| Re: Non-linear speedometer error [message #370653 is a reply to message #370652] |
Sat, 08 October 2022 20:41   |
 |
Matt Colie
Messages: 8547
Registered: March 2007
Location: S.E. Michigan
Karma:
|
Senior Member |
|
|
Bruce,
I did a short stint with Chrysler Introl way back when. Up until that time I thought I could sort of count on normal speedometers to be close and sort of linear. That ended the day I got assigned to work the calibration stand.....
They are not linear devices, the are not accurate and they are not even very repeatable. Isn't that about what you just wrote??
The guts of an eddy current speedometer (or any similar device) is a magnet that gets spun by the mechanical input. That magnet is spinning inside a cup. That cup has a pivot, a spring and needle. The plan being that the field induced by the spinning magnet produces an eddy current in the cup and that drags the cup along to deflect the pointer.
To try to linearize the system, that cup is made of pedals almost like a flower. These can be bent into or out of the field to calibrate the system. As you can also appreciate, as the spinning element bearings wear, all bets are off. Oh, I forgot that the alnico magnets tend to loose strength with vibration and the cup (I have forgotten what that is actually called in the business) can develop residual magnetism of its very own...
My advice??
Get a cheap or used GPS to station on top of the instrument panel and use that. You should be able to set it for KPH when you are at home too.
Almost an post text: I just remembered that an associate put a different transmission in his truck and lost the speedo cable. He bought a GPS speedo that was cheaper than the cable to correct the issue. Look on Amazon there are a few.
Matt
Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Near DTW - Twixt A2 and Detroit
|
|
|
|
GMCforum
Search
Help
Register
Login
Home
