Air Core Driven Speedo Needle Removal

[pokesmot]

I did a little research on the details behind how the speedometer in my gauge cluster works.

Its driven by an "Air Core Mechanism" or at least thats what the GM service manuals reffer to it as.

"The circuit employs a pulse generator producing timing pulses for a magnetically driven air core gauge. The timing pulses are produced in response to pulses indicative of the remote condition being sensed as, for example, wheel rotation or engine speed. "

So, if its pulse driven; one can probably be able to remove the speedometer needle, and then simply zero it out by reinstalling the needle with the engine running in park?

that won't make it any less accurate I think/hope?

I'm asking because I want my odemeter to reflect my cars actual mileage: not the mileage taken off the car where I got my donor GT instrument cluster!!

I have already devised a way to roll the mileage back without breaking the plastic bands which create silver lines between the numbers if tampered with... I used my base cluster as a test subject :)

so essentially my mileage will be legit again, and there wont be indication that it was tampered with.

Just unsure if its gonna mess with my speedo :(

[pokesmot]

I went ahead and rolled my mileage back to what it should be. had to remove the speedo needle in the process.


Going to take it to the shop I work at and hook it up to a scan tool, lift it in the air a bit, put it in drive.

I'll let it idle in drive; whatever the scan tool says the speed should be is what I'll set the needle at.

then I'll roadtest it, and make sure the speedo matchs what the scan tool tells me.


sucks my own personal scan tool for my car is in MPH not KMH so it makes it tricky to get it perfect :mad:

[rixGAphx]

Originally posted by pokesmot
Sucks my own personal scan tool for my car is in MPH not KMH so it makes it tricky to get it perfect :mad: Isn't your speedo dial 'dual-scaled'?

US cars have mph in large numbers around the outside, with marks at 5 mph increments. The kph scale is around the inside, with smaller numbers and no incremental marks.
So either scan tool, SAE or metric, would work.
I would imagine the Canadian cars are exactly reversed.

Or, is it the incremental marks that are causing your consternation?
Even without SAE increments on the dial (to match your scanner), you can easily get as close as necessary with the stock dial.
It's not like you're using this set-up for super-accurate time-speed-distance rallying.

The stock speedo isn't super-accurate, anyhow, since it's only intended as a close approximation of vehicle speed.
Afterall, regardless of selected tire size, the difference in 'rolling circumference' between a new tire and a worn tire gives a variation at the speedo of about 1.5 mph (2kph).
Switch tire sizes, and you can have a 10% variation at the speedo even with a stock drivetrain and cluster.

Oh well, just my 2 cents (US :D ).
I know this car is a labor of love to you, and I applaude your tenacity.

Peace,
-Rick

[pokesmot]

Originally posted by rixGAphx
Isn't your speedo dial 'dual-scaled'?

US cars have mph in large numbers around the outside, with marks at 5 mph increments. The kph scale is around the inside, with smaller numbers and no incremental marks.
So either scan tool, SAE or metric, would work.
I would imagine the Canadian cars are exactly reversed.

I know this car is a labor of love to you, and I applaude your tenacity.

Peace,
-Rick

lol,

thanks rick!

well, normally the cluster would have the MPH marks on the inside as I have the canadian version with KMH on the outside... but I have the glow gauge overlays on them as well, so the gauge only shows kmh, Id have to try and calculate in my head how many MPH = KMH (something like 1.6033 km in a mile?) :(

surprisingly enough my speedo seems to almost be perfect, or maybe I'm stupid and cant calculate in my head very well. However, when I pulled the needle it was at 0, and if you push the needle slightly past 0 (counterclockwise) there is a slight resistance. I made note of this before removing the needle and installed the needle as best as I could with the car running in park.

I think i got lucky and hit the speedo needle bang on when i reinstalled it.


Regardless, if any other user on these forums has the need to remove their needles for whatever reason, it is in fact possible to recalibrate them with a scan tool!

just so long as you dont physically modify the needles by trimming them shorter, because I noticed there was a small counterweight embedded in the base of the needle. I'm sure the counterweight is important to the operation of the needle.

[jcasemore]

I stumbled upon this guys converting program and I really like it. It's free and easy to use.

Also, you can just take the .exe file and use it anywhere. Off disk, flashdrive, whatever.

http://joshmadison.net/software/convert/