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MG Midget and Sprite Technical - Sprite Mk3 Flickering Rev Counter

Hi Everyone.
This is my first post here, I hope you can help!
I'm not overly savvy on all things electrical, this is our first classic and it's a bit of a learning curve, so laymans terms much appreciated :)

I have had the front wiring harness replaced on our 1966 Sprite. Unfortunately, since receiving it back, the rev counter now flickers quite abruptly when sat at idle. Going up as high as 4500rpm! It settles down when the accelerator is depressed, but it still isnt right. I know most will say to take it back to be rectified, however I'd like to fix it myself to help the learning process.

Some history. Although the car is a 66 MK3 it was re-engined with a 1275 and associated gearbox when restored back in 1997. At some point someone decided to fit separate spotlights incorrectly that shorted the loom somewhere melting a vast amount of the loom, hence why it needed replacing. When fitting the replacement MK3 loom from MGOC spares there wasn't the associated wiring for the tachometer apparently. So wires were run from the coil to the tachometer. It's a 4 spade sports coil I believe. Before the car went to be "repaired" I believe only 2 spades were connected. Now all 4 are.
The tacho worked before the repair. The car (and tacho) is neg earth and has an alternator. ...

Hope that's enough info....

Chris
C Fleet

On the front of the Rev counter, under the Smiths logo, there will be a series of text and numbers in white paint, that includes the letters of either RVI, or RVC, followed by some numbers.

Which is yours?

If original it will be RVI on a '66 car. This is current sensing.

RVC is voltage sensing, and was fitted to later cars, or as a replacement.

See the following wiring diagrams and you should be able to work out the connections for yourself.

Then you can start to fault find.

Assuming you still have points ignition, then Fig 1 and 2 can be read as points, instead of electronic ignition.


Lawrence Slater

Chris,
Is there any suggestion of the engine misfiring or hesitating when the needle is flickering? A twitchy needle is a common symptom of a low voltage failure in the ignition circuit. i.e. between ignition switch, coil or points and the interconnecting wires.

If there is no engine hesitation then it is likely a fault between the coil terminal and the tachometer that Lawrence has posted the diagrams for. It may be nothing worse than the connection at the back of the tachometer being a bit loose.
Guy

Thanks for the tips so far... Will check out the type of tacho either tomorrow or Monday. A bit perplexed as to why it worked before but not now? Anyways can anyone describe what the 4 connection spades on the coil are? I just wonder why the tacho was fine with two wires (spades attached) and that now all 4 spades are wired it's not.... I told you I was new! ;)

Oh and as for the engine, touch wood, it's running absolutely fine. In fact it was the first time I've run the car in 2 months and was suprised how smooth it felt....
C Fleet

If the engine is running fine that's good. To me it suggests that the flicker of the tach is just to do with the wiring between the tach and the coil - or at least with the connections at either end.

At the coil, if there are 4 spade connectors there are still just the two coil connections, the spade terminals are paired together. One will connect to the ignition and its twin to the tachometer. The other will be to the dizzy. Not sure what a 4th wire would be - unless it has something to do with a ballasted coil perhaps? Or possibly a switched connection to earth to act as an immobiliser?
Guy

Hi Guy,

That makes sense (I hope!) we keep the car away from the house so I will go and have a fiddle and look see tomorrow. I'll take some pics as well, just in case.
I do wish PO's wouldn't mess around with things. ....

C Fleet

Chris,

Could you post a picture of the coil connections?

One could be a radio interference suppressor. Looks like a condensor, and is essentially the same thing, but for the purpose of eliminating the electrical noise from the points.
Lawrence Slater

I've always had a problem with my taco's - have an Aldon Ignitor/ sports coil. At times the taco just swings wildly and after a bit settle down again - it can happen when happily bowling along a motorway etc. I have a spare reconditioned one and it does the same.
Both RVC's - have heard that a seperate feed not from the ignition may cure it, but not invesitaged yet.

R.
richard boobier

Well i think i've resolved the issue. Looking behind the dash it appears that 2 of the white wires heading from the coil to the Tacho were connected. Tracing this back through to the coil, there was another wire connected to the same side as the ignition. disconnecting this wire has solved the issue. The car is still running superbly. Thanks for your help!
C Fleet

Chris,
hi, welcome, glad you got it sorted

you'll find a copy of this a fabulous investment and if you read it thoroughly you'll know more than some long term owners (Ref: 0057) - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html

(it has a supplement for the earlier car too)
Nigel Atkins

This thread was discussed between 07/01/2012 and 08/01/2012

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