I don't make a habit of moving posts around, but here's one which may solve certain questions some may have regarding speedo calibration. Sorry if it gets a bit into electronics but there's no other way to avoid it. I posted this today on BB:
Well,well, here we go with the serious nitty-gritty stuff! It sure is possible to calibrate a speedo and make it perfect, provided it is the one corresponding to the gearbox in the car! This is a prerequisite if you also want the odometer to count distance accurately. To do the job correctly you will need a signal generator with a square-wave output and up to 1kHz max. This will simulate (on-bench) speeds exceeding 260kph for verification purposes. Remove the speedo from the instrument cluster. Notice the pcb (printed circuit board) and the little motor driving the needle. Supply 12v and some signal to the appropriate termials. Notice how the needle moves - and what makes it move. Observe the little flat shiny aluminum disc with lips (looks like a shallow pan) containing a round magnet disk. They don't touch each other. The magnet is fixed on the motor shaft and the pan to the needle. There's your adjustment point, the proximity of these two parts. Bring the parts closer to each other and you read higher speed. Increase the gap and speed decreases. There's only one way to do this. Since all speedos are optimistic you must increase the above gap to lower the indication to the proper value. Locate two screws holding the yellow plastic top frame and loosen enough so you can slide various thicknesses of paper (used as a spacer) to increase the distance after the screws are tightened again. Note we're talking about 0.05-0.5mm here or thereabouts (up to 0.020"), that usually does the trick.
Now you must work out the math to find which exact frequency you need for a given speed. To do this you must measure the exact rolling circumference of your tires by chalk marking on the tire/road, rolling once and measuring the distance. Work backwards from the tires to the gearbox pinion, remembering it has 6 teeth on it to signal the inductive pickup. The result will give you the frequency for any given speed you input.
I hope I haven't lost you through this, I've done this quite a few times and it works perfect every time. I also calibrate my tachometers but this is more involved as there always is non-linearity throughout the range.
Good luck if you are determined enough to do the above work!
Jim K.
JimGreek,
calibration of the speedometer unit as you described is certainly one way to calibrate the speedometer.
Certainly the technique you have described is quite involved. My only other suggestion/ alternative is to simply purchase this wonderful little unit from Dakota digital; the SGI-5. Complex calibration is now a thing of the past.
Cheers,
Marco
calibration of the speedometer unit as you described is certainly one way to calibrate the speedometer.
Certainly the technique you have described is quite involved. My only other suggestion/ alternative is to simply purchase this wonderful little unit from Dakota digital; the SGI-5. Complex calibration is now a thing of the past.
Cheers,
Marco
Marco
1985 Alfa 90
1985 Alfa 90
I bought one of the Dakota Digital speedo adjusters. Very easy to hook up, simple to adjust, worked fine, and allowed a good range! Then I found that the problem was an intermittent signal loss from tranny sensor. The Dak Dig interface has a little led that blinks when there is good signal and does not blink when there is signal loss. Nice touch.