This old tach is in my boys truck. I don't know what brand it is...but any idea how to wire it? It has 6 wires- blue, yellow,green, black, and two reds. Does it need one of those old "tach sending boxes" to hook it up?

Probally not.I have an old SW that needs a box and has just 2 leads.Did you do a google search on vintage tach wiring?If it has a light at least 1 if not two will be for that.Rocky

In googling...I'm guessing maybe green going to coil, blue and black are ground, reds are to ignition positive, yellow to lights.

Probally close to a newer tack wiring.Id say reds are hot.can you see what runs to the lights?Might eliminate at least 1 color.

Alot of wires, so it has electronics insides and is not a 2 piece design...
Suggest U take the back cup off and check inside as the circuit board is often marked by function such as 12V+, ground, lite, coil..

Just my $0.02...

So did I..

And found this..

http://www.jalopyjournal.com/forum/threads/tach-wiring-help.164893/

the diagram makes sense but I am curious why there is 2 seperate wires for ground and 2 seperate wires for hot as opposed to 1 wire for each & doing the T ing inside the cup

OZ...that's the same pic I was looking at!

Robert...I had same thought...I just guess they wanted the lighting and tach circuits separate???

Because the tach can be wired for a positive ground or negative, and 12V or 6V..
Means that internally the primary electrical circuits are floating above ground..
That way it is compatible with virtually all cars, 6V negative ground for VWs & some pre-56 US cars, 6V positive ground for certain pre-56 US cars, 12V negative ground for 56 and later US cars, 12V positive ground for 56 Studebakers/Packards. Today all 12V cars are negative ground.


Just my $0.02...

And don't forget the old Triumphs/English stuff with positive grounds too...


(Came to mind because I was just talking to a buddy in Oz about an old Herald. )

Noted

O.P., if you have the case apart, is there a switch for 4,6,8 cylinder[s] ?

Ok...I gave some bad info in my original posts...somehow I got confused from all the stuff I googled.

It actually has 2 red wires, one yellow, one blue, one black.

I found someone on another board from 2006 with a tach that looks the same...but I pulled the back off and mine looks different inside. Mine had a brown circuit board inside and it had a little dated slip of paper inside that had the year 1968 on it. (seemed early for a circuit board doesn't it?) but I could only take it apart so much without breaking it. I'll post a pic tonight.

Here's the link to the forum talking about the same thing...but how do I know which red goes to ground?. And I can't see if one red goes to the case or not like that pic shows as my tach is different inside:

http://www.hotrodders.com/forum/tach-wiring-help-91971.html

It has a dial switch inside that lets you choose 6 cyl or 8 cyl.

No 1968 is not to early for a circuit board , sounds like a wire is missing ?

I don't think it's missing wires....I have same 5 wires as the two guys on other forum. Just...how do I know which red is ground?

I cant see red being ground but that aside I would ohm red & see if it has continuity to the case/shell which would be what needs to be grounded I would think