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Strange fault with car lights

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danrogers

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My girlfriends sisters rear reversing was out, so she brought a new one, and asked me to fit it.

I opened up the housing and fitted the new bulb, but it did not work. I checked the bulb, not much resistance across it & filment looked in tact.

Put my meter across the contacts and it was reading 12v with reverse engaged on, put the bulb in and the voltage across it the contacts drop to zero, but still no light emitting from the bulb?

No I'm no expert but does this mean that there is not enough current to light the bulb? If so what is likely to be causing that?

thanks
 
1. There is a poor, corroded or rusty ground going to the bulb socket.
2. There is a fault in the 12 volts going to the socket from the backup switch circuit.

I lean towards #1. Those are my guesses and I am sticking to them. :)

Ron
 
hmmmm thanks Ron, I'm not so sure though because the car is fairly new and it uses the same ground as some of the other bulbs that are working?

what would be the easiest way to test it?
 
The "rear reversing" is out. Hm, is there a front one?
Is there only one bulb or the customary 2? What does 2 do?
Check the voltage(with the bulb in place at the reversing switch.
Is it a British car? Than 'nuf said!
E
 
Hi, lol don't know why I said rear. No as it is a small car it only has a single reversing light.
Not sure how to get to the switch?

Yep UK car!
 
A common cause for scenarios like this, can be high resistance in one of the wires supplying/grounding the bulb, due to moisture ingress to the copper strands because of damaged insulation. At the point where the insulation is damaged, the wire can be swollen like a snake that has just had a meal ;-) and will be green. The wire still has the capability of passing the small current required to indicate on the voltage meter, but not enough when a load, such as your bulb, is introduced.

Firstly, you should get hold of a schematic because newer cars aren't always wired directly from the reverse light switch, to the bulb holder - some vehicles have the switch connected to a BCM input (Body Control Module) then the BCM switches an output internally, or via a CAN message to another control unit, which does the switching.

The idea is to work backwards from the bulb holder, to the next connector in the circuit, using a multimeter and test lamp in parallel.

What make & model vehicle is it?

HTH
 
It is a smallish car (say: low price) I would assume that the reversing switch would not go through a computer control unit. Most likely it is a manual trany. There would be a switch on the trany or the gearshift. It is likely that there is a fuse somewhere. Check in the least read and most discarded book in the world: the Owner's Manual. E
 
It is a smallish car (say: low price) I would assume that the reversing switch would not go through a computer control unit. Most likely it is a manual trany. There would be a switch on the trany or the gearshift. It is likely that there is a fuse somewhere. Check in the least read and most discarded book in the world: the Owner's Manual. E

I'll use your post as an example against the least-spec'd variant in the range of vehicles I regularly work on...
This is a model year 2007 onwards, 1000cc engine (yep one Litre) 5-speed manual transmission, 3 or 5 door small hatchback.
It is quite cheap.
It is equipped with CAN-Bus.
There is no fuse directly in the reversing light circuit.
There is, however, a switch on the transmission.

Operation is as follows:
The N.O. contact switch on the transmission is wired directly to ground on one terminal.
The other terminal goes directly to the ECM (Engine Control Module) and does not pass "GO".
When reverse gear is selected, a CAN message is passed from the ECM to the BCM (Body Control Module)
The BCM then switches an output IC (High-Side driver) to supply power to the lamp.

Q. Why use CAN-Bus and multiple ECU's instead of wiring everything directly?
A. Many reasons, some listed below;

Far less copper wiring used, resulting in far less weight.
Switches with lower current carrying capacity can be used - less bulk, less weight.
Fault tolerance and error handling/reporting far superior to direct wiring. (Current-sensing resistors to detect and handle short/open circuits and report faults)
Bulb check facility using "cold pulsing" - where low current, insufficient to actually light a bulb, is passed through the circuit intermittently, to check whether the filament is intact.
Lamp switching facility, i.e when one of a pair of tail lamps in a single cluster is used as Stop/Tail (using PWM with a single filament bulb) and that bulb fails, the Stop/Tail function is switched to the other bulb.
Along with many other reasons.
 
Are you sure that the switch is making contact? I had a reversing light not work sometimes, and found that the switch was faulty. The switch contains a spring which is compressed by the reverse mechanism, and a rubber diaphragm that keeps the oil in the gearbox. The spring had got weak and the rubber had gone hard, so the switch didn't work. A new switch fixed it.
 
Until the OP comes back with specifics regarding the actual wiring/bus-system employed, our answers, unfortunately, can only be deemed as being conjecture.
 
Hi all, thanks for the replies.

It is a Ford Fiesta 1.4, 2004 or 5 I think. That is the only infomation I have at the moment, I have just been looking online for a photo of the light cluster but so far have failed. I will keep looking in the meantime. When I am with the car I will try and take a earth from one of the working bulbs

Thanks again!

PS. Thanks Mickster I did not know CAN-bus'es were used that frequently in automovtive applications
 
:DHee, hee, haw, haw, ho, ho.:D
Your car has a tiny 1 litre engine?
Does it have 3 wheels like in India?
 
:DHee, hee, haw, haw, ho, ho.:D
Your car has a tiny 1 litre engine?
Does it have 3 wheels like in India?

Might be time to get those eyesight correction operations re-evaluated....eh?
 
In post #8 the tiny engine was 1000cc. Did it grow up into a 1.4 litre engine?

We have gasoline in Canada and the USA, not petrol. Gasoline has much less taxes that your petrol but it is the same stuff from the same places at the same cost.
 
<snip>
the least-spec'd variant in the range of vehicles I regularly work on...
This is a model year 2007 onwards, 1000cc engine (yep one Litre) 5-speed manual transmission, 3 or 5 door small hatchback.
</snip>

"Your" implies ownership.
I do not own a vehicle with a 1000cc engine. (which, incidentally in this case, only has 3 cylinders too ;-) )
I do own a vehicle with a 1400cc engine, which sips fuel like a bird.
How many gallons per mile, sorry, miles per gallon does your behemoth get you?
 
My car uses less gasoline than many cars and huge SUVs.
It has a 2.2 litre 4 cylinder engine that sips fuel when cruising but uses a lot of fuel when I floor it.
My son has had some "performance" cars with huge V8 engines that waste a lot of fuel.

My govenment does not rip off users of fuel as much as your government does.
Today the gasoline price here increased 5 cents per litre to $1.29. About 30.1 cents per litre is taxes.
Your petrol costs more than double the cost of my gasoline because of your very high taxes.
 
My govenment does not rip off users of fuel as much as your government does.
......Your petrol costs more than double the cost of my gasoline because of your very high taxes.

Isn't that the truth!
We have had a fuel protest near me this last week, vehicles crawling along the motorways.

But we do have a lower rate of income tax.

IIRC, Canada's income tax is around 40% according to a Canadian guy, from New Brunswick, I worked alongside a few years ago.

Guess the respective Governments get to add to their coffers by every means possible...
 
Canada's income tax is geared to how much income a person makes and the deductions. From nothing to lots.
My wife and two kids were my dependents for years so I claimed them as deductions on my income tax payable which reduced it.
Now that I am a retired senior, my government pays me pensions and I don't pay any income tax.
 
I don't like the term "rip-off". The taxes we pay go mostly to the services we receive. Medicare, old age pension, roads, education and on and on. Compare that to some countries that do not "rip-off" and see what there is for the common people.
Did I just state that? Me, a right-winger! E
 
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