940 dashboard switch bulbs

[827Roverman]

Anyone know for sure which bulbs I need to get to illuminate the switches in my 1995/1996 940

[360beast]

I've only ever seen one type of bulb in 940s to be honest David.

[Forrest]

I’m pretty sure the switch illumination near the steering wheel, including the headlamp switch, is 30710781 (picture right)

Centre console illumination including heated seat switches, ash-tray, etc. is

Clear: 3545170 (picture left)
Green: 3523964 (later cars, picture centre, no longer available)

Volvo advise replacing 3523964 with 3545170 but in my experience the plastic part of the clear bulb is slightly fatter and a very snug fit in some applications.

The heater/ aircon controls use panel bulb 3537346.

[Laird Scooby]

Originally Posted by 827Roverman

Anyone know for sure which bulbs I need to get to illuminate the switches in my 1995/1996 940

Originally Posted by Forrest

I’m pretty sure the switch illumination near the steering wheel, including the headlamp switch, is 30710781 (picture right)

Centre console illumination including heated seat switches, ash-tray, etc. is

Clear: 3545170 (picture left)
Green: 3523964 (later cars, picture centre, no longer available)

Volvo advise replacing 3523964 with 3545170 but in my experience the plastic part of the clear bulb is slightly fatter and a very snug fit in some applications.



The heater/ aircon controls use panel bulb 3537346.

The two on the left have replaceable bulbs which can be replaced with :

https://www.ebay.co.uk/itm/LUCAS-T5-...K/153181883547



The right hand bulb is a wire-ended bulb welded to the contacts on the bulbholder. The wire can be cut allowing the dead bulb to be removed and then the holes for the wires drilled out using a 1mm drill in a Dremel. Then new wire-ended bulbs aka grain of wheat bulbs can be fed through and soldered in place or an LED can be fitted with a current limiting resistor - this requires one of the holes in the bulb holder to be drilled out to ~4mm to allow the resistor somewhere to sit.

There are 12V LEDs available on fleabay with built-in current limiting resistors. However these don't last long. The simple reason being they are designed in China by the Chinese who use the nominal voltage of the cars system i.e. 12V which is incorrect. They work out the value of the resistor based on 12V and allow maximum current (usually ~30mA) to flow in the LED which gives (Vcc - Vf)/I = (12 - 3.6)/0.030 = 280 Ohms.

What happens in reality is that Vcc is actually 14.4-14.0V (charging voltage) so worst case scenario, Vcc (supply voltage)14.4V (cold regulator voltage) - Vf (voltge across the LED) = 10.8V. So far, we're 2.4V extra in the system so the current limiting resistor won't do what it was designed to giving 10.8V/280 = ~39mA - 9mA or 30% more current through the LED than it was designed to take!

Normally for maximum brightness, only 20mA is needed. Therefore the resistor should be :

14.4 - 3.6/0.020 = 540 Ohms. I prefer to use 680 Ohms to give a little extra margin for error, a 560 Ohm resistor could be used safely but no real adavantage.

That said, the Vf i quoted of 3.6V is for green and blue (and also white) LEDs, most others are 2.2-2.4V. Substitute that value into the equation instead of 3.6V and this happens :

(14.4 - 2.2)/0.020 = 610 Ohms.

Now you see why i chose to use 680 Ohm resistors. Even on an LED with Vf = 3.6V the current is 10.8/680 = ~16mA which is still very close to maximum brightness but will last almost indefinitely.

[bob12]

Dave, as the normal bulbs are on the dimmer circuit are the led's affected by that or do they impinge on the dimmer to its detriment?

[Laird Scooby]

Originally Posted by bob12

Dave, as the normal bulbs are on the dimmer circuit are the led's affected by that or do they impinge on the dimmer to its detriment?

No dimmer problems to my knowledge Bob, mine have been in this one for 4 years and my previous 740 had them for about 2 years, no problems on either.

As for the dimming, yes the LEDs do dim but they won't go completely out like filament lamps do but to all intents and purposes, except on the darkest nights you'll think they're out if it's on the dimmest setting.

[360beast]

Not the best picture but LED bulbs courtesy of the great Dave himself. Just fitted and for the first time in god knows how many years my 940 has 4 working interior switch bulbs.


IMG-20200714-WA0006 by Luke Ryland - Flickr2BBcode LITE

[aardvarkash10]

replace the resistive dimmer with one of these cheap as chips pwm devices.

Job done. Everything will dim together.

[Laird Scooby]

Originally Posted by aardvarkash10

replace the resistive dimmer with one of these cheap as chips pwm devices.

Job done. Everything will dim together.

Nice find Ash, do they do a slide pot version as well?

[aardvarkash10]

^well within your skill range to re-engineer I think Dave!

I just got three of these to do dimmer duties on LED strip lighting circuits in the yacht. I doubt I'd want to put 5A through them, let alone 10A, but at $3.50 NZ Pesos a throw, they are cheap enough to burn out once a year...