Thread: Vintage Setting up a tach
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Old 03-24-2021, 07:03 PM   #4
142 guy
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Join Date: May 2014
Location: Saskatchewan, Canada
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Tachs tend to come in two flavors, voltage sensing (the most common) and current sensing (like the original Volvo tach's supplied by Smiths). If you purchase a modern tach, it will almost certainly be voltage sensing and the sensing line just connects to the negative terminal of the ignition coil assuming you are using the original 140 ignition coil and distributor. If you manage to find a current sensing tach the signal lines have to be connected in series between the coil negative terminal and the points so that coil current flows through the tach sensing circuit.

Planetman is correct about there being two drop in versions of the Volvo Rally cluster for the 140. The more elaborate one with 4 gauges, speedo and tach is rather rare and quite expensive when it is in good condition. The sensors for the cluster are specific to the cluster and hard to find if not included with the cluster. I searched around for a 4+2 gauge Rally cluster and the cost for nice refurbished ones with all the parts was significantly more than the cost of building my own cluster using SpeedHut gauges. The SpeedHut speedo was a GPS unit which eliminated all the hassle with drive cables and dealing with different final drives and tire sizes.

If all you want is a tach, I bet Planetman could fix you up with an original dash mount Smiths / Volvo tach.
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