Father wanted to take the BX home today, so a load was lugged
The seats weren't folded down properly (he insisted he knew what he was doing!) but still; non-estate-estate-car yo.
Anyway, speedometer. Again. The story behind this most recent borkage starts at the beginning of the month, coming home from the FoD. I was about 10 miles down the M1 coming home, and the speedo went from reading 70 to 120. I whacked it and the speed rose a bit more. 30 minutes and more whacking later, the speedometer is reading off the end of the scale. Rather suboptimal, so I laid the car up for the week and took the (welded) Micra to work. I reached out to the club to ask if someone had a speedo going spare, and they did! This was lucky as replacements on eBay were over £100.
I began by taking the binnacle out of the dashboard and flipping it over onto the steering column, a job I've become really good at as of late. I addressed an easy issue first- the sidelight lamp blew last Sunday and it was duly replaced.
I like the design of the lamp holders. You twist them anticlockwise to unlock and clockwise to lock in place. The only drawback with this is that it presses the contacts onto the traces on the binnacle, so they could eventually bend out of shape and cause the light to flicker/not work.
I took the speedo cable off and was greeted with shitty oily mess
Mmmm.... tasty filings.
The instrument panel was then extracted. Not much hardware is used on these. The real niggly bits are these grab washer things that are attached to the two locating pins on the bottom of the panel. The rest slots in and screws in pretty nicely.
The car then looked like this.
I could then move inside to go about with changing the speedometer itself. We've two 7mm bolts to remove on the back
And while I was there I also replaced another backlight. For some reason the car has a lamp installed in the glowplug socket, so I just swapped them over. The plastic front was then removed to expose the front of the instrument panel
Two 5mm self-tapping screws later and it's out! There's a fine mist of oil everywhere.
New and old:
I tried to clock the new speedometer, but unfortunately instrument manufacturers were getting clever around this time and there's an anti-tamper mechanism on the wheels:
The wheels have two sets of rounded bumps. Between them are these square nubs which have to all be aligned for the wheels to turn and count the miles. If they're misaligned, the mechanism either binds up or just rotates once each wheel reaches 9. I gave up and transplanted the wheel assembly from the fucked speedometer into the unfucked one. Interestingly, the new one has a fulcrum for the retention bar slot which made it easier to remove and refit.
Lovely. Now to install it.
...And this is where the pictures end . Sufficed to say it wasn't all that painful. I used brake cleaner on the speedo cable itself to clear out all the shitty oil to avoid a repeat of last time. I put it all back together and only* busted the HRW switch. Never mind. I have spares. I then did a road test to make sure it worked and indeed we're back to wobbly (but accurate) speeds. All that foreplay for nothing, eh?
Now some autopsy shizzle. Here is how the speedo shoult work internally:
The drive turns a bar magnet within an (presumably for eddie current reasons) aluminium cup or dish. There is a tiny airgap between magnet and dish because the magnet is pretty weak. The dish itself connects to a thin shaft with the speedo needle and two clock springs attached. As the magnet turns, it starts to drag the dish along in the same direction, however, the dragging force is counteracted somewhat by the clock springs. When the magnet RPM increases, the rotating magnetic field also speeds up, making the cup slowly turn, being regulated by the clock springs. It's a sort of really basic magnetic clutch. These springs are pretty weak because the magnets are weak and the needle is pretty light. This was useful to know when I looked at the old speedo.
The old speedo's insides is caked with metallic oily sludge. I'm fairly certain the sludge was ferrous as it loved to be stuck to the magnetic poles. This is a bit of an issue, as it means there's constantly a blob of oil bridging the magnet and the cup. When I tried to turn the drive with my screwdriver, it was super hard to turn and I noticed the cup was also turning more readily. I flipped the speedo round and the needle was over-reading like crazy. The weak clock springs did little to help: the more oil there was in the cup the more viscous the coupling was between the magnet and the cup itself, meaning there's less slip. I'm sure with some brake cleaner and Q tips I could fix it up nicely. I think this all came about from when I oiled up the cable when I installed it. There's some sort of packing on the speedo drive which failed when driving home and let oil that was sitting there leak through and onto the rotating magnets and gears, kind of turning the assembly into a fluid coupling. The new speedo looks like it's got a fair amount of grease packed in it so this shouldn't happen again. Shouldn't. I hope.