Speedo WTF?! Theories please

[GTSBoy]

Yuh, so while I'm usually the guy explaining how the speedo and related shit works, I recently experienced something which blew my mind and I can't think of how it happened. So, I'm casting it out to the braintrust to see if there's any reasonable explanation.

Here's the story. I drove the car (R32 GTSt, R33 turbo gearbox) away from home, and the first time I noticed the speedo was once I was out onto the 60km road about 1km and 5 turns from home. I'm doing maybe 60 km/h. The speedo is reading ~120 km/h. I'm all "WTF is this?".

I roll down the road and out onto the 80km section, where the speedo is gleefully telling me that I'm doing at least 150, even though I'm pretty sure I haven't even caught up to the mix of sheeple and mad dream boats doing anything from 70 to 100. I accelerate to see what happens and the speedo happily winds off the dial at 180.

I stop at some lights. Take off, and......the speedo is back to normal.

The brain is doing about 500 km/h trying to work out how this could come about. The R32 speedo is a cable drive. The connection is purely mechanical, right up until you get to the spinny magnet and drum bizzo in the speedo head. It's not like there could be an electronic fault that was double sensing rising and falling edges of the waveform or anything like that. The cup in the speedo head had to literally be getting dragged around against its spring with enough force to make it read about double what was really happening.

So, has anyone got any brainwaves as to what the bloody hell could cause that to happen in the real, actual, physical world?

 

The speedo drive was built from a Navara drive about 10 years ago when the car's big birthday transplants happened. The drive cable is only a few years old, having snapped after ~25 years of service. The back of the speedo housing is hale and hearty, no bits broken off IIRC.

[joshuaho96]

5 hours ago, GTSBoy said:

Yuh, so while I'm usually the guy explaining how the speedo and related shit works, I recently experienced something which blew my mind and I can't think of how it happened. So, I'm casting it out to the braintrust to see if there's any reasonable explanation.

Here's the story. I drove the car (R32 GTSt, R33 turbo gearbox) away from home, and the first time I noticed the speedo was once I was out onto the 60km road about 1km and 5 turns from home. I'm doing maybe 60 km/h. The speedo is reading ~120 km/h. I'm all "WTF is this?".

I roll down the road and out onto the 80km section, where the speedo is gleefully telling me that I'm doing at least 150, even though I'm pretty sure I haven't even caught up to the mix of sheeple and mad dream boats doing anything from 70 to 100. I accelerate to see what happens and the speedo happily winds off the dial at 180.

I stop at some lights. Take off, and......the speedo is back to normal.

The brain is doing about 500 km/h trying to work out how this could come about. The R32 speedo is a cable drive. The connection is purely mechanical, right up until you get to the spinny magnet and drum bizzo in the speedo head. It's not like there could be an electronic fault that was double sensing rising and falling edges of the waveform or anything like that. The cup in the speedo head had to literally be getting dragged around against its spring with enough force to make it read about double what was really happening.

So, has anyone got any brainwaves as to what the bloody hell could cause that to happen in the real, actual, physical world?

 

The speedo drive was built from a Navara drive about 10 years ago when the car's big birthday transplants happened. The drive cable is only a few years old, having snapped after ~25 years of service. The back of the speedo housing is hale and hearty, no bits broken off IIRC.

Don't the R33 tachometers have a tendency to go completely crazy with age? Capacitors can lose their dielectric and basically turn themselves into a short circuit, dry joints intermittently making contact on inductive paths can pump up voltages beyond what's normal, etc. It's hard to say exactly what is causing it without really getting into the guts of it and reproducing/testing it but it's nothing beyond belief IMO.

[GTSBoy]

Apples and donkeys. The R32 speedo is mechanical. How does a spinning magnet in a cup turn the cup twice as hard as it usually does?

[joshuaho96]

9 minutes ago, GTSBoy said:

Apples and donkeys. The R32 speedo is mechanical. How does a spinning magnet in a cup turn the cup twice as hard as it usually does?

I can still think of a number of weird things that can cause what you saw. It's basically just an inductor at the end of the day. RF interference might do it if nothing is wrong with the circuit. If something is wrong with the circuit flipping an inductor off and on with a smoothing capacitor connected to a load is basically a classic boost converter. The flipping of the inductor off and on to ground is usually supposed to be controlled by a transistor but in this case it could be literally anything. Tin whiskers, that kind of thing. Only way to verify is to get it on a bench and see if you can reproduce with known input signals.

