Page created: 14-Jan-06

Re-published: 1-Nov-09



Technical: Discovery I
Immobiliser Spider Unit

My Discovery developed an intermittent fault whereby the 12v power feed to the fuel cut-off solenoid was interrupted. Sometimes this meant it wouldn't start and on one occasion the engine simply died whilst I was driving. I immediately suspected the immobiliser as this fault is a relatively common problem and I guess it was only a matter of time before I'd have to address it on my Discovery. As a temporary measure I made up a lead with a crocodile clip on one end and a female spade connector on the other so I could supply 12v directly to the solenoid from the battery. This made it possible to start the engine, although to stop it it was necessary to lift the bonnet and disconnect the wire as the key no longer performed this function!

When Land Rover designed the alarm immobiliser system for the 300 series Discovery they added a component, commonly referred to as the 'spider' unit, as a second line of defense in addition to the main immobiliser. I've been led to believe that this was to satisfy some particular technical requirement of Thatcham at the time rather than any specific security issue. However, for all practical purposes this unit is redundant as all it does is duplicates the function of the main unit, i.e. interrupts the starter and fuel cut-off circuits. 

The 'spider' comprises a small circuit board inside a black plastic box buried in the centre of the dash and is prone to failure of the soldered joints on the board. It's sandwiched between the stereo and the heater and I'm guessing that the constant heating and cooling that it undergoes in this location leads to fatigue. This doesn't seem to be a problem that the main immobiliser suffers from and it, in fact, is still used as the immobiliser for the current Defenders, without the troublesome 'spider' unit. For the 300 series Discovery, Land Rover now produce a by-pass for this unit, part no. AMR4956, to solve the problem once and for all. It costs around 5.00 and I got mine from Dingocrofts.

Fitting the by-pass is simplicity itself - the difficult part of the operation is getting to it! In order to gain access it's necessary to almost completely dismantle the centre panel of the dash board, which in my case was made even more complicated by the extra switches and wiring I've installed.

The first step to getting at the 'spider' unit is to remove the stereo, clock, ash tray and in my case two banks of auxiliary switches, followed by all of the knobs from the heater controls. Removing the latter reveals two small screws that retain the heater control fascia, which must also be removed.

Next, remove 6 no. screws that retain the lower part of the fascia that houses the cup holder - there are three each behind the ash tray and lower auxiliary switch panel - and pull it forward out of the way. After that, remove the 4 no. screws that hold the heater control unit to the main fascia panel and then 6 no. screws that retain the main fascia itself. These are located: one each at the back of the clock and coin tray/auxiliary switch panel, two vertically inside the stereo aperture and two concealed behind the small trim cover immediately below the stereo aperture. These last two had me scratching my head for a while...

It should then look like this:

The 'spider' unit is the small black plastic box in the centre of the picture above, directly behind the heater controls. It's secured by two small screws, the rearmost one of which is particularly tricky to get at!

With the unit disconnected, it's a simple job to plug in the new by-pass harness, which looks like this:

Before putting everything back together I secured this harness with a cable tie to prevent it vibrating and causing any more problems. I also test started the engine, just to be sure it was ok.

Re-assembly was, as the saying goes, the reverse of removal and took a lot less time as I knew where all the screws went. It took me about two hours in total. I'm now reasonably confident that I've solved my intermittent problem, but I'll probably carry my emergency starting lead with me for a while, just in case...