Courtesy dome lights not working

Hi everybody,

I recently purchased a 1994 Discovery 300TDI with a few issues, most of which I managed to solve with the friendly supporters at aulro.com. There is however one issue that I can't seem to get fixed and I thought I try the landyzone to see if anybody has any further suggestions for me (for those following both forums, I apologise for the double posting). The issue is as follows:

Both courtesy lights aren't working. The research I've done so far suggests this could be one of the following: faulty multi function unit (MFU), bad ground, light bulb broken, on/off switch broken, issue with door switches. I have removed the whole front courtesy lights assembly from the car and when I connect my voltmeter to the two relevant cables, it shows 12V (which is 1-1.5V less compared to measurements at the front cigarette lighter). I have then hooked up my 12V battery charger to the removed assembly and confirmed that both the light bulb and light switch are working. The strange thing is that when I remove the bulb from the assembly and connect it directly to the wires (which showed 12V on the voltmeter), the bulb doesn't light up. I have a similar problem on the cigarette lighter in the boot. If I connect the voltmeter, it shows 12V but as soon as I insert a device like a GPS, the device is not working. It appears that as soon as a load is applied, it's not working. I'm not sure if the courtesy light and cigarette lighter problems are related or are on the same wiring. Some more background information: the light in the boot isn't working either (bulb is ok) and the MFU test sequence also indicated that there is a problem with the courtesy lights. Could it be a faulty MFU despite the 12V on the courtesy lights wires?

Attached images show my test setup:
First photo shows removed courtesy light assembly hooked up to 12V power source (you can only see the back of the assembly, but the switch is in the on position)
Second photo shows me testing the cables with the voltmeter.
Third photo shows that the black cable is bolted onto the roof. I'm not sure where the other cables are leading. There are two plugs coming out of the hole. The first (which I'm testing on the photo) was connected to the courtesy light assembly, it has a purple/brown and a purple/blue cable. Regardless of which of the two cables I'm testing, I always get the 12V. The second plug that comes out of the hole wasn't connected to anything (it has two black and one white cables). Maybe that is part of the problem?
In the fourth photo I have attached two wires to the plugs and again measure 12V.
In the fifth photo I make contact with the bulb from the courtesy light assembly and it won't light up.
With the cigarette lighter in the boot of the car, I get 12V by making contact with the red and black terminals of my voltmeter. And when I swap the red and the black parts, I get the same number but with a - in front of it.

I cleaned both the ground connections for the courtesy light and the battery, but no change. I also put a ground cable directly from the light assembly to the chassis. Didn't make any difference...

Any ideas would be appreciated.
Thanks!

sounds like yu have a high resistance in the feed to those items, so that is soon as it is loaded, yu are loosing the volts.

try running a wire direct from the battery to the +ve side of each item and then see if they work ok. then check all fuses and contacts/junctions back from devices to battery.

Oh bugger! And not even a simple hello in the Introduce yourself section!
Hang on.. where is it?... Ahhhh, found it....op2:

The Mad Hat Man said:

sounds like yu have a high resistance in the feed to those items, so that is soon as it is loaded, yu are loosing the volts.

try running a wire direct from the battery to the +ve side of each item and then see if they work ok. then check all fuses and contacts/junctions back from devices to battery.

Click to expand...

Thanks for the quick replies. I will give the wiring to the +ve side a try today and report what happens. I have checked all the fuses in the fusebox, they are ok. How would I find out where the courtesy light cable goes to? It goes in under the roof but I don't know where it comes out again. Sorry, am by no means a mechanic or electrician...

The Mad Hat Man said:

sounds like yu have a high resistance in the feed to those items, so that is soon as it is loaded, yu are loosing the volts.

try running a wire direct from the battery to the +ve side of each item and then see if they work ok. then check all fuses and contacts/junctions back from devices to battery.

Click to expand...

Ok, connected the dome light assembly directly to the red battery terminal and left the black cable connected to the ground in the roof and it worked all fine, i.e. light went on....

That's a good start, then .
Now yu need to trace the usual supply wires, which may not be that easy. Get a copy of Rave which might tell you which connectors etc you might need to check.

Just to confirm it - it might be worth reconnecting the old wires and monitoring the voltage as yu turn devices on and off. I reckon yu will see the volts drop off as the lights are turned on.

The Mad Hat Man said:

That's a good start, then .
Now yu need to trace the usual supply wires, which may not be that easy. Get a copy of Rave which might tell you which connectors etc you might need to check.

Just to confirm it - it might be worth reconnecting the old wires and monitoring the voltage as yu turn devices on and off. I reckon yu will see the volts drop off as the lights are turned on.

Click to expand...

Thanks Mad Hat Man. I'll see if I can trace the wires with the Rave manual and the Haynes book I purchased. Once again thank you!