Electronic vs mechanical speedometer

I need some help here.

In my 1980 400i auto (series 1) the speedo is mechanical with a drive wire going to the gearbox (I guess). My car came from the US and while the speedo shows both mph & kmh it is focused on mph and I drive in the metric part of Europe .

A few years ago I bought a brand new (NOS) kmh speedo that got stuck on a shelf for many years. Now I'm converting all gauges to LED so I decided to swap the speedo.

Somewhat puzzled I found that the new one was electronic. Missed that back when...

Is there a way to convert wire drive to pulse drive to use the new one? Or is it to sell it and find a mechanical unit. The latter would be a shame as it is brand new.

All input much appreciated.

//Rob

 

Hi,
Thank you for the input.

The speedo is most likely changed and I can´t see that there is a sticker on it. That would have been nice.

I have done some digging and it seems pretty straight forward to swap over. Dakota Digital makes a pulse generator that fits in the speedo wire mount. I hope it is small enough to fit on the side of the gearbox in our cars And they make a universal speedo correction unit.
https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=109/category_id=358/mode=prod/prd109.htm
https://www.dakotadigital.com/index.cfm/page/ptype=product/product_id=1192/category_id=712/mode=prod/prd1192.htm

The connection is easy, just plus and minus and a wire from the pulse generator on the gearbox to the electronic speedo.

The original Ferrari puls generator seem to be very expensive and also not very reliable, hence the Dakota Digital option. It is just fed 12V and is magnetic. Probably a Hall sensor but Ím not sure.

Do you know if the original Ferrari pulse generator is raising voltage, AC or DC, or if it is generating constant 12V pulses with raising rate corresponding to speed.

I would like to test my NOS speedo on the bench before buying needed parts to covert from wire to electronic but need to know what to feed on the sensor input.

Thank you
//Rob

 

Some updates for others attempting the same thing, ie moving from wire drive speedo to electronic.

The electronic speedo I have (kmh dial) work of 12v pulses from a sensor on the gearbox. This is the way they are setup on late 400i and 412's, as well as other F cars.

The speedo is fed plus and minus 12V and the pulse signal. The pulse signal normally comes from a Hall sensor on the gearbox and that sends 12v pulses dependent on the rotation of the output shaft from the gearbox, ie fixed ratio to wheels. I don't know if these are square or sinus pulses but my speedo only reacted to sinus pulses. That may also have been due to low voltage on the pulses in my test setup (being overly careful).

I believe that the original Ferrari pulse generator runs direct off the drive shaft whereas the wire driven speedos is driven via an angle drive and some gears. Going from wire to electronic "the Ferrari way" would likely mean taking the tail of the gearbox to remove the gear drive to insert the Ferrari pulse generator as far as I can understand.

I set up my electronic speedo on the bench to test that it worked and what pulses it needed. The number of pulses are constant per mile/km at a given rear axel ratio and given a certain tire dimension. My electronic speedo (in kmh) shows 100kmh @ 160 pulses per second (160Hz from a tone generator) on the bench. Hence, the sensor should produce 5,6k pulses per km and 9,1k pulses per mile (+/- a few percentage du to read-out on dial) for the speedo to produce the right reading.

The odometer confirms running in km as 60 seconds of 60kmh produces 1km on the odometer on the bench.

Since the original Ferrari pulse generator part seem to fail on occasion and is very expensive, I will not go that route. Plus the extra work with the gearbox wire drive.

I will use the Dakota Digital adapter that screws directly on to the wire drive. Pls see link in earlier post. The Dakota Digital pulse generator is set up to provide 8k pulses per mile so an adjustment is most likely needed. They don't list pulses per revolution so I can't calculate correction factor before buying one, as that will be dependent on the number of revolution for my specific car to produce 1 mile. According to my calculations using 3,25 final drive ratio and a circumference of 1925mm (Michelin 240/45/415) my car makes 0,6m per drive shaft revolution. Change in wheel size or final drive will change the ratio.

The unit mounts in the same place as the wire drive does, so my plan is to unscrew the wire and just insert the DD pulse generator, run the electric cable up to the dash, put a Dakota Digital adjuster box (pls see link in earlier post) between the pulse generator and the speedo, adjust and be done. I hope it is as easy as it sounds. I will probably leave the original wire disconnected and just cover the ends from dirt.

You can always run a speedo off a GPS pulse generator at roughly the same cost but that is not legal here. -And it would probably not give any reliable readings indoors or in tunnels.

It would be interesting to see if the US/miles speedos run the odometer in miles or km. I mean if the speedo scale is a sticker the odometer (bjwhite comment) would still show km (same number of pulses) which means that several cars out there have significant lower "real" mileage. The the speedo runs from pulses per second and the odometer from pulses per mile/km. So changing the scale with a sticker would only change the read-out of the speedo needle not the odometer.

Hope it helps someone.

Kind regards
//Rob