My 2002 TD6 is suffering from a slow power drain - 1.8 amps, and I think I've tracked it to the interior lights or the after-market head unit.
I'd be very, very grateful for any help, but in particular, can I disconnect the battery to put in a meter and live?

Nothing to stop you disconnecting the battery -ve terminal and inserting an ammeter in between the battery -ve terminal and the end of the -ve cable end you have just disconnected.
I suggest you have an ammeter with a range upto 10A initially which you can then set to a lower scale as the car ecu's shut down progressively.
You should then be able to pull fuses one by one in a process of elimination to identify the circuits on which battery drain drain is occurring.
After removing each fuse be patient to allow things to settle before taking an ammeter reading, upto possibly 20minutes in some cases.
You should be aiming for a battery drain minimum of about 30mA if you are lucky.
The common cause of battery drain on these cars are the satnav, the telephone, and the FSR for the blower fan and possibly a few besides. Not come across the interior lights causing a problem, be interesting to know what you find. Fitting led lights internally can resolve most problems due to low current requirements..
On reconnecting the battery you will need to do the steering wheel full lock to full lock to reset the steering sensor.
Best of luck.

Thank you. I like the idea of LED lights. Must I change the fittings or can they just be plugged in?

Replacement led bulbs can be obtained from eBay etc. no need to change fittings.
It's just a case of looking at each of the bulbs in the car and then buying a decent led version of the same bulb, they aren' t monitored so no need to buy the more expensive canbus versions.
From memory most of the bulbs are type 501.
Be warned, ther's a lot of bulbs.
When I changed mine I never bothered with the map reading bulbs I found they were very awkward to fit so didn't bother, never use them anyway.

I can't remember without going to look at my car. But don't the interior lights 'fade out' when you lock the car?

If they do then the LED bulbs wont like the dimmer, they'll still work, but their life might be shortened by the dimmer unit as LED's have to be dimmed in a different way to conventional bulbs.

my smd panels fade out.... ... - .- -.




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All my interior leds seem to quite happily fade out. It's a good point about the possible shortened life through dimming but mine have been in 3½ years to date, if they fail I'll just replace them, they're not expensive.

As i said they'll still work, but the bulbs (or the dimmer) might not last as long.

My understanding with LED's is in order to make them half as bright, you supply them with full voltage for half the time (in pulses), whereas a conventional bulb you just supply it with half voltage all the time. We've got problems with the manual dash dimmers in the trains at work where some of the bulbs have been replaced with LED's, it might be you can get away with it for the short time the fade out lasts though.

But I only did Electronics to A-level (a long time ago.) So perhaps my understanding is out dated.

You've just about got it right.
If you use manual dimmers of the type used for incandescent bulbs with led lamps the led lamps will fail in a surprisingly short period of time, although when this happens it doesn't seem to affect the dimmers too badly though.
As above if the leds fail I will just replace them.
Quite a few people on here have replaced the interior lamps with led versions and to date there doesn't seem to be a plague of failures, maybe next week.

I was more concerned about the dimmer than the bulbs, as you say the bulbs are cheap enough (and if they do blow next week 3 years use from them isn't bad), but i'd baulk at buying a new ECU.

Final victory. The Land Rover dealers charged me $80 to tell me that there was a power drain and quoted for a minimum of 10 hours and possibly 30 hours.
Local car electrician has borrowed a friend's OBD reader (his wouldn't read Range Rover), found it was the cooling fan, repaired with a non-RR fan, charged me $180 and sent me off rejoicing.

I hope.

The X5 has the same electric fan.