I managed to find some time today to get some calibration data for the speedo and rev counter for the my TF. I plan on replacing the dials sometime and don't want to try and guess the position of the pointers when I have to remove them to fit the dials. Some people have been know to use their GPS data on their phone while driving their car to try and set the correct position for the pointers, I wanted to try and avoid this and get them bang on. This data may come in useful for anyone wanting to attempt this sometime.
First I had to find a signal generator capable of producing a pulsed square wave signal of sufficient amplitude to drive the speedo and tacho. Fortunately Ebay came up trumps again and this little baby cost all of £2.78 from China - it looked like it met the spec for the job!
Capable of 1 to 200khz and putting out a signal of 12 volts, it looked perfect.
So it was time to connect up the signal generator and get the oscilloscope linked into it so we could see what frequency gives what RPM.
The oscilloscope is a device to accurately measure waveforms, this Picoscope connects to a laptop PC and gives good results!
Now its time to adjust the signal generator to give a nice round RPM value, I chose 5000 rpm for easy maths. This is easily set by adjusting a potentiometer on the signal generator PCB.
Then with the oscilloscope channel 1 (in blue) connected to the signal generator I got the trace of the wave on the screen. As a check I used the Oscilloscope's on-board signal generator to match the frequency of the signal generator that is feeding the tacho, then I read the frequency off from the on-board generator to get an accurate value of the frequency, shown by the red trace underneath the blue one.
the frequency of the pulses to give 5000rpm is 167Hz or 167 pulses per second. This converts to a nice calculation of 10000 pulses per minute for 5000 rpm. So the TF tacho picks up 2 pulses per rev from the engine!
Next it was a double check at a different RPM value, this time I chose 3000 RPM.
And the Oscilloscope trace gave a nice linear value of 100Hz or 6000 pulses per minute!
So now I have accurate frequencies to calibrate the tacho, next its time for the speedo...
First I had to work out where to connect the signal from the signal generator, a check with the electrical schematics showed the input is on pin two on the green socket on the rear of the instrument cluster. The yellow wire is the signal coming from the signal generator. The red and blue wires are the 12 volts needed to power up the instrument unit.
Now the signal generator is turned on and a useful speed set on the speedo, in this case 100MPH.
Now the waveforms are matched with the Oscilloscope's on board generator and a frequency of 111 Hz is determined for 100MPH or 6660 pulses per minute.
Then its time to check the linearity by picking another speed, this time 50MPH
And the frequency noted as 56Hz or half that of 100MPH - as one would expect.
Then as I couldn't resist it, I took the speed up to max - not many of you will see a TF with this speed on the clock!
Note that the Odometer is gradually ticking away as I have done the speedo tests - This one is actually only 5000 miles more than on my car, so if I use this speedo, I may change it over when the miles match.
So there you have the frequencies for the TF instruments. I can now remove the pointers from the clocks and put them back on and check the calibration is bang on when it comes time to fit the new dials...I am hoping to fit some rear illuminated ones - if they are still available from a source I found last year!
Sundance