬Abbey Road Time-Machine *eek*

 

[Bob_Rich]

Sometimes a corroded lead/terminal can go high resistance. It will let enough current through so that a meter will read it as full volts. Possibly only needs a few microamps of current to move the meter over to read virtually full volts. If you can when the meter is on can you then touch a bulb on to the leads as well? if it kills the reading and the bulb does not light then it look like an open circuit lead--but not that open circuit that the meter cannot read it

 

hope this helps best of luck

 

Bob

[LazerBrain]

Following on from the above... Can you put the meter in current reading mode in place of the relevant fuse? That should then give you some indication of how much current is flowing through the bulb - probably not enough to get it to glow. Obviously be careful as you won't have the protection of the fuse whilst you're running this test. I have a test cable that allows me to measure the current through a fuse whilst in circuit.

 

cheers,

 

Darren.

[JampJ]

I had pretty much the same problem, are you sure the brass bit is touching the lead bit on the bulb? I found that the bulb wasn't going in far enough to get a good contact on one of the bulbs. I ended up putting a bit of extra solder on the end. If you are getting the earth and volts, it must be a bad connection.

Cheers John

 

JFDI

(Just F*****g Do It)

[Unclefester]

The earth in the bulb holder is clearly fine, as the brake light filament is working. I'll try the suggestion made by Rob and Lazer.

 

The front right sidelight is working fine, it's on the same 5 amp circuit as the rear right, while the left circuit is 7.5amp and includes the instrument lights and I imagine also the numberplate light.....which is also working.

 

Lucy in the Sky with Diamonds..ooooh hooo hooo!!...

😬 😬Abbey Road Time-Machine *eek*

 

[Unclefester]

if it kills the reading and the bulb does not light then it look like an open circuit lead--but not that open circuit that the meter cannot read it

 

Bob, What would be the likely cause of this? The point is that this 5amp protected circuit also has the front right sidelight in it, and the front one's working fine. The car passed its MoT this summer so the light was ok then.

 

If this situation is the explanation, and I'm getting 11.6volts on my meter from the rear terminal, and the brake light filament works ok on the same bulb, where would I be likely to find the problem assuming it's corrosion in part of the circuit?

 

I'm also embarrassed to admit I don't know how to use my meter in "current reading mode"....does that mean putting the cables in different holes? I rarely use it and have mislaid the handbook.....

 

Lucy in the Sky with Diamonds..ooooh hooo hooo!!...

😬 😬Abbey Road Time-Machine *eek*

 

[Unclefester]

OK the current measurement across the 5amp fuse is 1.17amps.

 

This is with the light on sidelight only, and engine not running. The battery is ok and was charged recently.

 

Is this too low?

 

Lucy in the Sky with Diamonds..ooooh hooo hooo!!...

😬 😬Abbey Road Time-Machine *eek*

 

[Bob_Rich]

From what U say all other bulbs on that light pod and front circuit seem OK. There are two things that are specific to that lamp only. the actual bulb pin (the solder blob on the lamp) and the actual joints ( in line crimps or bullets) that go the the lamp circuit. suspect one of these has gone high resistance. depending upon your wiring layout there may be more than one joint in the line but the one under/near the rear wing would be the one I would suspect--it gets a lot of road crud. Another possibility is that the wire to that circuit has had a nick in the insulation and damp has got in and it has corroded through and broken the wire and gone open circuit save for a deposit of corrosion material enough to give you a voltage reading.

 

DONT use a current meter on what may be a voltage source ( i.e. the battery). If you probe a circuit for current allways put a load in to limit the current. I usually use a bulb wired in series for that.

 

Hope that helps "dry connections can be a mugger!!

 

best of luck Bob

[LazerBrain]

It's higher than I would expect if you only have 1 sidelight bulb on that circuit that is illuminated.

 

I hope I'm not teaching my uncle to suck eggs... I'm assuming (from the 2 decimal places) that you have a digital meter. The black probe should be connected to the COM socket on the meter, the red probe to the 10A socket. Generally there's one socket for volts, ohms and low current measurement and a separate socket for higher current measurement. The higher one is often unfused. Be careful, as the 2 probes are now effectively connected together so you could short stuff out... There should also be a matching setting for 10A on the meter that you need to select.

 

With the fuse removed, you can now put one probe from the meter in each of the fuse holder's connections. The meter is now completing the circuit instead of the fuse - and measuring the current passing through the fuse holder. With a digital meter it should not matter which way round you use the probes - it'll just pop up a -ve sign.

 

With the lights switched off, the meter should read close to zero. Switch on the sidelights and you should expect to see about 0.4amp for each 5W sidelight bulb. Is there a difference in the reading with the rear bulb in and the rear bulb removed? This difference will be the amount of current the rear bulb is taking.

 

cheers,

 

Darren.

[Johnty Lyons]

A very common problem. Ccts that give a good voltage reading BUT won't pass current

Quite common on Caterham rear light units as has been said is due to a dry type joint [corrosion probably ]. Check all connections and clean/renew and 99% of time it will be cured. I see this a lot on Marine nav light systems where every light reads over 12V but not one of them lights It's at that stage Mr Owner with his tiny DIY meter gives up and lifts the phone completely baffled 😬 😬

 

---

jj

MTM , N.I. L7C AR 🙆🏻

Membership No.3927.

240BHP 1900cc K Series 40th Anniversary

 

[Unclefester]

Thanks guys, that's all pointing in the same direction.

 

I gave myself a swift interweb tutorial on use of the digital multimeter and so did use the red in 10amp socket and the 10amp sector on the dial. I measured the current across the fuse with the rear bulb in and the front bulb lit.

 

I will try it with the rear bulb out, but as all other electrical oddities I've had have been explained by what you've suggested, I assume the problem is corrosion somewhere and will hunt it down, wherever it's hiding.

 

 

 

Lucy in the Sky with Diamonds..ooooh hooo hooo!!...

😬 😬Abbey Road Time-Machine *eek*

 

[Unclefester]

That was educational. I now have !

 

I didn't realise it was possible to get nearly a full 12volt reading which meant nothing in terms of actual bulb-lighting!

 

As you guessed, it was a bad connection where the rear light wiring plugs into the main loom just inboard of the rear arch, between the tank and the body.

 

Just touching the connectors was enough to get it going again, but I'll need to get the car on a lift in order to actually de-clag the connectors as access is very limited.

 

Thanks all!

 

Lucy in the Sky with Diamonds..ooooh hooo hooo!!...

😬 😬Abbey Road Time-Machine *eek*