MadaboutKitcars

I have almost completed my dashboard (pictures in the next few days) and will be shortly trying to callibrate the speedo.

Can anyone tell me how many pulses per turn of the propshaft I will get? I seem to remember someone mentioning 9?

Also can someone remind me of the diff ratio fused for a 2.5 E30 (no LSD) I am sure I have this written down somewhere but I am equally sure someone must know of the top of their head....

Cheers

Robin

Diff ratio depends on the model of the donor, ours was a 3.91 as it came out of a Touring. I think some of the others are 3.64 but I don't remember off the top of my head

The diff ratio should not be important as the speed sensor is on the output shaft, IIRC is 9 pulses per rotation. That plus the rolling radius of the tyre into the equation supplied with the speedo is what I used to calibrate.

Robin

This website explains the electronic calibration method:
http://www.caigauge.com/page27.html

Basically on the E30 you use Wheel circumference and the following formula.
Number of magnets x half shaft or wheel revolutions per mile or per kilometre = calibration number.

if you use this site it will give you tyre circumference for your vehicle.
http://www.miata.net/garage/tirecalc.html

I have 225/50 x 17 tyres on the back so circ = 81.2 inches. There are 63360 inches in a mile so 63360/81.2 = 780.3 revolutions per mile.

Now on the standard BMW sensor there is 9 magnets but check yours with a multimeter.

So back to the formula:
Number of magnets x wheel revolutions per mile = calibration number
9 x 780.3 = 7023

This is your calibration number 7023

Now you need to convert it using this table:
http://www.caigauge.com/page20.html

Not all numbers listed so go for the closest you can find
7031 = 0100100010

0100100010 this is your dip switch settings.


Hope this helps i did mine this way and checked it against TomTom GPS speed on the way to the SVA centre and i was on average reading 1-2% faster on the speedo this was comfirmed during the test.


Stuart

Thanks Stuart

I should have mentioned that I am not using the standard Marlin instruments - but anyway you confirmed my 9 magnets theory.

As you don't take the diff ratio into account I assume then that the measurement is made on the driveshaft side of the diff not the propshaft? For some reason I had always assumed it was the propshaft side. It makes the calculation easier anyway!

Kind regards

Robin

Indeed it is

So using Stuarts recommended web site (which handily converts tyre size into turns per mile) and my tyre size of 205 55 16 I get 811 wheel revolutions per mile which equals 7299 pulses.

With the ETB gauges you just enter the number of pulses per mile using the button on the front of the Speedo. Sounds too easy....

Robin

Just be careful which way you need to put the dip switches as I seen to remember when I did my landy speedo the dip switches had to be switched in reverse order to the way I thought they should go i.e. right to left not left to right... so make sure you read the instructions properly!