Aftermarket tach install

[dimitrisprophet]

Firstly, I appologize for making so many post recently. Although many topics are unrelated so I believe they deserve their own posts.

 

So my '83 Z24 didn't come with a tach on the cluster, and the p/o left a part store universal tach in its box with the truck.

 

I understand that I tach needs 12v, a ground, and a wire that receives the firing intervals. I know that I can get this signal from the negative side of the coil, although since there are 2 coils, do I need to hook it up to both?

 

I also know that every 720 has the wiring for a tach taped up even if it didn't come stock on the cluster. Is it possible that I can get this signal wire from the pre-existing harness behind the dash?

 

Many thanks 🙂

[dimitrisprophet]

So this is on a '86 D21, but I believe it has the z24 as well.

 

In the picture is the wiring harness that goes to the secondary ignition coil, and he has the tach signal wire attached to the blue with red wire.

 

Would this work on my 720?

 

[datzenmike]

On the '83...

Along side the steering column just under the dash are two 10 pin connectors. These connect the dash to the cabin harnesses. The connectors are White and a Blue one. The White connector has a tach signal wire that is Black/Blue stripe. The Black/Blue wire goes back and through the 2.2K ohm resister and to the ignition coil for the intake side plugs. NO you don't need to connect to both coils.

 

I don't know if your after market tach will work with this resistor or not. If not, you'll have to find the resistor that is in line, remove it and/or jumper around it. I think on the '83 the tach resistor is near the coils and the left front headlights. It's tapped up in line and looks like a lump in the harness.

 

Find this, and you have it...

 

[dimitrisprophet]

Okay, so I found the 10-pin white connector with the black/blue wire under the dash. 

 

I also found white wire connector next to the coils that looks like the one in your picture. Although, it isn't plugged into the black thing like your is (is that the 2.2k resistor?).

 

Should I splice the signal wire to the wire before it goes into the little white connector, and get a resistor that plugs into it? Or should I wire the signal wire to the black/blue wire under the dash?

[dimitrisprophet]

[datzenmike]

It should say 2.2K (thousand) ohms. Even if you did not order the optional tach it's cheaper to make ALL wiring harnesses with the resistor in place.

 

I cut one apart once and there is nothing but a generic 2.2K ohm resistor in there. You could buy one and wire it in if missing.

 

[dimitrisprophet]

Just in case someone else is trying to do the same, this is what I figured out.

 

For an aftermarket tach, you do not need the 2.2k ohm resistor. 12v red wire to 15amp fuse (keyed on) for power, ground wire to under the dash, green tach signal wire to the negative wire on the intake side coil.

 

Also, there is a black and a red wire going to the coil, but do not assume which is positive and which is negative. I originally tapped into the black wire and it didn't work. I pulled the rubber topper off the coil to realize that the black wire was positive and the red wire was negative. Connecting to the red wire made the tach work.

 

Question: Which wire on the dimmer harness should I tap into for lighting up the tach?

[datzenmike]

 To be dim-able a light must be connected between the Red/Blue and the Red/Yellow wires in the illumination circuit. If your after market tach only has one wire do what I have done in the past. Connect the tach light to the power wire. It will be on all the time the ignition is on, even in the daytime but you won't notice it.

 

 

The coil terminals are marked + and -.

[dimitrisprophet]

Sounds like a plan, thanks

 

And yeah I shouldn't have assumed that the black wire was negative and the red wire was positive, although I've never seen it be the reverse.. weird