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New tach places too much load on my harness

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I just installed a tach on my '66 Coupe.
I have an electronic ignition (no amplifier) that was in the car when I bought it.

The tach (VDO) is wired to the '-' of the coil and draws power from
the cont voltage unit that powers the other guages. (I actually just
spliced a wire coming out of the cont voltage unit).

All seems okay....except I just took it out for another test drive and,
while the engine was still cold, turned on the lights.
The engine cut out. At first I thought I had grounded the coil somehow.

Once the idle steadies out at 600rpm and everything is toasty, there's
no issue. However, while idling if I turn on a turn signal - the idle drops
but recovers (I'm sure it would die if the engine was cold as before).

any ideas? I suspect I'm pulling too much load. Everything was cool
before the tach went in.

(Side note - the temp guage is now reading much hotter. I had thought
about changing the thermostat but as I write this I suspect the reading
is inaccurate. As I recall; the switched live wire I tapped for the tach
went from the cont voltage unit to the engine temp guage.)

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You need to remove the tach power from the cont voltage unit, it is designed to steady the gauge readings not power anything else. The tach power should come from a 12v switched source such as the ignition switch or fuse panel.
The wiring in the car is of course 30 years old. I got to a point where the battery cables got hot when I would crank it, and at one point actually smoked. When you start adding things also consider the battery cables. Stock they were 4 gauge. I now run 2 gauge. Installed one of these kits:

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