sierrafery
Well-Known Member
Hi, you should be more specific which light, dipped beam, main beam, side light ...or all of them? ... did you check for voltage in the plug?
sierrafery
Well-Known Member
It's much more complicated, the lights are controlled through FETs no individual fuse for any side
Ok, I thought that was the high-beams and the dipped were each on a 5A fuse. Sorry.
OK, I know I am going to regret asking this, but in the interest of furthering the knowledge of myself and fellow members, what the ferk is an FET?
Field-Effect-Transistor when I was a lad. ;-)
sierrafery
Well-Known Member
Nothing to regret but better don't eat your mind to figure out how the D4's lights are working unless you are a highly skilled electronist, they made a simple thing like exterior lamps which can be achieved with a switch and a fuse(eventualy a relay) into advanced electronic circuit, just a flavour from the system's description:
CIRCUIT PROTECTION
Operation of the lamps is performed using overload proof Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). The MOSFETs can detect overload, load interruption with the lamps switched on and short circuit to positive with the lamps switched off.
The MOSFETs are protected against short circuits, removing the requirement for the lamps circuits to be protected by fuses. The MOSFETs respond to heat generated by increased current flow caused by a short circuit. Normally this would cause the fuse to blow. The MOSFETs react to the heat increase and cut the supply to the affected circuit. Once the fault has been rectified or the MOSFET has cooled, the MOSFET will automatically reset and operate the circuit normally.
If an overload occurs, the current flow is dependant on the temperature of the related MOSFET and can be up to 20 times the rated current of the lamp. The MOSFET heats up and deactivates the load applied to the circuit. When the MOSFET cools the circuit is once again reactivated. This thermal cycling occurs continuously in the event of an overload occurring.
A number of lamps are controlled by relays and these circuits are protected by conventional fuses.
BULB MONITORING
Bulb failure monitoring is performed by the CJB processor. The lamps are cold and warm monitored by the MOSFETs in order to detect bulb failure.
And they are also a PITA when they go wrong. KISS was forgotten by LR/JLR long ago. Sadly.
Unbelievably, I understood that. Well I understood the cause and effect but I don't have a clue how the MOSFET is made or how it works internally. I always understood how old fashioned valves worked, I kind of understood how transistors worked and appreciated the advantages of these over valves, but these MOSFET things just seem a little too clever for their own good. Why not instead replace the fuses with circuit breakers like miniature MCBs? All the advantages of fuses with none of the disadvantages of MOSFETS. Sounds like a version of "if it ain't bust then lets think up a crazy-complicated way to fix it". i.e. someone invented these things and just had to convince someone else that they always needed them. After all, how often does a blown bulb cause a real problem? Especially if you carry spares?
Having just read up on all this I am pleased to discover that human beings are not the only things to be "bipolar"!
That said, I am ashamed I did not know about them especially since they have apparently been around for a lot longer than I thought and the idea of them even longer.
As far as the headlights are concerned, they seem to just operate on a kind of feedback circuit, in which case, how can they ever allow a bulb to blow? And if they do, does that mean it is the circuit or MOSFET that is at fault or is it the bulb that was not manufactured properly? Or does the bulb blowing jigger the MOSFET and cause that to need replacing as well?
I think I will just go and lie down in a darkened room. I suddenly seem to be beginning to understand a bit about all this. Or am I? Hmmm!
Anyway, thanks for the time and the explanations.
Having just read up on all this I am pleased to discover that human beings are not the only things to be "bipolar"!
That said, I am ashamed I did not know about them especially since they have apparently been around for a lot longer than I thought and the idea of them even longer.
As far as the headlights are concerned, they seem to just operate on a kind of feedback circuit, in which case, how can they ever allow a bulb to blow? And if they do, does that mean it is the circuit or MOSFET that is at fault or is it the bulb that was not manufactured properly? Or does the bulb blowing jigger the MOSFET and cause that to need replacing as well?
I think I will just go and lie down in a darkened room. I suddenly seem to be beginning to understand a bit about all this. Or am I? Hmmm!
Anyway, thanks for the time and the explanations.
I have tried the near side lamp in the offside and it works fine , also tried the offside in the nearside and the dipped beam still didn't work so it appears the problem is in the light cluster. I have sprayed the connectors with electrical cleaner spray and still no joy, swapped bulbs to make sure it wasn't the bulb and that didn't work. Can you get the wiring for the lamp or will it be a case of buying a new lamp?
