LED Instrument Bulbs

[ptjs1]

Apols for posting in multiple sections of the BB. I'm just trying to find a member who may understand LED electrics!

After 12 years of frustration at the appalling design of instrument warning lights on a facelift XJS convertible (ie invisible!), I've finally decided to do a fairly cheap upgrade by putting LED coloured bulbs in place of the standard B8.5D bulbs.

I thought it was going to be fairly simple but have discovered that there is a potential issue with 3 of the bulbs. These are the Sport Mode illumination bulb, 1st gear lockout illumination bulb and the Check Engine Light circuits. For some reason they are linked into circuitry which seems to give the bulbs a constant current draw of .01v (as measured on my voltmeter) when not in use. This doesn't cause a problem normally as it doesn't seem to cause any illumination of the standard bulbs. But those who have changed to LEDS report that the bulbs then become faintly illuminated all the time. And apparently if you then leave a standard bulb in the Sport Mode slot, then the 1st gear lockout LED won't work (presumably because the circuits are deemed to be mutually exclusive).

Now I'm guessing that this is because an LED chipset can operate at much lower volts and therefore the LEDS glow with even .01v applied. My question is would an LED bulb with 2 LED units be less likely to illuminate with .01v applied than one with one LED unit? My naive logic says that it would still glow dimly with 2 units, but maybe there's something in LED current draw which would mean that the more LED units are in the chipset then the less likely it is to glow with a tiny voltage of .01v applied? Does that make sense? If so, I'll fit the more expensive 2 unit bulbs to the problem circuits.

I'd appreciate any thoughts on the above.

Tks

Paul

[MichaelSB]

I would be interested in a answer to your question as it is relevant to my current dash issue

[J44EAG]

I`m by no means an LED expert...far from it.

Would some for of resistor either in parallel or in series not soak the residual circuit current and knock out the LED?

CANBUS LED units also have an inbuilt resistor. Might that not help?

There you go...probably talking from outa me exhaust as usual!

Mike K

[PPV]

There was an article on this in Practical Classics mag recently. Unfortunately I binned my copy as there was nothing to keep it for after I had read it all. Perhaps you have a copy or someone else has remembered what was said. I think for some instruments it needed a resistor added to work.
Regards

[BrianG]

Paul, as a retired electronics engineer I can state that LEDs will NOT light with a voltage of 0.01V! The normal voltage for a single LED chip is between about 2.5 and 3.5V, depending on the LED colour. You are correct that an LED will glow with a much smaller voltage than a filament lamp but it is likely that your meter is not seeing the true voltage that is present because it may actually be pulsing to a higher voltage. In general you can't just replace a filament bulb with an LED without a resistor to limit the current flow - on a 12V supply if there is no current limiting resistor the LED will destroy itself. I would expect LED 'bulbs' in standard automotive profiles to have a built-in current limiting circuit or resistor inside; it may also have multiple LED chips wired in series, like the white 501 parking light replacements I've fitted recently, which will certainly raise the voltage at which it starts to glow.

I don't know the circuit that drives these warning lamps in the XJS as to whether some are driven from switches or driven from a combination of various ECU outputs. It is certainly possible that the inadvertent glow is caused by the LED bulbs you have tried if they only appear to have a single chip. These may 'clamp' an ECU output to a low voltage which can cause unintended paths for the other LEDS and generally cause strange results. It all depends on whether they have a common ground and are ECU switched to a positive voltage or are connected to +12V common and are ECU switched to ground, or maybe a mixture of both.