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MG MGB GT V8 Factory Originals Technical - 4 cylindre to 8 cylinder tachometer conversion

I have just soldered in a 470 ohm resistor across pins 4 and 7 of the tacho IC then checked the calibration. It is out by approx 50 - 60 RPM at 3000.
andrew robson

Ooops! I've posted the wrong value for the resistor. It should be 47 kOhm's and not 470 Ohm's. The true value was in fact 46.6 kOhm's
andrew robson

Would you change to a 46 or 48 ohm resistor to get closer? Or is that too gross a change?
Jim Stuart

Jim,

You would probably find that due to the tolerances of all the components on the circuit board of your tacho you will have to play around with resistor value.

As I mentioned in the second posting I used a 46.6 kilo
Ohm resistor; this was found by soldering a conventional style of variable resistor (too large to leave wired in place). I then "injected" a known frequency (12 volt square wave) into the tacho input terminal and adjusted the variable resistor until I got the correct RPM value on the tacho. Then I removed the variable resistor and measusred the actual resistance value across it. It was then a simple matter of soldering a resistor across pins 4 and 7 of the tacho IC.

To answer your question, the resistance needs to be approx 47 kOhms, as mentioned in my case I needed a 46.6 kOhm res this is 400 ohms less than the value quoted on the res itself. You have posted values of only 46 - 48 ohms.

regards
Andrew.

P.S If you need a copy of the pulse generator that I used to calibrate the tacho let me know. It has one IC, a couple of resistors and capacitors and is reasonable simple to make. You would need to find a frequency meter though.
andrew robson

Andrew,

If you used one of the small style variable resistors would it be acceptable to leave it in place?

Is there any advantage in using the fixed value resistor?

Larry
Larry Hoy

That fixed resistors are generally much more stable than variable resistors is the main advantage.
George B.

Andrew,

Thanks, you have answered my question.

At the MGV8 Convention I asked if anyone had measured the trim pot after recalibrating the tach for 8 cyl. Apparently no one had taken the time to desolder one leg and measure the value. Now the mystery is solved.

Carl
Carl

Larry,

George is right about resistor stability. I have a VDO aftermarket tacho in my Range Rover which uses a small variable resistor and it seems to drift every couple of years. This may seem fussy but I converted it to fuel injection and the fuel and spark calibration points need to reasonable accurate with regard to the RPM. A difference of just 20 RPM can have me adjusting the wrong table.

Andrew
andrew robson

This thread was discussed between 01/01/2001 and 14/01/2001

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