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MG MGB Technical - Tachometer Fit?

I have a Smith RV1433/00 tachometer from a 1971 mgb roadster. I want to put it in a 1973 MGB Roadster with a car number of 304806. Can anyone tell me if the wiring harness in the 1973 MGB is compatable with that tach? Thanks. Curt
C Pieper

Not directly, the tach and wiring changed at the end of the 1972 model year, unfortunately for you. For 1973 the tach changed to one with an RVC number and was voltage triggered, the earlier tach was current triggered. You will have to route a white from the ignition switch via the pickup on the tach to the coil +ve, and remove (and insulate) the white that is already on that coil terminal. Other than that it should work with the same 12v (green) and earth (black) connections as before.
PaulH Solihull

Paul,
Is there anyway you can show me this in a diagram, showing me:
1. the wires by color,

2. the point of attachment of the white ignition wire. Ny tach has two white wires on it.

3. You say to route a white from the ignition switch via the pickup on the tach to the coil +ve. What is the +ve?

I know this is asking a lot, but a rough diagram attached to an email would be really appreciated. Thanks.

Curt
C Pieper

Curt - http://www.mgb-stuff.org.uk/ignition1.htm and http://www.mgb-stuff.org.uk/tach.htm might help.

You say your RVI tach has two whites, do these have bullets or are they just chopped off? In either case the new white from the ignition switch will connect to one of them, and another new white will connect from the other to the coil terminal that should already have a white wire on it. Remove and insulate the existing white wire from the coil, and connect the new one to the terminal that came off.

If the whites on the tach have bullets then you might as well put bullets and connectors on your new wires. If chopped off you could either add bullets to those, or remove the old wire and route a single length of white from the switch to the coil feeding through the pickup along the way. The top drawing in the second link shows the arrangement for positive earth cars, for negative (which yours should be) the wire from the switch would come from the bottom right, loop through the pickup as shown, then come off top left to go to the coil.

Whichever you do the tach will only work with the wires connected/fed one way, not the other. However the wrong way doesn't do any damage, so you can try one way, and if it doesn't work (and you are sure you have the green 12v and black earth connected to the tach) then simply swap them over, in which case even if your whites on the tach *have* been chopped off, it might be easier to fit bullets to them rather than fiddle with the pickup.
PaulH Solihull

This thread was discussed between 24/08/2011 and 25/08/2011

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