Bentley Arnage Project - Speedo

When I went on the test drive my speedo was working fine, no sooner had I paid for the car, it packed up. It then started working again and worked perfectly for the rest of the 65 mile drive home, only to pack up again the next time I drove it. It has not worked reliably since and is now a high priority job. I bought a cheap sat nav as there are now dozens of average speed cash generators round here and the risk of inadvertently getting nabbed and taxed is too high to ignore. I can relax a bit with the speed alert warnings set but found out that it will only warn if my speed is 10 per cent over - that's cutting things a bit tight. You can change the set up to warn at 100 per cent but it doesn't save that setting across power down - bugger. Come on, Dave, grab the bull by the naughty bits and get on with it!

I had looked for some instruction and the guys on the Rolls Royce forum provided me with useful advice as to how to get the dash to pieces. It was a bit of a faff but nowhere near as bad as I expected.

First job is to set the steering wheel to it's lowest position, next you have to remove the organ stop heater controls, they just unscrew - 3 of them, you only need to do the ones on the right hand panel. With them off you now need to remove the chrome surrounds - these have 2 functions, they make it all look pretty but slightly less obviously they hold the wooden dash trim in place. They have little bayonet clips on them - they look a bit like an old car bulb - the sort you have to twist 90 degrees to remove. They are a wee bit fiddly as they are hard to grip but twist anti clockwise, push the wooden trim a bit and you can get your fingers round them a lot easier, mine wiggled out quite easily. You can now pull the trim out from the top, tilt it towards you and lift it out - simple as simple can be.

Next thing is to remove the screws that hold the top roll in place, this allows the dash top to be lifted slightly, which makes it easier to manipulate the speedo / tacho assembly out of it's location. For some reason Bentley decided that was too simple so they made two of the retaining screws for the panel rather difficult to get at. The two at the top are simple, the two at the bottom considerably less so, particularly the one on the right, which is totally obscured by the lower leather covered panel. Removal of 2 screws that held that on allowed just enough access to the hidden screws. With them out the panel came out very easily, the cables all had plenty of length on them to make getting at the multipole connectors very easy. With them removed the panel was out and taken to my workshop for further disassembly.

It's all held together with numerous torx screws and 4 clips built in to the white plastic rear part, you don't need to remove that though, as I found out, just remove the ones at the front that hold the black part and the perspex screen - they are bonded together, don't try and separate them. With the front part removed you have access to the speedo and tacho units, both just plug in to the motherboard via multiway connectors, hopefully the photos make things clear. The speedo unit wiggles out of place, be gentle with it and under no circumstances touch the visible part of the dial, I suspect it would mark easily. I cleaned the multiway connector with switch cleaner and then reassembled the unit - simple reversal of taking it to bits. The only way to know if it is fixed is to put it back together and try it.

I will say it doesn't work like I thought it would, I expected the needle to be on a spring but it isn't. The motor unit that actuates it appears to be a stepper motor with feedback telling the controller where the needle is. There does not appear to be a mechanical problem, the needle isn't sticking. There could be a problem in the electronics, I hope not as a replacement is ridiculously expensive - 5 or 600 quid for a second hand one. I will test it without putting everything back together but the challenge with intermittent faults is you never really know if you have fixed them. I have been there so many times in the past, I am fairly confident with this one though.

Reassembly is quite easy, getting those two screws in the bottom tabs of the panel is an absolute pain though.

I would recommend that before you start doing this, get yourself a few of those capless bulbs, while it is out you might as well replace the panel illumination bulbs, it's a hell of a pain to go all this way just to replace a blown bulb, new ones should last a good few years. They are retained with twist out holders, it takes 2 minutes to do, costs about a quid for 5, it's daft not to do them when it's all in bits. 

I reassembled it just to the point of having the panel secured in to the dash and all the electrical connectors replaced - I forgot to say at the start - it is very important to disconnect the battery before you start - the one to the left of the boot, you don't want to risk damaging anything. I didn't want to put everything back until I was sure all was good. I discovered that a little trick you can do is while it is all apart, very carefully move the needle by hand so it is at about half scale, when you reconnect power and turn the ignition on it should reset to zero, it just tells you the speedo module is talking to the motherboard. Mine was, it went back to zero and stayed there.

I took it for a quick drive, all I can tell you is that it's working now, I can't say it will keep working, such is the nature of intermittent faults. I will keep the sat nav in the car to use if it packs up again but for now it all looks good.

I noticed that there is only one panel on sale on ebay at the moment and it's nearly 600 quid so I do hope it's sorted, I suspect I may only find out when the car gets hot as that's when it seems to be most prone to failure and it is quite cool today. None of the pins looked like they were tarnished or damaged, I couldn't see any obvious dry joints. There was no obvious evidence of somebody else having been in there, the sticker that goes across both halves of the unit was still intact.

While it was apart I had a pretty good look at it, the capacitors all looked good, if you take yours apart and see leaky or swollen capacitors, now's the time to change them.