Common sense tells me that the sending unit for mechanical drive speedometers work on the principle that somewhere in the transmission a gear spins, and that gear is connected to another gear that either steps up (increases RPM)or steps down (decreases RPM) the RPM of the speedometer cable. This cable mechanically drives the meter.

What is the best way to convert a mechanical system as mentioned above to an electric system. There is a good article on Electronic Speedometers in this months (NOV 2002) issue of Classic Trucks . They go into detail about the different sending units for electronic speedometers. They listed the following - Signal generators or Hall Effect Generators/Switches, Vehicle Speed Sensor VSS, Magnetic Proximity Sensor, Signal Converters.

Has anybody done a mechanical to electrical conversion for the speedometer. If so what type of sending unit did you use and how did it connect to the tranny. What problems came up and how were they resolved.

Responses appreciated
I have a '69 F250 with a C6

Rob

 

I know that newer cars with AOD have electronic speedo. I also know theat the speedo gear on my 72 fits the AOD output shaft speedo gear. What I am thinking is you could get on of the sending units ad then youy would only need a copmputer to covert the signel. This would also meen a ghange of speedometer and odometer.
Robert
72 F-100 Ranger XLT W/ 86 5.0l W/ AOD
krazdndenver(No Email Addresses In Posts!)

 

I don't know anything about this as fact but I would have to think that a magnetic pickup on the driveshaft would be the easiest to install. My Nissan 4x4 uses the same principal for ABS wheel speed sensor. If you are a wiz with electronics you could use a speed sensor to provide pulse then count the pulses and convert it to a voltage for the speedometer. I haven't looked at how any of it would work I am just theorizing.

 

Rob,
I only have three minor comments. One might be helpful.
First, why do you want to do this? Is it to get cruise
control? Were you thinking of using a new speedo, or
using your old one? I'm merely curious. It seems that
some new Fords still use plastic gears to drive the
speedo cable. The Tremec TKO in my Torino is the same
trans used in the Mustang Cobra R's, and my TKO uses
two plastic gears and a cable (it's actually the exact
same plastic gears as in the old 1965 Toploader 4 speed,
and the T19 I just pulled out of my 1972 F100). You might
see how the Cobra R's speedo is driven (i.e., is it a cable
from the trans, or do the gears drive a generator, which sends
an electrical pulse to an electronic speedo? I honestly
don't know...just thinking out loud).

My second comment might be helpful. As I understand it
the speedo (but not the odometer) in our trucks uses a
spinning magnet (seperated from the speedo needle by an
air-gap) to "pull" the speedo gear "up." As the magnet
spins faster, its magnetic pull gets stronger, and hence
the speedo needle rises. Simply, the speedo needle is not
gear driven. My understanding of how this works comes
from the old, FTE truck list. You could search the archive
for the original discussion...I think I remembered it
correctly.

Finally, you might consult with a local speedometer shop. When I
lived in Cincinnati there was a shop downtown called
"Cincinnati speedometer." I once talked to them about fixing
a speedo problem. Perhaps there is a similar place near you
with knowledgable people.

Hope this helps. Let us know how this goes and what you
find out.

Ohio Bill