I have collected number of tach and speedometers, as there are some of the best looking gauges I have found.
The 6 cylinder Tachs I have been able to converted to read 8 cylinders.
The speedometer has been harder.
From these very pages I learned: " This is a pretty easy circuit to test if you have a voltmeter. Green wire to the transducer should have 12V with key on (check fuse #4, too.) Also, the green wire at back of speedo should have 12V with key on. Black wire from speedo should be grounded (ground point G14 behind speedo). If all of these check good then you can test the transducer for output voltage.
General concensus is that the transducer produces a square wave form AC output. So, with your voltmeter set to AC and the igniton key “on”, spinning the transducer cable (a drill motor would be nice for this) should produce some sort of a reading from the yellow wire. (Of course, checking the wave form would require an osciliscope so it’s not easy to check the quality of the output.)
OK From this I can figure it is a simple switch just switching the 12 Volts on and off. My 56 Studebaker had the same kind of sending unit for the tach in-between the distributor cap and the distributor.
And I am betting the speepo gear in the transmission is set to turn something like 1000 pulses per mile.
So I am figuring the transducer/switch is something like the same rate or double that…or??
That is what I am seeking the drive rates for both the cable driven speedometer and the output or pulse rate for the electronic speedometer. (I have a couple of both)
I have already mounted a Tach into the dash of my custom 93 Chevy van and want to add its matching Speedometer.
I know this is a little off the wall, and I hope no one minds my use of these great gauges this way.
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The transducer output is to match the pulse rate, speedometer, gearbox output axle turn rate, the differential ratio and the wheel diameter for correct reading, Rich…
I’m not sure that changing the transducer to an on/off switching is an easy option. However, the description in the manuals is that ‘the transducer turns 7 1/4 turn for each 6 turns of the road wheels’ (propeller shaft?) - for whatever it is worth…
There is some pitfalls in this; it refers to Jaguar original set up. Ie probably BW65/66, diff 3,058:1 and 205/70 15 tyres. However those are numbers to be taken with a bit of salt - and the transducer/speedo is a matched pair…
The number of teeth on the gearbox drive gear versus number on the transducer driven gear dictates the pulse rate - bearing in mind that the internals of the transducer contains 8 poles for pulse generation.
I don’t know the pulse shape, but pure sinus is simplest to produce - and the speedo is likely a pulse counter rather than reacting to ‘average’ voltage…?
Don’t know how you propose to drive the devise you plan - or the type of gearbox…?
What I am doing is replacing a 1993 Chevy 350 with a special Low RPM torque 383 fed by a 1985/91 Camaro/Corvette TPI intake system into a 93 Chevy Custom Van.
So I will have to deal with three VSS systems. First I will be leaving the 40 Pulse per one turn of the output shaft of the stock 4L60E transmission so the stock PCM can handle the transmission.
Then for the older TPI fuel controller PCM I will need a 2000 per mile pulse system for it.
So as I have already got a Jag Tach installed in my custom dash, I have long wanted the matching Speedometer to replace the 93 80MPH factory speedometer.
I am glad I looked into this problem at the same time.
To add to this problem is my plan to add an old Borg Warner Over Drive take off a 50/60s 3 speed transmission. This will cheaply allow me to reduce the engine’s RPMs to below 2000RPMs at highway speeds like 80MPH.
The engine is built for its peek torque at 2000RPMs the mileage sweep spot.
For all of that to work I will need to put the vans VSS system (or a good replacement) and the TPI VSS system to the output shaft of said OD.
Lucky there are aftermarket systems made for a lot of TPI engine swaps in a number of cars including Jags.
And thanks to Dakota Digital’s SGI-100BT: Universal Speedometer and Tachometer Interface I believe I can convert the 2000 pulse TPI System to the Jag’s 8000 Pulse and even fine tune or calibrate the speedometer to read correctly.
As I am retired I now have the time and have been saving up the parts for about 20 years…(I know I cannot believe it took so long) to build this super van.
Almost all other improvements have been done with this van, and I would still be driving it had not an oil line let loose and sized the current 350.
The special 383 was began in 2001, but buying this 93 put it on hold as I just could not swap out a perfectly good engine so everything stayed on hold until now.
I love this Van as a highway cruiser, but hate that it only gets 14MPG…so IF I can get her running at 20MPG on a level highway with no head wind I can then enjoy driving her again.
You will see how I tore out the dumb interior and rebuilt it. How I flipped the rear air and made it feed air though roof vents to every seat including the front seats.
There are a number of other changes now shown, as I made a hatch in the floor over the gas tank so I can replace fuel pumps without dropping a 32 Gal. tank.
I converted the Vacuum brake booster with a Hydrobooster.
Batteries (2) are now under the floor behind the driver’s door. One is used for night time use and the second is switched in to insure starting in the morning.
Heavy duty antisway bars.
Transmission and engine oil coolers behind grail’s old removed turn signal lights.
Special LED daylight running white lights with amber turn signal lights.
So I have done a lot to this van to make it the way I feel it should be.
Sounds like a cool project; the reason behind the low pulse rate was that earlier Jags used a reed switch type generator; then later a differential mounted sensor with very coarse teeth to match the pulse rate of the earlier system. It is always fun to improve a vehicle design that was limited by technology of the time (and fuel prices).
