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MG MGB Technical - Tacho Problem

I've searched the archive but cannot seem to find an answer to this problem with my 1970 BGT tacho reading.

It has an RVI 2430/01 (should be 2430/00) tacho which, up until yesterday seemed to be working fine. Today, it seems to work smoothly and correctly up to about 2500 RPM. Anything above that and it becomes a bit erratic and fluctuates by about 200 or so RPM whilst the engine runs smoothly.

I have compared its behaviour with a dwell/tach and the dwell/tach operates OK at all RPM. The connections on the back of the car's tacho are all OK.

Is this just an internal problem with the tacho or is there some other item in the ignition system I can check?

John
J Tait

What ignition? Electronic ignition systems are known to cause problems with this type of tach, some have to switch to the later RVC type. It's possible the signal is getting slightly marginal at the higher revs and the car tach is sensitive to this whereas an external dwell/tach isn't. May also be too marginal to affect running. Could also be marginal tach electronics of course. A 'scope should reveal which, otherwise borrowing another RVI tach (you can wire the two in series).
PaulH Solihull

Thanks Paul. It has the original ignition system type i.e. points/condenser and not electronic.

I can't detect any engine misfiring or reduced performance at all, but I was just wondering now if a slightly marginal tacho signal (as you mentioned) at >2500 RPM could be caused by dirty points or a slightly closed up point gap.
J Tait

Hi John,

The most common cause of failure in the RVI tachs is the .25 uF capacitor.
If you are confident around electronics, it is an easy fix, with either a .22uF or .27uF cap. Recalibrating is a bit more awkward,as it has to be done with the innards out of the case, as the potentiometer doesn't line up with a hole in the case.

Herb

Herb Adler

Thanks Herb.

I'm stumped when it comes to electronic stuff - other than switch it on or switch it off :). I'll double check the points (doubt that it's the cause though)and see if things improve. If that fails, I'll see if my friendly MG man has a tacho I can try before I buy or take my tacho to the instrument repair people.

John
J Tait

All fixed! The points had closed up (didn't check dwell yesterday when comparing tacho readings did I) and were pitted. They haven't been touched in 10,000 miles, so I should have checked earlier.

Anyway,


i put in a new set of points and the tacho is dead steady now. Car is running just that bit sweeter too. Looks like the tacho is a good early indicator of problems arising in the ignition system.
J Tait

This thread was discussed between 19/06/2011 and 20/06/2011

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