Instrument bulbs - what's this?

[cappedup]

So, dash is all ready to go back in, just waiting on some LED bulbs from Moss to arrive.

 

Everything back in place apart from one conundrum.

 

There are two remaining bulbs/holders behind the dash and only one home, the IGN light. Which never worked. (Assuming it should light before starting, with the oil light.)

 

The two remaining are both the same type, plastic shroud protected instrument bulbs, i.e. Not a dial illumination one.

 

One of them is Brown/White and Purple/Black. These wires seem to go to something sticking out of the block (some kind of sender unit) and the other goes to a connecter between two brake lines.... which was corroded off.

 

The other bulb wires are Purple/White and Solid Purple. I get purple to go to a fuse.

 

So:

 

Which ones the IGN light?

What is the other bulb/holder without a home?

 

In other 'bulb' news, the BRAKE bulb doesn't illuminate. Is that a switch on the handbrake?

Edited September 14, 2017 by cappedup

[Mark69]

Hi ,

That's the oil pressure switch on the block, not sure about the brake wiring on lhd cars .

But the bulb holder may be for the glove box.

Mark

Edited September 14, 2017 by Mark69

[Waldi]

The " brake switch" connection is probably the PDWA valve, installed on the LHS on a brass fitting just below the master brake cylinder.

Edited September 14, 2017 by Waldi

[cappedup]

Half solved the riddle.

 

The oil pressure and brake pressure warnings are linked together in the same system.

 

My brake warning wire is corroded.

 

I think the PO has the ign warning light in the wrong spot, and it's the oil warning which doesn't light.

 

Found the wiring diagram. The extra bulb is the boot/luggage lamp! Don't have that installed on the dash.

 

So I just need to fix the oil and brake warning lamps and I'm golden.

[RogerH]

The ignition bulb is white/yellow and white and must be a filament bulb.

 

Roger

 

Have you got this site http://www.advanceautowire.com/tr2506.pdf

Edited September 14, 2017 by RogerH

[cappedup]

Brilliant link. Thanks Roger.

 

Yes, I read that the ignition bulb must remain filament.

 

I'm impressed with the little instrument LEDs. Dash going back together today. The difference is great. Photos to follow.

Edited September 15, 2017 by cappedup

[Keith66]

Hi

 

Considering the change myself as i have all of the dash out at present so do you have any before after pics?

 

I read on the Moss site that the LEDs are not dimmable so if they are too bright its a bit of a waste of time.

 

But nows the time to do it if im going to bother.

 

Cheers

 

Keith

[tthomson]

LEDs are definitely dimmable, but typically through using a Pulse Width Modulation technique, as per my article in TRAction in 2009.

It is now relatively easy to find a suitable PWM dimmer for a fairly low price by looking on Maplin, Rapid Online, RS components, Farnell etc.

 

TT

[Steves_TR6]

I’ve just fited a special dimmer and leds to my tr6 dash

 

Two comments

 

1, the led dimmer that i bought cheap on ebay works great but isnt compatible with the negative earth bulb fittings, so i had to add new insulated fittings for the led’s then they work great. They were too bright un-dimmed.

2, try the ‘special’ led replacement for the indicator tell tale, available on ebay from auk supplier, on the 6 the polarity swaps over with left/right indication but a ‘special’ led is available and works really well, so much brighter than the old bulb.

 

Steve

[Keith66]

TT

 

Could one of those be used to replace the standard rheostat but use the same knob so it looks original?

 

I'll dig out the 09 copy of TR Action.

 

Cheers

 

Keith

[Steves_TR6]

Thats exactly what i did Keith.

 

The electronic dimmer has a 1k pot to control the dimming, i replaced the tiny one supplied with a larger one, cut the long plastic shaft to suit, and bolted it into the original bracket behind the dash. Original knob refitted and looks great.

 

Steve

[RogerH]

I know I'm going to get kicked for this one but lets's give it a try.

 

LED's operate at about 2V. The LED's that you get for the instruments are connected direct to the 12V + car supply.

 

In order to do this there is a series resistor built into the bulb. This drops the extra 10V+ whilst allowing apprx 20mA through (it may be less currect - 10mA perhaps)

 

So if a series resistor is used to give a certain brightness then perhaps adding another resistor to the existing series resistor will dim it slightly more.

Brightness is relative to series resistor value.

 

Replace the Rheostat with a rotary two position switch and hook up the LEDs to position 1 that is a driect 12V+ feed and use position 2 that goes via the extra resistor.

 

This is what I have done with my LED dimmer for the OD warning LED. (a simple switched extra series resistor)

 

 

It should work - but!!

 

Roger

[RobH]

You are certainly right Roger if it is just a bare light-emitting diode, but its not always that simple. Many complete bulbs with LED elements incorporate a driver chip which stabilises the brightness with varying input voltage so you cannot dim those just with a series resistor. Instead you have to pulse them on and off (too rapidly to see) which is what the dimmer box does. That is what TT was referring to above. Pulse width modulation means that the shorter the 'on' time the dimmer the bulb looks because our eyes average the light output over time.