I was hoping to get a definitive answer to the wiring for a Smiths inductive loop rev counter, but cannot do so!
The original Smiths rev counter instructions and various wiring diagrams for both MG Midget and MGB, which used the same type as original equipment in the 60's, ALL show the rev counter white wire is connected in place of the feed from the ignition switch to the non-CB side of the coil. For -ve earth cars this should be the +ve terminal. This is as I think I remember it when fitting one to our Minor in the very early 70's.
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This diagram is for the 'Universal' version - the OE one had a single connection for the fused supply, instead of the multi-voltage and +ve/-ve connection block.
There are marker sleeves on the white wire (can see what looks like the black one in the picture). For -ve earth cars the black marker end of the white wire goes to the non-CB connection on the coil, instead of the original lead, and the red marker end goes to the ignition switch to replace the original coil feed. The green wire in the picture goes to a fused ignition feed (A4 on the fusebox).
If this does not work, try swapping the ends of the white wire around (someone may have fiddled with the loop, which the Smiths intructions say do not do!).
This type of rev counter can have problems with electronic ignition systems, such as the Aldon Ignitor (Pertronix), which is why ours has had the innards converted to the later version without the loop.
Now the BUT! Speedy Cables, who now supply Smiths gauges, show that the connection is made between the coil -ve terminal and the points connection on the distributor (for -ve earth).
Which one is right?
..
Did they not have a vacuum gauge to go with it ? 
