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Mk4 Ls400 Battery Disconnection


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I want to remove the centre console of my LS400 to put new LEDs in the temperature displays. I seem to remember last time I tried to disconnect the car battery the alarm kept going off, and in the end I gave up.

Is there a special trick to it? Put the key in the ignition first, or something?

Thanks

Jon

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When I replaced my battery I had the car unlocked andd the doors open. I dont remember the alarm going off, although it did when I reconnected the battery. Just have the key ready when you plug it back in. Press the unlock button when it starts going off.

I've opened a post about these temperature displays. I'm not so sure they are LED's but please let me know how you get on as I would love to get mine sorted.

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I've opened a post about these temperature displays. I'm not so sure they are LED's but please let me know how you get on as I would love to get mine sorted.

I've got it in bits now. They are bulbs unfortunately. Very hard to replace. Do you know where I can get them from?

They are 3mm PCB mounting bulbs. 17 for display and switch illumination, with green rubber covers. One without the cover, to illuminate the passenger seatbelt warning light.

They do use PCB surface-mount LEDs to light the indicators in the switches, and in most cases they're doubled up too. 3 LEDs for the "Dual" indicator, and 2 each for the Front, Rear, Recirculate, Fresh Air, Auto and Auto indicators.

I've attached a work in progress photo. I've taken a lot more, which I'll upload in another post when I've finished. I had to do a lot of dismantling to get access to the bulbs.

It looks like the only sensible way to replace the bulbs under the LCD displays is to cut them off the PCB. If you wanted to do a nice desoldering job I think you'd have to desolder the displays as well, which means 16 pins each for the temperature displays, and 25 pins for the centre display. As it's a double sided PCB this will be extremely hard to do without destroying the LCDs.

Jon

post-7174-1258289791.jpg

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OK, I have formulated a plan. I have managed to desolder the two broken bulbs, it wasn't as awkward as I thought. I just applied a bit of heat while prying the bulb with a small screwdriver.

I've reassembled the console but before doing that I soldered a couple of wires to the place where one of the bulbs was. This is so I can measure the voltage when it's back in the car, and also experiment a bit with other bulbs / LEDs before opening it all up again to replace them properly.

post-7174-1258295935.jpg

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OK, everything is reassembled and working now. With the engine running I measure 8.3v when the brightness is high, 5.3v when low. I think they are almost certainly 12v bulbs, underpowered to make them last longer.

There are several stockists of 12v 3mm "grain of wheat" bulbs on eBay, I think I'll try those first.

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Fantastic help so far.....

Question is, do I want to do this.... Hmmmmm. I'm OK with a soldering iron and I guess it looks do-able.

Is it just 2 led's per display? I have the right hand temp, and centre display out. How many is this going to be?

From what you're saying I'm guessing that none of the backlights on the unit are easily replaceable?

Thanks very much for this. I've been looking for help with this for a while

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Is it just 2 led's per display? I have the right hand temp, and centre display out. How many is this going to be?

The centre display has two bulbs, the outside displays have one each.

From what you're saying I'm guessing that none of the backlights on the unit are easily replaceable?

They are all surface mount 3mm bulbs, after intensive googling I've concluded that in the era of the LED nobody makes SMD incandescent bulbs any more. I've ordered some small green bulbs from CPC, I'll test them on the wire I've left sticking out of the dash before I fit them. I bought enough to replace all of the bulbs, I still haven't decided whether to do that or not. It will mean all locations have exactly the same brightness and colour, but on the downside it'll be a bit of a faff to do.

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Any reason why a standard 3mm LED shouldn't work?

Well you can't usually drop in an LED for a bulb. You change the brightness of a bulb by changing the voltage, but LED brightness is best controlled by modulating them. i.e. turn them on and off many times per second, and the brightness depends on the ratio of on time to off time. On for 1ms and off for 1ms = 50% ratio, so 50% brightness. Also you have to be careful not to pass too much current through an LED, it drastically shortens their life expectancy.

I measured the voltage across the temperature bulb terminals, it was 8.3v bright and 5.3v dim. You'd need a resistor to drop that 8.3v down to something like the 3.2v max VF for the LED. A 160 ohm resistor would do it. It might work, I'll give it a try. I've got bucket loads of clear 3mm LEDs. I'll solder a 160 ohm resistor to one of them and test it in the dark on the way home from work tomorrow.

Even if it works, it will be quite awkward to fit an LED and a resistor in the space where the original bulb went.

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That's what I guessed. Thanks for the info though but yes, space is probably a real problem.

Wondering if I do this whether to go for Blue bulbs instead (throughout) as I'd probably make a day of it and change them all

Thanks for all this info. You've been incredibly helpful with a problem that I didn't know how to start going about.

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