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MG MGB Technical - Tacho fault

The tacho on my 71 roadster has developed a fault recently, It works perfectly up to about 2500 revs and then swings violently backwards and forwards until the engine revs die back again, I don’t normally push it much further from this but would like it working properly. Any advice would be appreciated.
Trevor Harvey

When you say "until the engine revs die back again" do you mean the engine is cutting out or it is running normally and the tach only stabilises when you reduce the revs with the throttle?

The former is the tach showing you there is an ignition problem which could be anywhere from the ignition switch through the coil and points/electronic trigger, the latter is 'just' a tach problem.

Incidentally do you have points or electronic ignition?

A 71 would originally have had an RVI tach (printed on the dial) where the ignition feed to the coil passes through the tach to the coil, so bad connection in that circuit will affect the engine as well as the tach. 73 on were different.

If the engine runs normally at 2500 and above it could be a bad connection in the 12v or earth supply to the tach, but if the problem occurs consistently at 2500 then it's more likely to be internal.

You could open it up and have a look for any dry joints or stray connections, there are also repair modules available to convert it to more modern electronics - the original components particularly the capacitors suffer from ageing. Or send it away to be repaired. More info here http://www.mgb-stuff.org.uk/gauges.htm#tachprob
paulh4

Hi Paul,
The engine runs normal, It’s just the tacho that plays up around the 2500 revs. I can’t say the engine missfires or anything like that when the tacho is jumping around. I have been using an electronic ignition system for around 15 years and have been very pleased with it and I recall had to have some work done to the tacho to get it to work with the electronic ignition originally.
I’ll check the other items you suggest, thanks for your advice.
Trevor Harvey

Electronic ignition especially on the RVI tach usually needs to be powered (usually the red wire on negative earth cars) from a white at the fusebox rather than the coil +ve so only coil pulses are passed through the tach and not the currents drawn by the electronics. If you have already done that then it's likely to be the other things I mentioned.
paulh4

Thanks once again for your advice Paul.
The tach is powered from the coil and I will alter that and check the other things you mentioned when the car is a bit more accessible as it is laid up still for the winter.
Trevor Harvey

_Ignition module_ needs to be powered from the fusebox instead of the coil.

The tach should already be powered correctly.
paulh4

I Paul can you clarify your advice, you advised the tach should be connected to the white on the fuse box, mine is connected to the coil as is the electric ignition module, are you saying this is wrong, sorry electrics is my weak point but I try.
Trevor Harvey

Your tach should be powered (its 12v supply) from a green wire behind the dash, that is left alone.

If the red wire from the electronic ignition module is connected to the white at the coil then that needs to moved to white at the fusebox.

There should be one white on the coil +ve. Prior to 1973 that came from the ignition switch via the tach pickup and as well as powering the coil it provides the signal for the tach. That should be left alone. The tach should have RVI on the dial.

From 1973 there was still one white wire on the coil +ve but now it comes direct from the fusebox as the signal for the tach is done differently, and they had RVC on the dial.

Confirm which tach you have as it may have been changed. Someone could have changed the innards from RVI to RVC ... but we won't even go there.

Also conform how many white wires you have on the coil +ve, and where the red (usually) wire which supplies 12v to the electronic ignition module is connected.

paulh4

This thread was discussed between 10/02/2025 and 24/02/2025

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