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MG MGA - MGB Tachometer circuit diagram

Have any of you who have fitted a 5 bearing 18V engine to their MGA have a circuit diagram of how the neg earth electronic tacho should be connected?
I have seen a few but most of them are a little vague when it comes to showing where the impulse loop is actually connected on the coil at one end and to the ignition/battery at the other.
Any clarification on this would be verymuch appreciated.
Cheers
Colyn
Colyn Firth

Is the Workshop Manual diagram insufficient?


M F Anderson

be aware that the direction of the current in the loop is also important with those tacos. If it doesn't work, try reversing the connection
dominic clancy

Thanks Mick and Dominic,
I now get it, that is exactly the information I needed.

I had been mistakenly under the impression that the tacho required a separately connected loop to operate.

I didn't realise that the ignition feed to the +ve terminal on the coil has to be diverted through the tacho end route.

Much appreciated
Thanks
Colyn
Colyn Firth

Colyn,

I am not sure what you mean by "a mistaken belief" in relation to having a loop.
The Workshop Manual drawing, in the inset top left, shows the loop.

Mick
M F Anderson

Mick
I suppose I wasnt too clear in my post.

I mistakenly thought that, AS WELL as having the standard ignition and contact breaker wires, the tacho needed another wire from the positive side of the coil which loops through the back of the tacho which then returned to the CB side of the coil. I thought this additional loop was wired in parallel to the standard ignition connections.
It seemed odd to me and I wasnt happy with it which is why I asked the question. It most likely would have let lots smoke escape if I had done it that way!

Your diagram made me realise that all I have to do is divert the wire from the ignition switch through the tacho and then on to the positive (IGN) side of the coil.

I hope this makes more sense.

Colyn
Colyn Firth

Colyn

Just out of idle curiosity which instrument do you have? Mine is RVC 2415/00AF - negative earth. I just have a couple of wires out of the casing (excluding the two lights). I have one to the coil and the other somewhere into the bowels of the dashboard - my back playing up so not in a state to lie on my back to investigate further. One of the prices I have to pay occasionally for playing silly b*****s with ejection seats.

Steve
Steve Gyles

As it turns out, most of the above has proved to be irrelevant because when I took the tacho out of the box, it turned out to be a later RVC 2415/00AF like Steves, which doesnt need the loop connector.

As Steve has said previously, there are just two terminals on the back of the RVC tacho, a bullet connector that is connected to the CB(-ve)terminal on the coil and a spade connector that is the +ve supply.

I wired it up temporarily today and it worked just fine with my electronic ignition set up, so I now just have to install it into the car.

Thanks again everyone.

Colyn
Colyn Firth

I have to tackle the same. This explains it very well:

http://www.mgb-stuff.org.uk/tach.htm
Hansueli Ryser

This thread was discussed between 01/05/2016 and 04/05/2016

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