Okay lets go back to reading some info, look at
Sci-Tech Encyclopedia:Speedometer which is at
URL

http://www.answers.com/topic/speedometer

This article describes three types of Speedo's
mechanical analog, Quartz electric analog, and
digital microprocessor. There is another speedo
type call Eddy Current Speedo. Check out this
as well. Its well documented and is used today
by the Analog Gage Calibration industry.

The Quartz uses a permanent magnet generator,
a permannt magnet mounted in tranny/drive shaft.
This produces a sinewave type of signal. This
signal is per the article proportional to the
vehicle speed .... with 4004 Pulses per mile
or 2503 per Kilometers.

On the digital microprocessor speedo the article
points out that the same magnetic sensor iis used,
but the Datsun 280ZX uses a Slotted LED Sensor with
a Wheel that has 26 slots. You may recall that
is the same signal input I use for my external Speedo.
Also the back logic board in the Z's digital Dash
is a NeC uPD7507c Microprocessor, and 3-chips a
HD-14566BP, a HD-14520BP, and a HD-14584BP.
Baicly thes three chips are the dgital Speedo logic.
The Timing to the Speedo comes from the Micro's
Program and the Speed Sendor is the Slotted LED
assembly that is driven by the Speedo Cable.

So as I interpret the system it will display the Z's
speed till it reaches The 85MPH Limit which is
loaded into the Program Memory. I don't know the
location, but if you have the ability to step the
Micro through its Progam Memory and you decode the
intructions, you will see the loop.

My suggestion is Don't Scramble the Program...
Just add a 1 to the 85 Limit and the New Limit
will be 185-MPH. The digital dash should No Longer
be a issue to all of the Z owners who want the
Correct Speed Indicated from the Digital Dash.

Hope the above is is helpfull to those you who have
the digital dash....

Terrymac

 

Hmm I might take apart my digi dash and take a look. Tho arnt the chips soldered to the board?

 

I believe that lww said that we can just switch it to KPH and send it in to recalibrate it so that it reads in MPH. It will still say KPH but it would be in MPH. I also heard someone say that you will have to mess with the speed sensor in the tranny in order for that to work. IDK, just thought I would bring that up.

 

Theres a speed sensor in the trans? I thought it used the cable for the mileage thing then picked up the speed from that?

 

Maybe it was the cog inside the tranny, that was connected to speedo cable, he was talking about you would have to change. It wasn't making too much sense to me, I didn't see why you would have to change that (the cog) if you got the speedo recalbirated. That is point of sending it to the recalbiration shop, right?

 

I think theres gears in the mechanical part of the speedo as well. But it might be simpler to play with the gear in the tranny

 

they make de soldering kits but its possible since its a part that could potentally go bad that they legged chips are acually in a socket and the socket is soldered to the board and not the chip it self meaning it would be easy to take out.

 

Ah wouldnt have known that as I've never taking that digidash apart lol

 

I just bought 79 cluster from bleach to swap with my digi cluster, problem solved.

 

Can I have your digidash?

 

Yeah, I have two 79 speedos but, one of the just has the speedo and tach and nothing else. I gotta figure out what I need to pull off a spare 80-83 cluster before I can swap them out. It might be awhile and I will give it to you. I think I might be able to get 15 bucks for it somewhere but you can just have it. Warren gave me a lot of misc Z stuff and I will just pay it forward.