Speedo WTF?! Theories please

[GTSBoy]

Add front upper control arm bushes to the list of curses.

At least we have glass headlights.

[niZmO_Man]

magnet got stuck near your speedo drive

[GTSBoy]

It's still doing it at odd intervals. Mostly on the first drive of the day, but tonight it was fine on the way to the pizza shop, but was reading nearly double as I drove away from the shop.

Factor is not 2x. Is about 170/90 = 1.9:1 or thereabouts. Is most disconcerting having the speedo wound off the dial while still only doing ~110.

I think I'm going to have to spend a day taking it apart after Xmas day and putting it on the healing bench to see if it will fix itself.

[Butters]

Wouldn't reducing the gap between the magnet and cup increase the reading ?

IE some metal filings finding its way inside and would give an intermittent experience depending on where it is sitting. 

The other failure point would be the spring ?

[GTSBoy]

Yeah. The spring was the leading theory, but clearances could also be responsible.

It's hard to understand how the spring could fail in a way that does it intermittently. Maybe it's gummed up with something, maybe it's broken in some weird way. Hence the proposed disassembly. Am hoping that some aerosol maintenance (spray solvent followed by silicone lube) is all that is required, because who the hell could tell if the clearance was correct, right?

I hope I don't f**k it up. This speedo is literally accurate to the km/h across pretty much the whole range (at least for the 99% of the time it's not being possessed by the devil).

[niZmO_Man]

Reminds me of when I put a Pintara cluster in my R31 Skyline and it would rev to 9 with a blip of the throttle

[GTSBoy]

Quote

Typically these threads end up in a wild goose chase then information like that comes out at the end that explains it all.

OK. Thread satisfaction time. As usual, it turned out to be something slightly unexpected. Observe what I saw when it came out.

Top end of speedo drive cable.

And mating receptacle on the speedo.

That greasy sludge stank of gearbox oil. So it would appear that the o-ring on the Navarra-Skyline hybrid speedo drive (for the R33 turbo box to run the cable drive) is buggered. Pushing oil slowly up the cable. At least the cable is super dooper well lubed now. But I'm going to have to pull the speedo drive out and fix the o-ring before it pumps any more gunk out into the speedo. To whit....

It managed to pump a reasonable amount into the cluster. And...

You can see it on the outside of the alloy cup that carries the magnet. One can only presume that there was a stack of it inside that cup, in the gap where the copper speed cup runs. A good spray with cancer-in-a-can seemed to blow most of it out. A quick lick of PTFE in a can onto the various worm gears and such in the odometer train (replacing the epic amount of obvious tranny fluid that they'd managed to pick up as a fortuitous lube) and I threw it back together. There was a lot of either unrelated or possibly related filth inside the cluster, so that got a bit of a wipe also.

Will go back in tomorrow morning. I'm not squatting in the car in the dark with the mozzies trying to find all the screws. I shall report on behaviour and hopefully preserved single km/h accuracy after a couple of drives.

Oh, FWIW too....this is a 252k km old cluster. Everything inside was just absolutely beautiful (apart from the grime). The hairspring on the speedo, every other thing in there - all just in perfect condition. So "dying of old age" does not appear to be a thing, for things that are not abused.

 

[GTSBoy]

2 drives down with no shenanigans. Am convinced that the grease explains all. Now all I need to do is make sure to fix the drive end before the problem comes back. Is it still called procrastination if I just forget?

[Duncan]

On 13/12/2022 at 10:00 AM, Neostead2000 said:

......it may have failed from age and causing the needle to read higher sometimes, or if there's debris or stuck grease around it.....

 

On 25/12/2022 at 9:45 AM, GTSBoy said:

......Maybe it's gummed up with something, maybe it's broken in some weird way. .....

the answers were here, they just didn't seem likely. I'm surprised the shaft was such a good screw pump

[GTSBoy]

4 hours ago, Duncan said:

I'm surprised the shaft was such a good screw pump

It's not at all clear how fast the grease moves and how long it has taken to put enough muck into the speedo to be detected. It has been ~10 years since the conversion, and whatever the seal is at the drive end might never have been any good. We could just be seeing the result of a really slow process. Or, we could be seeing the result of a cable drive being a better screw pump than any could believe.

it's also not clear what the seal arrangement is. It could be an o-ring or it might just be a reverse screw/helix thingo. Will have to pull it out to find out. But I think I will have to combine that with an oil change, because I think that pulling the drive out will result in something else coming out too.