tachometer help

[38plymouth]

I'd love to know what the RPM's on my flathead are. I still have the stock 6 volt positive ground system. I was reading on the net and I saw that some people claim a 12 volt tach will work with 6 volt cars. I happen to have a sun super tach that is 12 volts. I think I might try it out but I'm not sure how to wire it. It has four wires:

black-ground

red-12 volt

green-distributer

white-lights

How would you wire this thing to test it? Would I reverse the red and black since I have a positive ground? Does the green go to the coil???

[dirty dan]

From what I remember on a Sun tach...

Red - positive (+).

Black - ground (-).

Green - connect to the coil's negative (-) terminal.

Usually the fourth wire is for dash lights.

[greg g]

Connect the green wire to the side of the coil that goes to the distributor. That is the trigger wire that detects the pulses as the points break the circuit and fire the coil. If you hook it to the ignition side, it won;t see the make break pulses.

[38plymouth]

Is red still positive and black negative then or should I reverse those because of the positive ground? I hate electrical stuff.

[greg g]

What I might do is hook the tach power up to a seperate power source like a jumper pack, or maybe a 12 V charger, seperated from the car's system, just ot test it. with it powered independently, you should be able to see if it will pick up the signal from the distributor.

There might be a setting on the tach for 6 and 8 cylinder cars. If I remember correctly an 8 cyl setting will work but the rpms will not be reflected accurately, as the tach needs to see 8 pulses before it counts a revolution.

Way back when I had a sun tach on my AH sprite. It was Positive ground, but 12 v. I can't remember if there was anything special about the power hook up. Since it has its own power and ground you might be able to hook it to the battery directly and independently from the car's system. Also I thik I read somewhere that the chasis will act as ground irrespective of the of what the polarity is.

I think powering it from an independant 12v source will let you see if it works, then sombody else may be able to chime in as to how to wire it permanently.

[38plymouth]

I set it for a 6 cylinder engine and hooked the green wire to the coil, red to ground and black to 6 volts and it works perfect. I wish the tach looked older, it doesn't really fit with the car.

[Reg Evans]

I have a these. They're from a boat. I wonder if they would work. They only have 2 connections on the back and a light socket.

[H Suhling]

I hooked up my tach with green wire to distributor side of coil, red wire to ground ,and the black wire to the coil wire terminal on the ignition switch for power. The tach is a Sun tach for six cylinder 12 volt neg. when engine starts tach goes to about 800-900 rpms but when you rev up engine it doesn't go back down to idle speed then continues to climb from that point on.

Any ideas? Maybe a different power location. Or maybe a bad tach.

Thanks

Harvey