From the first post:

Its really not that complicated, just a cup with a magnet spinning in it.

 

A speed sensor is, for the most part, a magnetic reluctor or hall effect type sensor. The exciter ring/reluctor can be driven in many ways, but ultimately read the same. There is usually a square cut, toothed wheel that spins, in very close clearance to a stationary magnet/sensor. The movement or the teeth passing the magnet, produces a square wave voltage pulse, that is read by a computer, which in turn sends out a signal to an electronically controlled speedometer. Obviously the faster the reluctor spins, the faster the frequency of the pulses. A crank position, cam position, electronic ignition in our post 79 rx7, the CAS in FC's all work, the only difference being for those type of sensors, something is done to create a reference point for the computer to determine its position, usually a single tooth missing in a set # of teeth, or multiple sets of reluctors and sensors being driven by a common source, but with different #s of teeth (but here we get into a rising and falling edge of the pulse, helping determine time)

 

Good description in general but it doesn't apply to 1st gens. No computer is involved
with the ignition or speedometers in our cars (except maybe the SE ignition).

What your describing is a digital electronic speed sensor. May use a lot of the same
tech as the analog one used in our speedos (spinning magnetic pickup) but then
converts that to a digital electrical signal.

 

Thanks for the detailed explanation.

I still need to determine whether the speed sensor (i.e. the component within the speedometer that's converting the rotation of the cable/magnet to the electrical signal used for cruise control, over-drive and power steering) can actually be "fixed" or replaced without having to swap the speedo...

I'm getting a replacement gauge cluster to help me with exploratory surgery.

 

From experience, you won't be able to fix the mechanism very easily unless its a
real obvious problem. I had an issue and tore mine down and ended up swapping
a 79 speedo into my SA to get more than 80 mph and a working speedo.

You need the special equipment mustanghammer mentions and expertise to really
fix it right. Make sure you get the donor speedo first before tearing it apart.

 

You could always get a good speedo and roll it back (or forward) for the mileage if that is your concern.

 

Yeah...this seems like an awful lot of pain to go through just to try to get my cruise control working.... :-(

 

...How hard is it to do that...?

I've got visions of trying to roll back the mileage and then never having the odo work right thereafter.

 

my bad, I thought he was inquiring about speed sensors in more modern use w/ electronic speedo's

 

Mileage is easy to change and it will still work fine. I did mine with no problems.

Also I'm pretty sure the cruise control doesn't work off the speedo. Its vacuum based I think.

 

...I wish it were that simple...

I have another thread on here describing my tale of woe with my cruise control, but in summary:

1) My CC isn't functioning;
2) My ACC checker tool indicates that: vacuum test passes, combo switch test passes, clutch & brake switch tests pass, and speed sensor signal test passes.
3) I've tried 4 different CC CPUs without success. It seems rather unlikely that *all* of them would be bad (but then my luck hasn't been great so far).
4) Although the speed sensor shows passing on the ACC checker, others that have had similar experiences indicated that swapping the speedo fixed their problem...

So....here I am looking for ways to avoid having to swap out the whole speedo in the hope that the "speed sensor" component alone is something that can be changed.

I'll do a test-swap with a different instrument cluster just to see if that fixes it. If not...I'm not sure what else to do.

 

Oh, now I see your issue. I have no clue how the 7s CC works, SAs don't have
em Sorry.

Its very easy to just swap the speedo out from one console to another one.
Going deeper and trying to swap just the mechanism from one speedo to
another is probably much harder (might be impossible due to they way they
work).

 

Thanks for all the input and feedback to my speedo questions, problem solved.

I resurrected this thread with questions re: speedometer functioning while investigating my non-functioning cruise control.

1) The CC problem was fixed by swapping out the cluster, as it turns out my original cluster was significantly corroded at the electrical connection points between the meters and the circuit board. I put the old speedometer into the new cluster-->problem solved.

2) The "speed sensor" *does* appear to be a removable/swappable component of the speedometer (although I didn't have to change mine out). It's a Hall Effect magnetic sensor located on a removable bracket mounted underneath the speedometer cable connector (a greenish glass cylinder ~1/2" long).

3) From it's location on the speedometer, the Hall Effect sensor is in very close proximity to the drum containing the spinning magnet used to drive the speedometer needle, so the sensor is able to pick up each rotation of the magnet and generate an electrical signal. This electrical signal is taken from the speedometer from two connections directly beneath the cable connector on the speedo unit itself.

Again, thanks for all the information, clarification and correction provided. This was bugging me since I bought the car, such a relief to have it fixed and understand why....

 

Hello guys,

so recently my RE Amemiya Cluster's RPM needle started to play when I was cruising, sometimes it goes to 0 while in motion, also the odometer screen when off blank, so maybe my speedometer cable went bad, I tried searching for a speedometer cable for RX7 FD but I couldn't find one, all I can find is up till '92, FC models. What about the FDs? do they use speedometer cables? and where can I find them?

Please help

 

Try the 3rd gen section maybe?

 

^ Yeah, 3rd Gen Section is your best bet. I have no idea what the FD's run off of for a speedometer input.