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LED bulb for front fog lights

752120
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Re: LED bulb for front fog lights

Postby 752120 » Mon Mar 30, 2015 11:28 am

Oddly Ivor, I had an intermittant fault with a rear fog light on my 93 my. Even though I never use them, the bulb failure light appeared on the dash whether I had the main light switch on or off and or the fog switch. This was not the case with the fronts on my 90my. Curiouser and curiouser.

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Re: LED bulb for front fog lights

Postby nlcrane » Mon Mar 30, 2015 12:53 pm

Equinoxstudios wrote:I'm just waiting for a nice evening after work to fit the pair of 50W LED bulbs I have sourced to my 94MY foglight units. In theory they shouldn't baffle the BFM's as they should draw the necessary amount of current to satisfy them. I guess I'll find out soon.

BTW I've never seen the bulb fail light illuminate to indicate a failed bulb unless that bulb is meant to be on (i.e. you only know that you have a duff headlight until you actually turn on the headlights).

So Norman are you saying that the BFM's actually know the bulb is duff before you try to turn it on? If that is true then the BFM is measuring the passing of a small current through the filament - too small to make it light up but big enough to know that the bulb is there - that's what we call "quiescent" current :ugeek: .

If the BFM only indicates failure when you turn the bulb on then the BFM is not measuring quiescent current but instead is only measuring the current drawn by the bulb when voltage is applied.

So how intelligent are our BFM's?


After closer inspection of the schematics I see that the BFMs do not use the usual current (or lack thereof) method to detect a failed filament. Instead they use a voltage threshold measured across a resistor in series with the actual filament. The voltage dropped across this resistor is measured in a comparator circuit in the BFM fail logic, so in order to have this voltage available for detection the lamp in question must be switched on. You are quite right Ivor.
In the control industry detection of indicator filament lamps is always done by the quiescent current detection method as this is more trustworthy and it also means that blown filaments can be detected when not actually switched on.
Evidently Jaguar, although excellent in some aspects of their electronics, were not up to speed with the latest technology of the time.
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baldilocks
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Re: LED bulb for front fog lights

Postby baldilocks » Mon Mar 30, 2015 8:15 pm

Nick and Ivor

Mine does not seem to work that way. After my bird strike I had the lamp fail light come on immediately but I had no lights on. To find out what was wrong turned on sides and mains, no problems, then tired fogs and they didnt come on. SUbsequently I found the drivers side fog had knackered filament, passenger side was totally smashed along with lens etc. So mine detects bulb fail once ignition is on only, does not need the lights themselves to be used.

Ivor, sounds like you are going down the road I was looking at, do you have aphoto of the LED H3 and is it branded?
93 4.0 Sovereign, James, 687486, 94MY
93 3.2 XJ6, Jerry, 94MY
93 3.2 XJ6, parts supply, 94MY

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Re: LED bulb for front fog lights

Postby Equinoxstudios » Mon Mar 30, 2015 10:11 pm

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Re: LED bulb for front fog lights

Postby baldilocks » Tue Mar 31, 2015 12:01 am

Thanks Ivor, same design I was looking at, but rated at 80W equivalent. Would be very keen to hear how it goes as the engineer in me is aware the light source is at a different point so will affect how well the reflector works. Given the original fogs are far from the greatest it is debatable if could be much worse and may well make them better.
93 4.0 Sovereign, James, 687486, 94MY
93 3.2 XJ6, Jerry, 94MY
93 3.2 XJ6, parts supply, 94MY

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Re: LED bulb for front fog lights

Postby Equinoxstudios » Tue Mar 31, 2015 12:38 pm

Well, the light output simply isn't comparable. The LED replacement is poor. Only two of the ten LEDs in the unit are pointing forwards, the remaining eight are shining out the sides. and because they are not at the focal point of the reflector they may as well not be there!

So this 50W unit ends up with about 10W of LEDs shining out the front. The shadow line is nice and crisp since the LEDs are near enough a point source but the LUX levels are way down on the standard quartz-halogen H3.

I could try experimenting with a reflector optimised for the side facing LEDs because without them it's pretty damn useless :cry:
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Re: LED bulb for front fog lights

Postby nlcrane » Tue Mar 31, 2015 6:28 pm

I don’t suppose there is a way to test the fail function of the BFM with the LED fitted......for example ...fogs on / LED removed...dash fail lamp on?
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Re: LED bulb for front fog lights

Postby Equinoxstudios » Tue Mar 31, 2015 7:28 pm

Hi Norman, I've only tried mounting the LED units into a spare foglight I have in the workshop - I wasn't going to fit it to the car until I was happy with the light output.

So no verdict yet on whether the BFM throws up a fail...
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Re: LED bulb for front fog lights

Postby nlcrane » Tue Mar 31, 2015 8:37 pm

Equinoxstudios wrote:Hi Norman, I've only tried mounting the LED units into a spare foglight I have in the workshop - I wasn't going to fit it to the car until I was happy with the light output.

So no verdict yet on whether the BFM throws up a fail...


OK, understood.
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Re: LED bulb for front fog lights

Postby baldilocks » Wed Apr 01, 2015 12:35 am

Equinoxstudios wrote:Well, the light output simply isn't comparable. The LED replacement is poor. Only two of the ten LEDs in the unit are pointing forwards, the remaining eight are shining out the sides. and because they are not at the focal point of the reflector they may as well not be there!

So this 50W unit ends up with about 10W of LEDs shining out the front. The shadow line is nice and crisp since the LEDs are near enough a point source but the LUX levels are way down on the standard quartz-halogen H3.

I could try experimenting with a reflector optimised for the side facing LEDs because without them it's pretty damn useless :cry:


I was thinking that was a likely outcome given the reflector design. Comparing the LED with the H3 is it possible to detemine the depth difference between the side LEDs and the filament, I was wondering about mounting the LED bulb further to the rear so as to get it back to the focal point, maybe just a couple of thick washers under it to get spacing right, if that makes sense?
93 4.0 Sovereign, James, 687486, 94MY
93 3.2 XJ6, Jerry, 94MY
93 3.2 XJ6, parts supply, 94MY


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