[GTSBoy]

It's not a circuit! It is a spinning magnet dragging around a metal cup. It is a physical device. There are no electronics involved.

[Duncan]

Generally with a weird problem like this I'd just wait for GTSBoy to post up

[Murray_Calavera]

Good excuse for a IC7 dash or similar?

[BK]

I get where you're going. My understanding is the cable spinning has a direct influence on the needle via magnetism right ? But it has to overcome the needle zeroing retaining spring tension to move at all right ? Only way I can explain is something, somehow that spring tension changed / decreased when it was bugging out. If that was the case at a real almost 0km/h car not moving but cable still spinning scenario, the gauge would have been not at 0 and deflecting around.

Apart from that, fked if I know.

[robbo_rb180]

Have had a similar issue in an old HQ. Turned out the drive cable spring had failed, never cut it open to see exactly but was doing random speeds most of the time and close to normal once in a while. 

[joshuaho96]

5 minutes ago, robbo_rb180 said:

Have had a similar issue in an old HQ. Turned out the drive cable spring had failed, never cut it open to see exactly but was doing random speeds most of the time and close to normal once in a while. 

Could also be this.

29 minutes ago, GTSBoy said:

It's not a circuit! It is a spinning magnet dragging around a metal cup. It is a physical device. There are no electronics involved.

It's still a circuit, just a really simple one. The metal cup is technically an inductor.

[GTSBoy]

1 hour ago, joshuaho96 said:

It's still a circuit, just a really simple one. The metal cup is technically an inductor.

I'd have to be driving underneath one of the UFOs from Close Encounters of the Third Kind for that sort of inductive interference!

[Ben C34]

On 12/12/2022 at 9:11 PM, GTSBoy said:

I'd have to be driving underneath one of the UFOs from Close Encounters of the Third Kind for that sort of inductive interference!

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

[BK]

14 hours ago, joshuaho96 said:

 

It's still a circuit, just a really simple one. The metal cup is technically an inductor.

I think he would argue its not a circuit - I would. A circuit would be current flowing through a conductor, an electrical connection between points. That's not what's happening here - there is no coil or wire with current flowing through it to move the needle, it's the eddy currents created purely by the magnet acting on the electrically conductive cup. Eddy currents do not flow in a circuit from start to end, they swirl around in a concentrated area in conductive material creating their own magnetic field.

That's why I mentioned the wound spring on the needle, which is a force to oppose this electromagnetism that moves the needle. If the force that moves the needle is correct, to me the.only way the needle to move further than intended is for the tension of the spring on the needle to decrease or slip allowing more movement.

That's the how a mechanical speedo needle could move more than intended, whether that's what's happening to this gauge, dunno.

[GTSBoy]

^^ Yuh, precisely. The behaviour exhibited by mine was so weird that I struggle to come up with better ideas than the possible UFO.

[admS15]

Voodoo 

[Dose Pipe Sutututu]

just broscience brain here, somehow the speedo mechanism itself has been come overly magnetised from some huge inductor near by?

maybe it's 5G interference LOLOLOLOLOL... (this is a joke for anyone that thinks I am being serious).

[Neostead2000]

Yup, look for a mechanical fault around the hairspring assembly, it may have failed from age and causing the needle to read higher sometimes, or if there's debris or stuck grease around it.

Also UFOs do like to sight see around down undah so that's also a highly probable scenario. Don't know which part you're from but especially the coastlines around Western Australia are notorious for this. 👽

[BK]

1 hour ago, admS15 said:

Voodoo 

R32s definitely are cursed - broken speedo cables, a/c actuators that always stuff up, a/c vents made from plastic for maccas kids toys, interior door handles that turn to talcum powder, guaranteed dash clock deaths and the dreaded dash pad bubble.

[Dose Pipe Sutututu]

2 hours ago, BK said:

R32s definitely are cursed - broken speedo cables, a/c actuators that always stuff up, a/c vents made from plastic for maccas kids toys, interior door handles that turn to talcum powder, guaranteed dash clock deaths and the dreaded dash pad bubble.

Sounds like you've described R33s too lol

[BK]

43 minutes ago, Dose Pipe Sutututu said:

Sounds like you've described R33s too lol

No way ! Only thing I've ever had to do on that list in nearly 20 years on my 33 is the a/c vent mode selector actuator. None of those other things are an issue on a 33R generally but on the 32s it's almost guaranteed.