MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

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 MGB Technical - taco for 71 needs earth but where

Hi
I have recently bought a late 71 gt with a non working tacho, my elec diag shows an earth, but the tacho doesnt have one. Have tried earthing the tacho body, no joy.
Anyone know where the earth wires should go onto the tacho? Also have only 1 green wire in, elec diag shows 2, or do they join up before hand?
Any help much appreciated.
Patrick
p healey

Patrick, Here's a link to the MGB wiring diagrams. Scroll through them until you find the 71/72 diagram. Look at the back of the tach and it shows three wires. Yo take it from there. Good luck and hope this helps. PJ

http://www.thebriton.com/mgbwiremaster.pdf
mgbwiremaster.pdf (application/pdf Object)
Paul Jennings

Patrick,

For further infos, please visit the following site:
http://www.mgb-stuff.org.uk/
Hammer & spanner
Electrics
Tacho...
You may find all what you need.
Guy RENOU

That mgbwiremaster.pdf diagram actually shows four wires, but you have to blow it up to about 150% to see the fourth wire, which is hardly convenient.

Only 1 green wire is needed, it may be that on some models the green is then daisy chained off the tach to some other componeent making two. The earth is connected to the case, usually to a spade rivetted to the back of the case, but can also be via a tag secured under one of the thumb-nuts that holds it in the dash. This also daisy-chains to other components and must go back to a good earth on the body, usually just under the screen behind the dash by the wiper motor. The 71 (and earlier) also needs two white wires in the pickup, one from the ignition switch the other to the coil. If any of these wires are missing or not conencted correctly at either end the tach will not work, nor if there is an internal problem of course.
P Hunt

The back of your tach does look like this?

http://kwbcc.com/RVI2430-00.jpg

Not sure I've ever messed with a non North American tach. Also, it's been awhile since I messed with a second generation tach. It's got four screws on the back, so it might be similar to the first generation electric tach in one respect. The screws need to be tight for the case to act as a proper ground. Particularly the two that don't attach to the case. Is that other hole show a rheostat that you can use to calibrate the tach? If so, that's cool. The easiest place to ground the tach is via an eye connector over one of the two mounting studs (after you put on the big funky metal thing that holds the tach in place).

I've found it easier sometimes to attach that eye directly to the screw that has the tab from the case. That tab is the ground path from the case to the screw and from there to the works frame.

You'll need to be able to pull the ground wire through the hole in the dash enough to attach to tach before pushing it through the dash (I consider that slightly easier that trying to get it attached by feel under the dash to one of the two mounting studs). It's definitely more trouble to remove the tach when done this way, so it's a toss up.

It's also possible one of the wires has come loose inside the case, but I'd bet on a loose screw first.
Dave McCann

That tach case has been modified to some extent, this is a UK tach of the correct era. The spade for the green wire is with the two bullets for the white wires, the spade for the earth is by the Smiths guarantee label.

P Hunt

Thanks to all for your help.
My tacho is the RV 2430/00 looks more like Dave's photo, certainly no earth spade as in Pauls photo, have looked at your website Paul, it's excellent and very helpful.
Will play with the tacho this weekend...
Thanks again
Patrick
p healey

This thread was discussed between 03/03/2010 and 05/03/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now