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MG Midget and Sprite Technical - Tacho reads high

Well it might be graet for 'perceived speed' but when I couple up a multimeter and check the reading teh car's tacho reads up to 1000rpm higher, and the speed which I have checked in the past by timing mile markers, agrees with the multimter not the tacho.

Voltage regulator problem or is there some adjustment to be made in the tacho.

I'm running Lumenition therefore trust the correct number of signals are coming from the engine.

Thanks, James
James Eastwood

Rev counters had a hole in the rear of the case with a pot for adjustment. If your meter reads 'revs' (and you believe it to be accurate) then you can adjust accordingly.

Or you can use a normal 12v battery charger to calibrate the rev-counter... it should read 3K rpm when connected to ignition white/black (method of connection is more complicated for the current-type of r/c).

Be sure to let the counter settle down, i.e. run for 10 mins before making adjustments.

I set mine in this way in around 1999, and can verify it's highly accurate even around 6K using the Emerald read-out / change-light.

A
Anthony Cutler

Hi James, I adjusted my tach as as above by using a stubby screwdriver into the rear of the tacho.

With the multimeter connected and engine running it only took a couple of minutes to fine tune (I was 1000 rpm too high).

Make sure that you test it throughout the rev range as it may read ok at 1000/2000rpm it could be well out by the time you hit 4000/5000rpm.

Thanks
Mark
Mark Whitmore

Great thanks I found it and have attempted to make a correction. The pre-set pot tracks are probably a bit dirty hence I'm struggling to get it where it needs to be. But I know where it is now thanks.

Regards James
James Eastwood

James,

If you think the pot is dirty consider giving it a dose of contact cleaner spray, in my experience it often works wonders on such items. IIRC Maplins and the like stock it marketed as Servisol IIRC.
David Billington

Anthony,
How does the the battery charger, rev counter thing work then? Is it something to do with the AC 240v at 50 hz stepped down to 12v? I take it that the 12v is still oscillating or pulsing at 6000 (2 times 3000, each cylinder firing once every two revs) times a second causing the tacho to read 3000. Is it just a case of connecting the positive to the terminal at the back and the negative to the case?
Thanks
Dave
Dave Brown

Hi Dave

Your analysis is correct. The r/c is powered from the ign 12v as normal; the battery charger 'pulsing' nominal 12v is applied to the spade terminal (for voltage types) or current sent down the loop on the back of the r/c (points closed).

I have an article that was published some years ago on this, which I could forward be email.

A
Anthony Cutler

Anthony,
That would be great. gungehead at btinternet dot com is my email address.
Thanks
Dave
Dave Brown

Can anyone explain this? (Bear in mind that my speedo needs recalibrating due to my gearbox being a type nine, that's this winter's job.).
At say 3500 rpm in top (5th) when I first set off, ie engine just warm, the speedo will give a certain reading, but half a hour later ,engine fully warmed up the speedo reads about 10mph lower for the same rpm. Is this a tacho fault due to some sort of resistance build up, or is it the speedo, maybe the cable binding as it warms up? Apologies for hijacking the thread.

Bernie.
b higginson

The rev-counter takes 10 mins or so to settle down... the needle deflection is determined by the 'on' time of a monostable, set using an RC network... these comps have temp dependency.

A
Anthony Cutler

Anthony. Thanks for such a concise answer.

Bernie.
b higginson

This thread was discussed between 04/07/2011 and 07/07/2011

MG Midget and Sprite Technical index

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