Tachometer Wiring
#1
I installed a vintage Dixco 3 wire tachometer and wanted to make sure it is wired correctly.  The wire colors are green, red and black.  Right now I have it wired as follows and the tach is working even the light:

Green wire- negative side of coil
Black wire - grounded to body
Red wire - cluster illumination 

A friend of mine thought the red wire should go to a 12 volt source so I plugged it into a switched accessory and the tach still works but the tach light does not work when I turn on the headlight.

I thought when I first installed the tach I found the correct wire diagram online for it but now I can't find it again.  If I find anything for a Dixco 3 wire tach the wire colors are white, black and green.

As I mentioned the tach appears to be working properly even when I turn on the lights the tach seem to work and not effected by the load of the light. My friend thought the tach would not read correctly when I turned on the lights because there is no 12 volt source.

Any thoughts on how I have this wired?  Is the way I have wired right now to the negative side of the coil provide a 12 volt source?  

Mike
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#2
My Autometer tach has 4 wires.

Green - Trigger from negative side of coil, ignition box tach terminal, etc.
Red - 12V switched power source.
Black - Ground
White - Light

Are you certain your tach isn't missing a wire?  I'll bet your tach is an older technology design and not the "Air Core" design of modern tachs.

Autometer does make tachs with modern technology and old school looks.  But they are not inexpensive.  I will say, after I switched to a new Autometer tach, I was surprised on how much quicker they respond to engine RPM changes than the older technology.  I like the one shown on the top.  But I installed the one shown on the bottom only because it blends in with the 1969 Mach 1 interior a little better.

The way you had it originally connect it's getting a voltage source when the points are open.  But, that's less than 12V because of the resistor wire so I don't if that will any affect on accuracy.  If it were mine I'd open up the tach and see where that red wire goes.  If it's for the light it would be directly connected to the light socket.  Do those old tachs ground through the body of the tach?  Opening it up would verify that as well.

[Image: atm-2897_ml.jpg]

[Image: atm-2894_ml.jpg]
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#3
Thanks Mike!

I will have to open up the tach and see what is going on inside.  I found something online that shows the Dixco 97 tachometer having green, black and white wires.  Maybe there is a chance someone spliced in a red wire in place of the white. From what I have been reading if these are not wired correctly they do not work.  Plus the more I research this the more confused I get. I might pick up one of the Autometer tach's like the top one in your picture.  But I like the vintage look of the one I have in the car now.


Attached Files Image(s)
   
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#4
Hey Mike as far back as I can remember all tachs had 4 wires. And they were always red 12v hot, black ground, green negative side of the coil, and the white went to one of the dash lights, so it would be controlled by the dimmer function of the headlight switch. If it only had 3 wires? I would think the red should go to 12v hot, black ground, and green to the negative side of the coil. The light in the tach would simply be illuminated all the time. you wouldn't notice it during the day, and at night you wouldn't be able to change the brightness, with the dimmer function. So if you dimmed the dash lights, the tach would stay bright. I'm not 100% sure but that's all I can think of. Hope it helps. 

JTS
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#5
(12-18-2018, 12:56 PM)JTS71 Mach1 Wrote: Hey Mike as far back as I can remember all tachs had 4 wires. And they were always red 12v hot, black ground, green negative side of the coil, and the white went to one of the dash lights, so it would be controlled by the dimmer function of the headlight switch. If it only had 3 wires? I would think the red should go to 12v hot, black ground, and green to the negative side of the coil. The light in the tach would simply be illuminated all the time. you wouldn't notice it during the day, and at night you wouldn't be able to change the brightness, with the dimmer function. So if you dimmed the dash lights, the tach would stay bright. I'm not 100% sure but that's all I can think of. Hope it helps. 

JTS

Thanks JTS...is there any risk of burning out points running it with no 12v hot?  As 1969 Mach 1 mentioned I am getting my power supply when the points are open. When I plugged into a switched accessary which would give 12 volts hot the light did not work. I need to put a test light to that and make sure I am getting power at the connection.

Mike
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#6
I'd open up the tach.  The case is usually very easy to remove.  If that red wire goes to something other than only to the light socket, then JTS is right for the wiring and the light is meant to stay on all the time.
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#7
(12-18-2018, 08:56 PM)Mustangmike Wrote:
(12-18-2018, 12:56 PM)JTS71 Mach1 Wrote: Hey Mike as far back as I can remember all tachs had 4 wires. And they were always red 12v hot, black ground, green negative side of the coil, and the white went to one of the dash lights, so it would be controlled by the dimmer function of the headlight switch. If it only had 3 wires? I would think the red should go to 12v hot, black ground, and green to the negative side of the coil. The light in the tach would simply be illuminated all the time. you wouldn't notice it during the day, and at night you wouldn't be able to change the brightness, with the dimmer function. So if you dimmed the dash lights, the tach would stay bright. I'm not 100% sure but that's all I can think of. Hope it helps. 

JTS

Thanks JTS...is there any risk of burning out points running it with no 12v hot?  As 1969 Mach 1 mentioned I am getting my power supply when the points are open.  When I plugged into a switched accessary which would give 12 volts hot the light did not work.  I need to put a test light to that and make sure I am getting power at the connection.

Mike

I think I found something that will take all the guess work out the wiring.  A guy I work with has this tach he said he would sell me.  It's basically brand new.  He decided to go with a tach that has a shift indicator light


Attached Files Image(s)
   
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#8
There you go!  I have the same tach in my Mach 1, only in the satin black finish to help it blend in with the black interior.  I'm very happy with it.  Extremely quick response, accurate, and easy to read when you're driving.
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#9
(12-19-2018, 10:04 AM)1969_Mach1 Wrote: There you go!  I have the same tach in my Mach 1, only in the satin black finish to help it blend in with the black interior.  I'm very happy with it.  Extremely quick response, accurate, and easy to read when you're driving.

He is bringing into work tomorrow for me to look at.  Shouldn't the needle be at zero RPM's?  I just picked up on this.
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#10
With the modern "Air Core" technology type of tachs the needle is at whatever RPM the engine was when shut off.  I don't understand why.  But that's how they operate.  It will drop to zero by simply cycling the key and not starting the engine.

So his motor was at 1300 RPM when shut off.  If that's a hot idle it has an aggressive cam.  And when you grenade your motor at 7000 RPM, the needle will tell your wife what you were doing.  Laughing
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