Illumination Control Module

The fault:

In my case revealed itself by smoke and a smell of burning from somewhere near the dashboard; I was miles from home and emergency action had to be taken.

This proved to be failure of the Illumination Control Module that is a complicated way of adjusting the intensity of the dial lights.

The story:

Day 1

I had been enjoying my Sport for the last few days when one evening, on the way home from work, smoke came out from the dash area.

Quick dash to the boot to disconnect the battery - no 10mm spanner in the toolkit (sort that in the morning!) - but the under bonnet toolkit luckily had one.  Disconnected the battery, smoke slowly subsided.

The only clue was that the dial lights had gone out shortly before the smoke appeared.  The dial lights had failed a few days ago but were OK after replacing the blown 10A fuse (passenger-side rear footwell fuse #2); maybe it was a warning of things to come.  The blown fuse should only feed the dash lights and, for some reason, the heated mirrors.

Whip off the under-dash cover; out with the instrument pack.  Nothing hot here.  Fish around in the wiring -!!!!!!! - ouch! There is a red-hot blue connector (FC4) leading from somewhere up inside the steering column and also a very hot unit (can't find out what it is by the roadside) in the lower end of the steering column lower shroud.

Fortunately I had been quick enough to isolate the battery before the wiring turned into a bonfire.

So - disconnect the hot blue connector (ouch again) and reconnect the battery.  Drive home no dial lights, instruments, flashers or main beam - but what remains seems OK and the engine goes!

Get home, disconnected the battery and left it to think over what might have happened.

Day 2

Further research has isolated the problem to a low resistance (0.75 ohm) somewhere in the switch-pack etc illumination supply wiring - the circuit that goes all over the car to operate the lights in the door switches and dozens of other places.

The illumination control module is well and truly fried trying to push power into the fault.  I will investigate further tomorrow and no doubt for some days after!   Fingers crossed there aren't any more wires fried; the ones I can see look OK though they got rather hot.  The car is nearly in kit form with so many panels off.

Hopefully I shall have better news shortly!

Day 3

OK - story against myself.

Having dismantled half the car and disconnected all the relevant plugs and sockets, I found that there is no leak (ie melt-down) in the harness.

When I connect the lights up, there appears to be a very low resistance suggesting a short circuit in the wiring.  But I forgot temporarily that a cold lamp has lower resistance than a lit lamp and with upwards of 20 lamps in the circuit, the cold resistance would be very low.

The resistance of one lamp cold is 13 ohms; of 20+ lamps would be around 0.75 ohm which was the reading I was getting and thinking I had a short.

Under operating conditions, with each lamp taking .1 amp (1.2 watts) 20+ lamps should take a current of 2+ amps and this was then proven using a current-limited 12 volt supply (can't be too careful).

So back to the smoke - the illumination module was well fried.

What if the module itself had failed? By now it was too late for the MOSFET power transistor which regulates the variable voltage to the dial lights - it was blackened and dead short! Several of the wiring connections were melted and shorted together so no clues there beyond clear evidence of excess current.

The disconnected plugs in the switch lighting circuit were reconnected one by one and all seems well as the current rises to a steady 2.4 amps.

In its dying throes the module could well have taken excess current and so overheated itself and the close-by wiring.

It is interesting to note that the illumination module had probably been changed before as it had a 2002 date on it - also it was not secured properly and there were tie-wraps holding everything together.