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MG MGB GT V8 Factory Originals Technical - Tachometer calibration/help

Ive got my tach apart and soldered in a potentiometer to calibrate it. I was wondering if anyone knows how I can hook the tach up to a function generator to calibrate it. It is a '74 so it has one green wire 12v and a white with black tracer from the neg side of the coil. This is where it reads the pulses that make it work. Can I simulate this with a FG? Any ideas?

Try this link
Jim Lema

Probably, but as the tach operate off 12v pulses it would probably be best to calibrate it at that too. If your FG doesn't output 12v you will have to interface it, a transistor and a couple of resistors might be enough.

But have you not got a digital multi-meter with tach function? Unless you are going to be regularly red-lining it I would have thought a side-by-side would be sufficient, and if you *are* regularly red-lining it you should probably have a rev-limiter anyway. If you have a vary-speed drill you could drive the distributor from that and get 'real-life' pulses to the tach and multi-meter without having to rev the engine.
Paul Hunt

The FG is good up to 15V DC. Ill try it out on Th and see what I can do.

That should work, but don't forget two things: The MG tach isnot vey sophisticated, and the coil generates spikes in the primary that your FG won't have. These could throw off your calibration.

Jim Blackwood

Jake try this circuit insted.
Look under the "My digital tach circuit".
I have it in two V8 conversions and they just work.
Just put it in and forget it. Its been running more then a year now, and no problem.

Manfrieds right,
Zac's method, or rather someone elses method that is superior to Zac's own that he reports for us, works very well. One of those "I can't believe how cheap and easy , there must be a catch,things" .
I calibrated mine by running the motor at various set RPM's and making and breaking the conection. ie 4000 rpm, connect wire and adjust to 2000, disconect wire to check etc... If I recollect it was about 2.4Kohm in the end, across Legs 3 and 7 of the chip if my memory is right.
Watch out that they sell you the right resistor. On the third attempt I made the shop check that it was a infact a 10 turn 100kOHM variable resistor, not the variety of others (100ohm, 1kohm etc)they tried to sell me.

Peter not that one, I used the electronic circuit with the divider. No adjusting, just connect and drive away.

Hi guys, I've read the previous thread with great interest.

Can anyone help with a similar problem, how to recalibrate an MGB tacho for a V8 conversion only using a 1970 tacho. This has only two resistors, one cap, one tranny and one diode, oh and a coil connected to the coil LT and ignition switch. The tacho also has a green wire connected from the fuse box.

I'm using a Range Rover distributor if it helps.

A.S. Kenning

Hi A.S.Kenning

Please contact me off line. I may have a large (ex-Costello) tacho you could buy. It doesn't fit my rubber-bumper V8 conversion.


P L Hills

You can use a tach from a '73-'76 and just run a wire from the neg side of the coil to the input on the back of the tach, plus it looks the same as those used from '68-'72. I just today finished calibrating mine, ('73 variety) the potentiometer worked like a charm. Final resistance across the IC is around 47K ohms, I used a 39K and a 10K 10 turn pot in series. A fuction generator can be used to calibrate it, you just need it to put out a 12V square wave with a 6V DC offset. This is hooked up to the input on the back of the tach, the tach also needs a 12V DC source and a ground. At 100 hz the tach should read 1500, 200 hz = 3000 and 400 hz = 6000, calibrate accordingly. I also got a schematic from Andrew Robson on how to build a pulse generator with a 555 timer that will produce a 12V square wave from a 12V DC source. Ill post a pic of the schematic on my webpage soon if anyone wants to see it.

Incase any one else is reading this, that variable resistor should be across leg 4s and 7, my mistake.
You can tell which is which because there's a largish blue cylinder (a capacitor I think) already soldered across leg one and seven.

I would like to see a pic of the schematic for the pulse generator using the 555 timer. Sounds like this could make calibrating the tach much more easy. Also, what is your web address? Thanks in advance.

T Johnson

This is something that's been very much on the bottom of my list of things to do but as the thread is running.....
I have an early style (i.e. large) 4cyl pulse tacho that was converted to 8cyl. It works fine except that in the Winter/Spring it's plain lazy and under reads. Once the car has run for a while it starts to respond normally. In Summer there's no problem. I don't know enough about how these things work to strip it and rebuild. Is there an easy way to rectify this or should I just take it to Speedy Cables in London?

Thanks, David.
David Canning

Well I worked the bugs out of the pulse generator, I had a resistor going to a wrong pin. Anyways it works great and puts out an 8.6V pulse. I was able to verify with an O scope and the already calibrated tach. Ill post a picture of the schematic soon, Ive also written an excel sheet for calibrating 4, 6, 8, and 12 cyl tachs using different resistance values dialed into the pot. Check under the gauges or electrical sections soon.

This thread was discussed between 09/03/2004 and 08/04/2004

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