Stoopid Question
Discussion
Hello All - I'm just trying to understand something fundamental which I don't get. I hope someone might be kind enough to set me straight?
If a light fitting has only two cores going to it (presumably negative and positive) does that mean a typical car would not know if those wires are connected to anything (such as a light bulb) ?
In other words, if I see for example I instruct the car to turn on the rear fog lamp but I don't see the telltale light up in the dash. Does that mean:
a/ The car tried to turn on the rear fog lamp but it didn't come on.
b/ The car did not try to switch on this light at all
Finally why does the bulb need two cores at all given I thought the chasis was "negative" and so wouldn't a single wire be enough (as long as the second pin on the bulb is connected to the chasis) ?
If a light fitting has only two cores going to it (presumably negative and positive) does that mean a typical car would not know if those wires are connected to anything (such as a light bulb) ?
In other words, if I see for example I instruct the car to turn on the rear fog lamp but I don't see the telltale light up in the dash. Does that mean:
a/ The car tried to turn on the rear fog lamp but it didn't come on.
b/ The car did not try to switch on this light at all
Finally why does the bulb need two cores at all given I thought the chasis was "negative" and so wouldn't a single wire be enough (as long as the second pin on the bulb is connected to the chasis) ?
Unfortunately you haven't said what car this is on, so most of this is generic info.
The lamp and the telltale most likely won't be on the same circuit. Depending on the car, you could have several ECUs in the signal path - one at the steering column area to handle the request from the stalk or button, a body control module to take the message from the column module and figure out where it needs to go to, and finally a front or rear control module to act on the message and actually close the circuit to light the lamp.
Most but not all cars will possibly also have some kind of lamp check circuit, which if rudimentary halogen bulbs are used, is generally a voltage pulse down the wire brief enough to measure, but not long enough to warm the filament up. LED stuff is handled differently (I believe based on resistance of the circuit) which is why you see retrofitted LED bulbs flashing every now and then, as the lamp check is testing the circuit and the LED energises much quicker than a halogen bulb.
If you're not seeing the telltale light up, are you trying it with the headlights or sidelights on? Most fog lights won't turn on by themselves - they need the primary lighting circuit to be active first.
(Of course, if we're talking about a car not made within the last 20-25 years, then this probably won't apply)
And the last question - most regular rear halogen bulbs have their ground through the casing, with one or two pins on the bottom for the positive connection. They're generally grounded via a common ground on the lamp board, which goes to a chassis ground via the wiring loom.
Bulbs like the 448 run exactly in the way you describe, with the lamp holder being connected to the chassis and the bulb only having a positive connection.
The lamp and the telltale most likely won't be on the same circuit. Depending on the car, you could have several ECUs in the signal path - one at the steering column area to handle the request from the stalk or button, a body control module to take the message from the column module and figure out where it needs to go to, and finally a front or rear control module to act on the message and actually close the circuit to light the lamp.
Most but not all cars will possibly also have some kind of lamp check circuit, which if rudimentary halogen bulbs are used, is generally a voltage pulse down the wire brief enough to measure, but not long enough to warm the filament up. LED stuff is handled differently (I believe based on resistance of the circuit) which is why you see retrofitted LED bulbs flashing every now and then, as the lamp check is testing the circuit and the LED energises much quicker than a halogen bulb.
If you're not seeing the telltale light up, are you trying it with the headlights or sidelights on? Most fog lights won't turn on by themselves - they need the primary lighting circuit to be active first.
(Of course, if we're talking about a car not made within the last 20-25 years, then this probably won't apply)
And the last question - most regular rear halogen bulbs have their ground through the casing, with one or two pins on the bottom for the positive connection. They're generally grounded via a common ground on the lamp board, which goes to a chassis ground via the wiring loom.
Bulbs like the 448 run exactly in the way you describe, with the lamp holder being connected to the chassis and the bulb only having a positive connection.
mike42 said:
If a light fitting has only two cores going to it (presumably negative and positive) does that mean a typical car would not know if those wires are connected to anything (such as a light bulb) ?
No, most modern cars have bulb check facilities as above.mike42 said:
In other words, if I see for example I instruct the car to turn on the rear fog lamp but I don't see the telltale light up in the dash. Does that mean:
a/ The car tried to turn on the rear fog lamp but it didn't come on.
b/ The car did not try to switch on this light at all
No, and no, it likely means the warning lamp on the dash is defective or the circuit fuse has blown.a/ The car tried to turn on the rear fog lamp but it didn't come on.
b/ The car did not try to switch on this light at all
mike42 said:
Finally why does the bulb need two cores at all given I thought the chasis was "negative" and so wouldn't a single wire be enough (as long as the second pin on the bulb is connected to the chasis) ?
Plastic bumpers don't have a chassis earth & that is where you lamps may well be, even if they are mounted to the car's steel body, they still need a wire to get from the light lense to the car chassis.Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff