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

I am installing a Rover V8 into a1978 MGB and I need to know how to modify the existing tachometer to read the V8 RPM correctly.
Thanks
Manny
Manny Beltran

Send it to a company who recalibrate them!
Chris at www.octarine-services.fsnet.co.uk

Visit Leon Zak's website:

http://zaks.com/mgb/

<<<The only part you need is a 100K 10 turn pot. I had one in my junk box, but they're available from Radio Shack, I wouldn't think that they are over a couple bucks.

The pot gets soldered on pin 4 and pin 7 of the only chip in the tach. There's a mark (usually an indentation in the chip case) on the corner of the chip that is pin 1. If you hold that so it's in the upper left corner, the pins are numbered 1-8 counter clockwise.>>>

Carl

I did the same 100K 10 turn pot modification. The only thing I did different was to ass some wire to the pot so the pot could hang outside the tach. That way I was able to adjust the tach with it in the car. Connected a hand held tach to the coil with the engine idling adjust the tach in the car to what the hand held tach is showing.

I would like to know the resistance value so we could just solder in a resistor and eliminate the need for the adjustment. It's on the list of things to do.
Jim Miller

I have recently modified my tacho circuit and used a 47 kOhm resistor. In fact the actual value was 46.6 kOhm due to its tolerance. After checking the calibration my tacho showed approx 50 - 60 RPM higher at 3000.
andrew robson

Andrew,
Is the resistor the only mod you did, or did you modify the circuit as mentioned by others above?
Stainzy

Stainzy,

I originally build the circuit featured in Leon Zak's site. When I wired it in I ran a pulse generator through my distributor, (the engine is not quite ready to start), this in turn ran my electronic ignition system and fuel injection, however, I feel the high back EMF produced on the coil negative damaged the new tacho conversion circuit. Perhaps it was intended for points ignition with half the back EMF. In hindsight I should have clipped the high voltage with a couple of zenor diodes.

To answer your question; all I did was wire the resistor across the Tacho IC then checked the calibration with a home made pulse generator and Fluke 78 Multimeter from 500 to 4000 RPM.

One thing I must mention is the tacho input is now wired to the "Crankshaft Reference Signal" of the engine management system which has a 12 volt square wave output instead of approx 300-350 volts off the coil negative.

Regards,
Andrew
andrew robson

This thread was discussed between 23/12/2000 and 02/01/2001

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