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Like others, My 70 Mach 351C has an Electronic Control Module to the left of the fuse box.

The large hose goes to the top port of the Vacuum Control Valve (3 port water jacket vacuum valve)

The small hose goes to a ported vacuum source on the carb

This system should have a speed sensor and a thermal switch - I can't find either. It looks like the speed sensor should go to the spedometer or be in line with the spedo cable (I don't know), mine goes no where - it is unplugged. The thermal switch connects to the grey wire (through the harness) and to ground. I may have one somewhere but I can't find it.

It looks like this system should work with a dual or single diaphragm distributor. My car has a single but I'm not sure if that is correct.

If you know anything about this system or have a 70 mustang with a 351C that you believe to be correct, please contact me or leave a post.
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My cougar came with a 300 hp C and had the box as you say. The temp
sensor is in the door jam of the driver and there is a short extension on the
speedo cable under the dash that has the speed sensor output on it.
I use the 351w out of a 70 Mach 1 and the speed sensor system sounds to be the same as yours (of course I don't use it in my 65 2+2). Anyway, the canister on the distributor was a dual originally and from what I remember the retard function (interior port on vacuum canister) was engaged when the outside temp. was over a certain level. IIRC the other poster is correct and the sensor is in the door opening .It's been a while so I'm a little fogged on the exact details but I hope this helps.
P.S. of course I'm assuming that the system for the W and C were the same.
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