Reposted from:
http://www.lrenthusiastforum.com/
The Definitive Alarm Spider Post
Right I decided it was past time I did a proper "how to" on this as it crops up quite regularly, bits of info are scattered around different posts on the forums.
A common failure on Discovery Series 1 vehicles (both Tdi & V8) with the two-button alarm system (i.e. all vehicles from 1996 model year to 1998 model year) is a component called the "spider".
Symptoms of this failure are typically an intermittent non-start or cutting out while driving (or both), which usually gets progressively worse until it is happening very regularly. Typically you will find that leaving the vehicle from a few minutes to a few hours will allow it to be started, only for the fault to repeat later on.
The spider unit is a small black box of relays, controlled by the immobiliser & alarm, mounted on a panel just in front of (toward the front of the vehicle) and below the radiocassette slot - a very inconvenient position which is designed to be very inaccessible to thieves in a hurry. This contains relays which isolate various key circuits of the vehicle; the feed to the fuel solenoid, and the feed to the starter solenoid on a 300Tdi, and ignition, fuel and starting circuits on a V8. A combination of heat, vibration and a not very good quality circuit board eventually results in a dry solder joint on the circuit tracks inside this unit, which often then get hot and burn out completely. It happened to my 1996MY Discovery V8i, the first one I ever saw with the problem in about 1998/99, and it took me about three months to find the fault by which time I was not in a very good mood ... the purpose of this post is to prevent anybody else having to relive the level of irritation I faced with a vehicle which would sometimes start, sometimes not, sometimes cut out while driving and then come back to life while coasting to a stop, sometimes die at junctions, sometimes die for an hour or more at a time.
So far, I have seen the fault affect both the fuel and cranking circuits on different 300Tdi vehicles, and the ignition circuits on my V8. I assume it can also affect the other circuits on the V8.
Diagnosis.
Before you do anything else check all the fuses and that the battery is OK - it's really very annoying to pull everything to bits only to find a blown fuse somewhere! The next thing to do is to check that the immobiliser is not actually active - if it is, there will be a red "key" symbol in the dash when you try to start the engine - and if there is then the problem is elsewhere, perhaps the alarm ECU, key fob programming or whatever.
First you need to identify and check the offending circuit. Does the vehicle crank and not fire, or does it not crank over?
If it cranks but does not fire, then the problem is in the fuel solenoid circuit on a Tdi, or ignition/fuel on a V8.
Take a digital voltmeter (DVM) and check if the fuel solenoid (Tdi) or ignition coil/fuel pump (V8) has a 12 volt supply to it by connecting the meter to the relevant blade connector and earth and turning the key to position 1 (on). If there is a 12 volt supply to the circuit, then the fault probably lies elsewhere. (Note that a dry joint may not always provide a perfect break in the circuit; on my own vehicle, it caused a voltage drop at the coil which was only apparent once you started cranking. As a result all appeared OK at rest, but when you tried to crank there was no spark. As I "knew" there was power to the coil, I didn't look at this any further, so it took rather a long time to find out that the power consumption when cranking was causing a voltage drop at the coil, which in turn caused the ignition amplifier module to cut out).
The connectors you are looking for are:
- The small blade connector on the back of a 300Tdi fuel pump, down by where the injector pipes come out
- The + connector on the coil of a V8
...if it's the V8 fuel pump you're on your own as I haven't had that out!
If there is no voltage at the circuit in question, then the next stage is to take the spider unit out - jump down to Repair.
If it does not crank, then the problem is in the starter circuit (Tdi or V8, much the same on both). Take the thin lead off the starter solenoid on top of the starter motor, and stick a DVM between it and a good earth. Turn the key to position II (start). You should get 12 volts across the meter. If you do, then the fault is most likely with the starter motor ... but to be sure, put the lead back on, stick the meter probe into the back of the connector, and operate the starter again. If the voltage is still present but the starter isn't doing anything, it will be the starter. If the voltage does not appear under load, it will probably be the spider unit. Time to take the dash out...
Repair
Removing the spider unit
This is fun. Really fun in only the way that a dashboard with lots of fiddly little screws and bits that you can drop in inaccessible locations can be... allow 2-3 hours at least. You don't need many tools, in fact I think only a couple of Philips screwdrivers, a soldering iron/solder, and something to make some clock keys out of, plus the radio removal keys for your particular type of radio.
I suggest you get hold of a workshop manual for this as it makes life easier if you know what order to take things out in and where all the screws are hidden but a brief rundown is below -- sorry if I have forgotten anything but it's a while since I took one out.
- Disconnect the vehicle battery
- Remove the radio (you will need radio keys for this) and its cage
- Remove the ashtray & the blank switch panel on the opposite side (they just pull out)
- Remove the clock on one side and coin tray on the other (easiest way is to get 2 flat bits of metal about credit card thickness, slide in above and below the clock to release the retaining tabs and it just slides out)
- Remove the twin pop out cupholders (about six screws)
- Pull off the 3 rotary heating/ventilation controls
- Unscrew and remove the plastic surround to these
- Unscrew the heater controls so you can move the unit around
You will now be pretty much at the stage of being able to take the black centre console panel out ... so take the screws out and wriggle it out. It tends to catch on things but will pop out with a bit of a tug. The face vents etc come out with the console.
Behind all this you will find a small black box screwed to a metal face which slopes towards you at about 45 degrees. It has two screws holding it in and a 10 pin multiplug coming out the RH side which is covered up by a metal security plate so you can't easily disconnect it. Take the unit out. This is a "sealed for life unit" which it is not possible to repair. Ignore that sentence.
You will see that on the end of the box where the connector is, the plate is a press-in fit. Get a small flatblade screwdriver and lever out the end plate, working your way round until it pops out. The whole circuit board will slide out.
Look on the back of the circuit board (the solder side). Chances are the fault will be immediately obvious, if it is a little blackened and burned, but if not, examine all the solder joints carefully, if the unit has failed in the normal way there will be a "dry joint" (where the solder joint has cracked up making a bad connection) on one of the larger pins on the board. Basically all you need to do is to get a decent fairly high powered soldering iron with a small point on it (I use a 40 watt) and re-solder the broken connections. Depending on how much you enjoyed taking the unit out, you might also want to do all the other relay and connector pins while you are in there - it makes sense.
As an option of course you can replace the unit, I think it costs about £40 or so and the part number is on the unit, usually AMR4889.
Put the box back together once it has cooled off a bit, and connect it up. At this point it makes sense to test the system before you put everything back in ... so reconnect the battery and fire it up - hopefully everything will now work!
Assuming it does the remaining steps are simple:
- disconnect the battery again,
- throw all the dash back in,
- congratulate yourself on fixing a unit that isn't supposed to be fixed
- reconnect battery, set clock & radio code etc etc
- Robert is your mother's brother go for a beer
One for the technical archive I hope, once somebody points out something I am bound to have forgotten somewhere
I cannot begin to explain the aggravation this caused me and the amount of work that went into finding one teeny little dry joint on a circuit board inside a sealed unit for which there is no circuit diagram hidden right in the guts of the dashboard ... so I thought it was worth sharing the detail in order that others don't have to go through the "learning process" I did!
