Fixing the wiring of my tach - I'm dumber than a blonde!
Hey guys - So I've got this 72 Plymouth Scamp. It has a Sun Tach (model ST-602) tach already on it. The tach works. But it is wired up hot to the battery + post. Luckily, there is a disconnect about a foot off the battery. When it is connected, the tach is ON, and the light is on. It will drain the battery. It doesn't matter the key switch or anything else. So, last night I tried to figure it out.
In summary ... I SUCK at anything electrical. I'm a mechanical engineer for YEARS. But when it comes to electrons, I have no idea what they're gonna do.
Here's the deal. On the back of the tach it has the following connections:
- Volts 12 (nothing terminated to this tach post)
- IGN. SW (terminated via a black wire, routed to battery + post)
- The bulb conductor is looped around and crimped into this IGN. SW eyelet.
- Trigger (terminated via a blue wire, routed correctly to ignitor)
- GRD / NEG (terminated via a yellow wire, routed up under the dash to a ground, although I don't prefer it's location, it is on paint)
When the connection is made to 12V under the hood, the tach light is on continuously, and when the car is driven, the tach works correctly.
Last night while messing with it, I disconnected it from 12V under the hood. Then I changed the black wire from the IGN. SW post to the Volts 12 post. I then attempted to connect it back to the battery under the hood. It didn't like that, and demonstrated it's objection with a loud pop and sparks all up in my face. At that point I felt as stupid as I always do when dealing with electrons.
I know what the "Volts 12," "IGN. SW," "Trigger," "GRD / NEG," and all that mean. I'll just be damned if I can figure out how the electrons are going to feel about me messing with them.
Seems to me that, I should disconnect the black wire from the IGN. SW and put it on the 12 Volts post. And then, add a new wire to the IGN. SW and terminate that wire to a 12V switched source (like the ignition of course). At that point, I'm assuming the switched source will keep the circuit to the 12V side of the tach "open" when the car is off. And only close the 12V circuit when the ignition is switched ON. This would turn the tach and it's backlight off when the car is off ... no?
2 questions:
- Is my previous paragraph correct?
- Without tearing into the steering column, where could I find access to switched power?
7milesout
