OPTICELL Lighting unit

Have you every wondered why your instrumentation lighting to the Aircon, Temperature, Hot/Cold Air slider, and Road Lights Switch are very dim or non-existent?

The answer to no lights at all is probably a blown bulb, 12v 6Watt, in the Opticell unit. The glow-worm brightness of these displays is another matter altogether! Eds note: This article assumes that the fuses, wiring and bulb holder are fully functioning and that the Opticell fibre terminations are not damaged or missing, we are talking about updating an existing, working, unit not the repair of a faulty system.

The Lucas Opticell unit is an ingenious item of the era the car was designed in. It is basically a small box with a lamp bulb and several fibre optic flexible light guides to carry the light to the various instrumentation points. The concept is great – no fiddly little bulbs to blow.

Warning: If you are not competent with basic mechanics and electronics then avoid any further reading of this section

.You will need to open up the section underneath the ‘ski-slope’ and extract the Opticell unit which is sited alongside the gear change lever, disconnect a lot of switches and optical fibres, modify a circuit and then put everything back together correctly! Refer to the Repair Operations Manual (ROM) that corresponds to you Jaguar for instructions on how to get it all apart.

Perhaps these displays worked well when new but the one I investigated was in dire need of attention. Having tried cleaning of the optics and attempted to find a new bulb and tried temporarily lashing in a 10watt bulb I came to the conclusion that a serious increase in brightness was required.

The option to simply install a bigger wattage bulb won’t work as the heat generated cannot escape and meltdown of the Opticell will quickly follow. A different solution had to be sought. A possible solution would be to dispose of the Opticell and all the fibre optics and install a separate LED into each and every instrument light point but was rejected due to the amount of work in doing so.

Having noticed that very high brightness LED’s are coming onto the market, 3Watt Luxeon Star types but not yet economically available in the UK I ordered one from an overseas source in the form of a small torch. A quick hand held test with the torch shining its beam directly down a single fibre showed promise as the indicator could be seen in overcast daylight.

Encouraged by the promising results I just had to go on. The torch solution turned out better than I had expected, as its outer diameter was just a little larger than the inside bore of the Opticell unit and was very nearly a push fit. The old light bulb and reflector housing of the Opticell was disposed of. There is a plastic guide that protrudes inside the Opticell inner bore that needs to be removed – a ‘dremel’ type tool is ideal for this. The push fit didn’t quite work so a little heat from a hot air gun just around the edge of the unit softened the plastic enough to force the torch body into the Opticell bore.

The electrical characteristics of an LED are not the same as an incandescent bulb and just connecting an LED across a 12v car battery is a very unfair contest – the LED will lose every time! Some sort of current limiting device is essential. The torch itself does not appear to have any current limiting built in but does in fact utilise the internal resistance of the batteries to do the job. Do you remember Ohms law? There are various help pages around on the www that will calculate a resistor value. For my purposes something around 18 to 24 ohms would have been just right but the closest I happened to have in my bits box was 30 ohms so that was the one I used. The heat dissipated by the resistor is significant and needs to be about 5-watt rating and to avoid a fire hazard needs to be fitted somewhere the heat can safely be dumped.

The end result was a significantly brighter instrument display with no visible signs of tampering with the original integrity of the build of the vehicle.