MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Tachometer

I just had electronic ignition installed in my '58 coupe. Now the Tach doesn't work and the shop cannot figure out why. Tach is positive earth. The mechanic had some kind of connection added to the alternator and attached the tach to that, thinking it would solve the problem. Well, the tach worked but showed 4500 rpm at idle.

Anyone have an idea about this? Do I need to remove the tach and have it recalibrated? Mechanic has no idea what to do.
Pat10

Pat, just been through a tach repair so am full of the knowledge!
If yours is an inductive input tach (type RVI) it will not work on certain electronic ignitions. The RVC type works, as it uses a voltage input, not a current type.
Art Pearse

Why do you have an electric tach in an MGA coupe?
Art Pearse

I have electronic ignition in my negative earth, alternator 1800V engined car. As Art says RVC. Mine is a Smiths RVC 2415/00AF.

Steve
Steve Gyles

How can I determine if it is RVI or RVC?
Pat10

It's printed on the dial face.

Steve
Steve Gyles

Pat10,
look for (very) small numbers and letters on the dial. (ETA: Steve posted whilst I was typing).

But more info required -
. which electronic ignition (make, model, type)
. do you mean wire attached to an alternator or control box dynamo?

I think it may be that the mechanic doesn't know about these things may be at the heart of the matter.

High resolution, good quality, in focus, photos of your tacho and engine bay connections may help.
Nigel Atkins

Art,
you might have missed PaulH's last post to the other thread -

"The pertronix one has been superceeded by powering the electronics from the white at the fusebox instead of the white at the coil. It's the extra current from the electronics passing through the tach pickup that causes the problem - hence reducing the pickup to half a turn. It's a bit hit and miss, powering the module from elsewhere removes that extra current altogether."

It was in reference to me posting attached PDF (which bear in mind is wrong now by Paul's account).
Nigel Atkins


Just leave the original power wire to the coil in it's original position but don't take the ing. module power from there as well
power the module(dist) seperately from either direct from the ign. switch or from any ign. switch operated power source (but NOT from the original coil power wire)

willy
William Revit

Hi Pat, you say that the car is fitted with an alternator so it should be wired negative earth and with a positive earth tachometer it will never work. If the tacho worked previously with the alternator fitted then it has probably been converted to negative earth, if the alternator was fitted at the same time as the electronic ignition the mechanic should have checked the electrical system and converted the tacho as well.
I Hazeldine

Pat, what model tachometer do you have?
Art Pearse

Hi Art,

the model is RVI 240I/00B
Pat10

The tach was in the car at the same time the alternator was installed. This was a long time ago (12 years or so) when I lived in Portland, OR. Also had a rebuilt MGB 1800 motor, a 5 speed tranny & disk brakes installed. I no longer live there or the car would be going to them. Everything worked all these years until the electronic ignition (which was for a positive earth car) was installed.
So, that's the only difference that has happened.

Pat10

I will send this to the mechanic and see what he says.
Thank you!
Pat10

Thank you. Got it.
Pat10

IF the car has an alternator it has to be neg. earth and the tacho must have been changed to neg earth
If you have a Pos earth ignition module it shouldn't start -if it's going it also must be a neg earth unit
Being an rv1 tach , if it was working before the electronic ign was fitted and the car runs ok with the electronic unit fitted just wire it separately as i mentioned earlier and it will be fine
William Revit

Thank you, William. When I can get it back to the garage, I'll let you know how it goes. (I don't know how to do it).

Pat
Pat10

This thread was discussed between 26/05/2021 and 30/05/2021

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.