tach wiring

[Otto]

Hello everyone,

I'm still wrenching away and I should be getting my freshly rebuilt engine soon. I have a 1972 Mach 1 with factory tach. My hasn't hasn't run in about ten years, which leads me into my problem. I'm checking my wiring and connectors and I can't find my factory connection. I had an MSD ignition but my ignition box disappeared. I'm trying to figure out if the I disconnected the tach wiring from the harness or if I can't find the connector. Can someone please help me out with pictures of the connector and where it meets the wiring harness?


My best guess is that it connects to the piece I'm holding.

 

[midlife]

Factory tach is in-line with the pink resistor wire and the wire going to the coil (positive side). With most aftermarket ignition systems, they provide an adapter for the Ford tach. Your MSD ignition box probably has (or had) that adapter.

 

[Otto]

Thank you for the quick reply. Is this what you are referring to?

 

[jbojo]

Are you going to return the ignition back to original or still use an ignition box? What distributor were you using? The wiring changed depending on what components that you were using at the time. If the tach wasn't working it was probably because you did not have the tach adapter wired in with the MSD box that Midlife mentions. The MSD box produces a square wave 22.5* output for the tach signal and doesn't work with the OEM unit. You can have RCCI modify your tach so that it will work without having to wire in that Tach adapter if you still want to use the MSD box. Those Tach adaptors are about $70.00 if you decide to keep the MSD system.

Thank you for the quick reply. Is this what you are referring to?

The wire color would be red-light green stripe that goes to the + side of the coil. At the back of the tach its a 2 pin plug with the same wire colors. This is the stock setup, you would have modified it for the ignition box.

 

[Otto]

I am buying another msd box. When I used it originally, I changed the connector on the wire for the tach to plug into the msd box and it worked. The reason i did that is the msd manual indicated it could be done. That is the only modification I made. The problem is I can't find the wire that I plugged into the box.

Forgot to add I'm using an msd distributor as well

 

[jbojo]

The diagram at the bottom of page 10 of the attached doc shows how your unit was probably wired up. At the upper left would have been that red-light green wire. There are some issues if you did wire your systems up this way and your tach would have not worked.

The connector would have been a right angle single wire rubber boot that pushed on to the + post of the coil. My guess is that you cut that connector off to install the MSD box.

6425.pdf

 

[Otto]

I think I'm going to hold off this problem until I install the engine. It's frustrating to pick up a project after do many years of it sitting idle. Thanks for the help

 

[Otto]

It's still bugging me. Can someone post a picture of the wiring that was mentioned, going to the coil and the tach wire? I have no idea what I changed years ago and I would like a visual reference for what is stock

 

[jbojo]

I don't have a pic but on the LH side of the engine compartment by the bulkhead you will see a flat 4 pin connector and the second pin will have the red-light green wire coming from the tach. This wire goes to 2 places, the + side of the coil and the left post of the starter solenoid.

EDIT: When the key is in the start position the solenoid is supplying 12v to the coil and the tach is disabled. When the key is in the run position the power to the coil is coming from the Tach through the resistor wire so the voltage is 9V, the solenoid is off so it is no longer supplying voltage to the coil.

 

[Otto]

Jbojo, I can't thank you enough. I think I solved the mystery. When looking at my wiring, I noticed a male connector but I knew it was connected to the tach output somehow, which is also a male connector. The male connector is the tach wire! I was missing something in the equation, the msd tach adapter. I completely forgot I used one until i saw a picture of the adapter. Mystery solved and my brain is now fried. What a headache it is to pick up a neglected project after so long. A million thanks Jbojo.

 

[will e]

Do you have to do an engine break in? If so, I suggest with the distributor removed you hook it all up, turn the ignition to the 'on' position (not start) then hand spin the distributor and confirm spark.

 

[Otto]

Thankfully no. It will be broken in on a dyno then checked for power output. I never thought of that though. Thanks.