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MG Midget and Sprite Technical - 123 -tacho puzzle

Help! I just fitted my new 123 distributor. The car runs well enough BUT when I switch on the ignition, my rev counter jumps right up to max (indicated revs). When I start the engine the tacho seems like its working correctly but it is not. The needle rises and falls with the revs but, for instance, when I set the idle speed it would not drop below 2500 rpm even though my stobe light tell me it is only doing 1000 rpm.

I should just explain that the 123 distributor normally comes as a negative earth version with just two wires(and that is what I wanted). In my case I was supplied with one with three wires that can suit either positive or negative earth depending on how the wiring is connected. The official wiring instructions only relate to the negative earth version but the supplier provided a wiring diagram for my version after I queried this. I can't upload the diagram as it is in PDF format and I can't fnd a way to convert it to jpeg (I'm working on it).

Maybe some of you electrical whizzes can make sense of this and suggest a solution.

Chris
Chris H (1970 Midget 1275)

Just maybe here is the wiring diagram referred to above. Chris


Chris H (1970 Midget 1275)

I have been advised by the supplier that the problem arises because the coil is wired through my tacho (apparently the same problem has happened with E-types fitted with 123). I can solve the problem if I provide a positive 12v feed from an ignition controlled and fused source (and disconnect the distributor from the positive terminal on the coil). What's the best way to get such a feed? If I inserted a switch into that feed would I also have an anti-theft device?

Chris
Chris H (1970 Midget 1275)

I wouldnt do a switch, my fear is that would create a double ground....(how do you spell FIRE?)

remeber your fuel pump is on this same circuit also.


Im assuming your car is negitive,

From what your saying you need to find a circuit in the fuse box that is only active (hot) when you turn on the key...(whipers, turn signals, ect) and run the red wire to that source...But Id look for a better way, make something more dedicated....hopefully someone here can help with a creative soltion.

Prop
Prop***The End in 2012

Hi Chris If you are negative earth then where have you taken the feed from for the unit (red)? Did you take it from the coil connection? If so then I think the supplier is right. The power to the unit should be from the fuse which is live when the ignition is switched on. This will mean that the current flowing through the tacho is only supplying the coil and not the unit as well. Hopefully this will mean the "triggering" measured by the current flowing through the coil will be cleaner.
Hope that made sense and hope it works.

If it is positive earth then the black wire after the unit could be routed through the tacho which I think would then also cure the problem.
Robert (Bob) Midget Turbo

Chris,
If I am reading it correctly, their solution would simply bypass the tacho, leaving it sitting at 0 at all times. Not sure how that constitutes a fix. Of course, you could convert to the 1972+ tacho which connects to the - side of the coil. Mine works just fine with the 123/Mini.
David "no, you cannot have mine" Lieb
David Lieb

David

I don't think they are saying bypass the tacho, only to take the positive feed for the 123 from another source. The feed to the coil through the tacho remains intact, the black wire goes to the -ve/CB terminal on the coil and the blue wire goes to ground.

Chris
Connect the red wire to the 'white' side of the ignition switched fuse.
Dave O'Neill 2

Thanks for the confimation David :-))
Robert (Bob) Midget Turbo

Sorry Dave, I should say! :-))
Robert (Bob) Midget Turbo

OK, that makes more sense. In that case, I would agree with Bobert and Dave O. I would also say that if that doesn't do it, you might try using that white circuit to trigger a relay that would switch a clean 12v from the starter solenoid.
David "relaying good news" Lieb
David Lieb

The supplied instructions said provide a feed for the 123 from the +ive side of the coil. The expert at SC Parts advised that this was only to provide a feed for the microprocessor so the solution is to find another power source for the 123 and leave the coil supplied by a live feed through the tacho as originally intended.

However, I don't understand what is meant by "the white side of the ignition switched fuse". Does the "white" here refer to the colour of the wiring? I found a spare switched 'tag' on my ignition switch. Could I take a feed off that (it is, however, further away that the fuse box).
Chris H (1970 Midget 1275)

The white wire on the ign switch is the ignition 'live'. There is a spare spade on the switch IIRC.

The white wire does to the connector block above driver's right knee; one white then goes back to the fuel pump. The other goes to the fuse box (2nd fuse from bulkhead); the spade on the fuse box has another white that goes straight to the coil. All this is unfused.

The white/black on the 'points' side of the coil goes to the points and tacho.

White and White/black is ignition unfused.

At the fused side of the box from the white is green. It goes to the voltage regulator for the fuel guage, flasher unit and tacho (IIRC - all things that come on with the ign).

So green is the 'ignition' circuit - fused part.
Anthony Cutler

What Ant says is partially correct.

The white from the fusebox does not necessarily go directly to the coil. On earlier cars - pre '73-ish? - the white loops through the tacho before going to the coil. The white/black from the other side of the coil only goes to the dissy.

This is why you are having an issue with the tacho, Chris.

As Ant says, the ignition switched fuse has white wires on the unfused side. Connect your red wire to a spare terminal on this side of the fuse...there should be a spare IIRC.
Dave O'Neill 2

Thanks for all the help. I took a feed from the terminal on the fused side of the ignition circuit (green)to the distributor. That has done the trick - tacho now works OK. Now need to road test the new dizzy but must wait for snow and SALT (!!) to go before doing so. Chris
Chris H (1970 Midget 1275)

This thread was discussed between 30/01/2009 and 03/02/2009

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