Instrument Binnacle Lights - bad earth? - where?

[Laird Scooby]

Originally Posted by eeedelli

Oooh! That looks good! Can you remember which type/size of base they are and where you got them? (Wow - you have a working fuel gauge too! ;-) )

The reason for resistors is that it would make the lamps last longer. I suppose if bulbs were substituted with LEDs then that would be less of a problem, though LEDs can get over-driven too, of course.

Although those LEDs must have a resistor built into them to limit the current to the correct value so as to work on a supply of 12V (as the LED forward voltage would normally be around 1.2V), the instrument bulbs appear to be fed directly rather than via the voltage regulator, so may well be getting around 14V when the engine's running, which would shorten their lives if the built-in resistor was calculated for 12V (i.e. 17% over-driven).

Ha-ha!!!! Don't be fooled by that fuel gauge, it was a fluke!

It had run out earlier in the day (having spent most of its time on empty even when full) and the only petrol in the tank when that was taken was the remains of a gallon i had in the can i used to carry in that one (my last 740GLE, shown in my Avatar) in case of such incident.

The bulbs are 501 and 286 for the filament type, T10 and T5 LED respectively.

However, there are LEDs and there are LEDs, not all are the same and a high mCd figure won't necessarily give you a good light output for various reasons.

Some T5 LEDs for your perusal that should work nicely :

https://www.ebay.co.uk/itm/T5-Dash-G...s/313012611144

Pick your colour and quantity, i'd suggest 10 for the instruments and minor switch illumination. I'll give my reasons shortly as to why i suggest these.

https://www.ebay.co.uk/itm/UK-x2-SIL...v/223207446001

Some T10 LEDs, again pick your size and quantity, if memory serves there are 4 in the 740 cluster.

The reason i've picked those is first they are COB - Chip On Board which gives a larger spread of light from the actual source and in both, they have a diffused lens which spreads the light further round so it more closely mimics the original filament bulb in housings that don't have reflectors, such as headlights. However, for headlights it is much more critical and there are very few LED headlight bulbs that actually work correctly.

Back to the instrument LEDs, while the ones i've suggested aren't the cheapest, they should give the best results. I couldn't find the ones i'm currently using listed which is a shame but those are the nearest equivalents.

While almost all LEDs are now made in China, there are a few reasons why i've picked UK sellers.

First, you won't have to wait on the "slow boat from China", second they're more likely to work and stay working as umpteen posties across the planet won't have kicked, dropped, thrown etc your small package of a dozen or so LEDs repeatedly and will have originally been shipped as a bulk order to the seller so are generally more likely to have been handled more carefully.

Third, if you happen to get some DOA units, you are more likely to be able to get it sorted easily. Remember though LEDs are polarity conscious so try removing the bulbholder and turning it round before condemning the LED for not working. Also given the age of the cluster, there could be other problems on the cluster as well.

The resistors you mention, with normal filament bulbs they are 1.2W (100mA) and with the cluster and minor switch illumination, if memory serves there are 10 in the circuit fed by the panel rheostat. That's a total of 12W or 1A so to drop them down by 4V in the feed to give ~half power, you only need a 4 Ohm resistor. That is based ona 12V supply, which you correctly point out is actually 14V with the engine running.

That brings me to the LEDs and your mention of the current limiting resistor fitted to them in the factory. In China where they're made, someone seems to have forgotten the simple fact the alternator charges at 14V or more. As such, they've designed the LED bulbs to run at maximum forward current of 30mA at 12V supply voltage.

However, most red LEDs have a Vf of about 2.2-2.4V, green and yellow LEDs are usually 2.4-2.6V and blue and white LEDs are usually 3.4-3.6V but let's work on the blue/white LEDs to work out what value resistor they've used.

12V - 3.6V = 8.4V. At 30mA, the resistor will be 8.4/3.6 = 280 Ohms. This is not an NPV (Nearest Preferred Value) so the alternatives are either 330 or 270 Ohms, give them the benefit of the doubt and go for the 330 option :

8.4/330 = 26mA approx.

Now work that out using 14V to find the actual current through the LED :

14-3.6 = 10.4V/330 Ohms = 32mA approx.

As you can see, this is nearly 10% over the maximum forward current which isn't recommended for continuous use anyway.

Transposing the normal maximum current for longevity and full brightness of 20mA and do the sums again, working on 14,4V (which is what the alternator gives on a cold voltage regulator, remember we're going "worst case scenario" now) :

14.4V - 3.6V = 10.8V/20mA = 540 Ohms.

At 12V (engine off) this will give a forward current (If) of 8.4/540 = 16mA ish. As the NPV is 560 Ohms, using one of those would drop it to ~14mA, maybe 15mA so would last indefinitely.

As such, i'd suggest running them with the rheostat on a lower setting than maximum to prolong their lives. For any that aren't on the rheostat circuit, it depends how many and whether it would be easier to splice a resistor into one feed or do each individually.

[bob12]

The main feed for the VDO cluster comes in on the plug below the speedo. You will see that the blue plastic circuitry has a + and - printed in it. It is the same plug that takes in the speedo signal from the rear diff.

As for the dimmer it is not a simple rheostat but a rather 'nasty' piece of circuitry with all sorts inside, a couple of chips plus resistors and and capacitors! From experience it is exceptionally difficult to take apart!

The 3 light bulbs for the cluster illumination are usually 504's (3w W2.1 x 9.5d). Control panel (switches) are usually 1.2w W2 x 4.6d. Both are capless.

These pics might help.

Bob

[Laird Scooby]

I couldn't remember if they fitted that nasty electronic dimmer Bob, thanks for the photos and for reminding me there are 4 501 bulbs for illumination.



Seeing the insides of that electronic dimmer gives me an idea how it works but i might be wrong on the exact principle. That said, the chip in there, the LM2901 is a quad op amp i think and i know i've seen the same in other parts of the car.

Stupidly over-complicated!

[bob12]

Originally Posted by Laird Scooby

I couldn't remember if they fitted that nasty electronic dimmer Bob, thanks for the photos and for reminding me there are 4 501 bulbs for illumination.

........................

Seeing the insides of that electronic dimmer gives me an idea how it works but i might be wrong on the exact principle. That said, the chip in there, the LM2901 is a quad op amp i think and i know i've seen the same in other parts of the car.

Stupidly over-complicated!

I'll dig it out from the garage soonest and let you know what it says on da chippie!

Thanks for actually counting the number of cluster illumination bulbs and reminding me there are actually 4!! Bob

PS - enlarging the dimmer innards I see the chip on the pcb is a LM2902N but, can't see those under the heat sink on the metal cover as they are hidden. B

[Laird Scooby]

Originally Posted by bob12

I'll dig it out from the garage soonest and let you know what it says on da chippie!

Thanks for actually counting the number of cluster illumination bulbs and reminding me there are actually 4!! Bob

PS - enlarging the dimmer innards I see the chip on the pcb is a LM2902N but, can't see those under the heat sink on the metal cover as they are hidden. B

My mistake Bob, it is a 2902 and the one i was actually thinking of was a 2904 :



Same family of ICs though, i obviously remembered the number from the data sheet when i looked up the 2904 in the speedo converter unit above.