OK, so I threatened to do it and Tom talked me into it, so here goes:
Pulling an instrument cluster apart.
Primarily on how to swap one of the gauges (in Tom’s case, the fuel gauge) but would apply to any of the internals really. If you don’t know how to get the instrument cluster out, drop the knee panel, undo the 4 screws (two at the top, two at the bottom obscured by the knee panel) that hold the instrument panel surround and as you lift it out, unplug the connector to the fuel flap release. There’s 2 connectors to the bottom of the instrument cluster. With ignition off and the key safely inside the house, unplug these two connectors. There’s a screw in each corner that secures the instrument cluster but they don’t have to be fully removed, just unscrew them until the instrument cluster is free. Once all 4 are free, lift the cluster out.
To take it apart, completely remove the 4 screws (as they also hold the front and back together), unplug the connector to the speaker marked 1 (which is what gives you the annoying beeps as well as the tick, tick from the indicators) and using a fingernail, unclip the translucent back cover at the points marked 2 along with one on each side.
Hinge it upwards (there’s another fixing in the centre that will pop out) and it will come off (and tear the security seals along the top) and expose the main pcb with the ribbon cable going to the display. So you’ll see this:
Notice the 4 groups of 3 screws (circled in red), these are what hold each of the individual gauges in place and also provide their electrical connections. Loosely sit it on the translucent cover and turn it over. On the top you’ll see two black lugs which will pop open if you press in with your thumb and push it out and you can hinge the whole front out.
The front is in three parts, the clear outer, the black surround and a plate that goes around each gauge. With that off you are left with the white plastic body with everything attached. If you want to change one of the gauges you just undo the 3 screws that hold it to the pcb and out it comes.
The mileage information is held in one on the chips on the main pcb, so as long as you retain that, everything else can be changed. I know there’s at least 3 different style of gauges, some have graduations every 2 miles on the speedo, some every 5 miles and then there’s the ones fitted to later cars with a dark green background rather than black as on the earlier ones. There would be nothing to stop anyone swapping all the gauges and the message centre display as long as you keep the original main pcb. If you were to swap the main pcb and it has a lower mileage than is stored in the BeCM, then the mileages can be synced but be aware it will always take the higher one of the two. So if you’ve got a low mileage car and you get a spare panel from a higher mileage car and swap the pcb, your mileage will suddenly shoot up. Incidentally, the BeCM only stores to the nearest 100 miles too, so if your BeCM says you’ve done 125,800 and your dash says 125, 870 and you sync to a lower mileage pcb, you’ll lose 70 miles (so don’t do it just after the MoT).
Putting it all back together is just the reverse of taking it apart but be aware when refitting it that there are lugs that fit into holes next to the screws that hold the cluster in place so you need to wiggle it about a bit to make sure they slot in place. Don’t forget to plug the speaker back in either. Don’t turn the ignition on with the instrument cluster out or unplugged (hence the warning about leaving the key in the house) or you’ll be dealing with SRS, and possibly dashboard, faults.
Calibrating the gauges.
While you’ve got it out, you may as well calibrate the gauges if you are really sad. The BeCM sends a digital signal to the cluster which uses digital to analogue converters to drive the gauges. As they are electromechanical, there can be differences from one to another so if you were to swap a gauge you might find it will read slightly differently to the old one. With the instrument cluster out, break the yellow paper seals so you can remove the translucent plastic cover (which will also mean unplugging the speaker) and if you look along the top of the circuit board you'll see a row of three blue trimmer pots on each side. These are used to calibrate the gauges.
On every P38 I have driven except my ex-plod with a calibrated speedo, they all read over so an indicated 70mph is usually a true 65-67 mph (for type approval purposes and speedo can legally over read by up to 10% but cannot under read at all). To calibrate the speedo you first need to know how far out it is, by how much and in what way by comparing the speedo reading against a sat nav. Remember that the GPS on the sat nav will only be 100% accurate when travelling at a constant speed on a flat, straight piece of road.
If you have a GEMS and a Nanocom (it may be the same on a Thor but I don’t have one to try it on) that makes it very easy as in the ABS menu there is a setting (ABS- Diagnostic-Utilities-Speedo) that causes the speedo to read around 50 mph (Nanocom documentation says it will make the speedo read 100 mph but on all that I've tried it on it reads about 50 mph). So you can set that, make a mental note of what it reads, then drive holding your speed at that same point and compare your actual speed on the GPS compared with what the speedo says. Then drive at different speeds and check them against the GPS so you have an idea of how far out it is at different speeds. You’ll now have your baseline figures so can adjust from there. The trimmers you need are marked VR3 and VR4.
VR4 deals with Offset and adjusts the reading by the same amount over the full range. So if it permanently reads 5mph over, then you adjust that with VR4. It is pretty rare for the offset to need to be adjusted. VR3 deals with slope so if it reads OK at 20 mph but the error gets progressively greater the faster you go, adjust the slope until it is correct over the full range. With the instrument cluster surround out, the cluster itself in place but not screwed in and the translucent cover partly loose, you can get in there with a very short trimming tool (I used the screwdriver bit from a set of the interchangeable bits) but be careful as you only need to give the pots a tiny bit of movement (many years of tuning radio transmitters for maximum power means I've got pretty good at it). On a GEMS, once you’ve seen where the needle sits when driven by the Nano all you need do is use it to drive the speedo to the figure and adjust VR3 so the needle is reading what your actual speed was at that reading.
If you feel like having a play while you are in there, VR1 deals with the Temp gauge reading, VR2 the fuel gauge reading while VR5 and 6 do the same as 3 and 4 but on the rev counter (can't remember which is which though but you have the same offset and slope adjustments on those two). Bear in mind that the fuel and temp gauges are fairly heavily damped so won’t respond immediately to any adjustments.