Electric tach question

[Dyna]

I have a 1951 V8 flathead in my car. Electrical system converted to 12V. Just had the engine re-built and thought it would be a good idea to install a little tach under the dash to keep an eye on the rpms. I bought a cheap off shore tach and installed it. With the pulse wire on the -ve side of the coil the tach needle jumps all over the place,with the pulse wire on the +ve side of the coil the needle pegs at 10,000 and doesn't move. Black wire is grounded, red to fused circuit, white to ignition switch. Have missed something in the wiring? Or did I waste my $s.
Thanks.

 

[41Joe]

Is it one like you would get at Autozone?
Okay, stupid question, please don't be offended. There is a switch on the back of those types that change it from 4 to 6 to 8 cylinder. Is it on the right one?
From what I can see, I think the pulse wire should be on the - side of the coil.

 

[BillM]

I agree on the pulse input connected to -ve assuming you are now 12 volt negative ground.
You might try a filter at the coil connection such as this:

 

[40cpe]

I don't know your wiring colors, but the pulse goes to the "-" side of the coil, there is a ground wire, a wire from the ignition, and a wire for the illumination light. There shouldn't be two "hot" wires going to the tach itself. Did you not get a wiring diagram with it?

 

[Drbrown]

Agree with all above. Have '47 converted to 12v neg ground. Tach is wired off neg side of coil.

YES, check the switch setting on the back of the tach.

Also, recently my tach was showing crazy readings when I had a Pentronix ignition module. I had replaced the module and tried resistors and a diode to correct the tach, but none corrected it .... until I replaced the distributor with a distributor with a stock points set-up. Tach immediately read correctly and has since then. Who knows what evil lurks in our ignition systems.

 

[Dyna]

Thanks for the replies. Switch on the back is set to 8. When I first wired it I ran the yellow wire to my dash light circuit, when I turned everything on the tach wasn't lit up,couldn't see anything (LED backlight) only lit up if I turned the lights on. Moved yellow wire to ignition and tach lit up when power turned on. My electrical setup is a little different (12 volt), I have a master switch that turns on all power to fuse box,nothing works if its not on. Ignition switch only controls power to the coil. Here's wiring diagram that came with it. Any further ideas?

 

[40cpe]

I don't understand needing the illumination on to see the tach unless it is digital, but you referenced the needle. Do you have a points distributor? There have been recent posts about needing a diode in the pulse wire to steady the needle.

 

[Dyna]

Stock points distributor. I will try to research diode. Tks

 

[40cpe]

http://www.fordbarn.com/forum/showthread.php?p=1409872&highlight=diode#post1409872

 

[Drbrown]

I'm attaching a "Pro Tach" 5 wire diagram. It illuminates when the car lights are turned on.