Hi all,
So, some quick back story.
I had an '81 that had a 300 I6 and a DuraSpark II ignition. I installed an instrument cluster with a tachometer into it many years ago.
A few years ago, I removed the DuraSpark II ignition and installed a Davis Unified Ignition HEI conversion distributor/ignition.
I hooked the wire that originally went to the negative post on the coil to the TACH connection on my HEI, and my tachometer worked perfectly.

Jump forward to now.
The '81 was totaled, so I now have a 1985 302 that was EFI. It also has a tachometer.
I have since pulled the 302 out and installed my 300, along with the HEI ignition, and a Holley Sniper EFI.
The EFI has a tachometer out wire that I'm supposed to hook to the tach signal for the dash.

However... I can't remember how the tachometer was hooked up in the 81 and no longer have it to reference.
I have all the same plugs on the driver's side fender (both the EFI 302 and the DuraSpark II ignition had the same plugs) but don't know which wire sends the tachometer signal to the dash.

Can anyone enlighten me? I've found a few wiring diagrams for the DSII ignition, but none seem to point me in the right direction.
Thanks!

 

On page 26 in this link Start & Ignition - Gary's Garagemahal (the Bullnose bible) you will see the tach wire is a black/lightgreen stripe wire.

One page 29 in the same link above, you can see in the 5.0 fuel injected engine the tach hooked to a orange/yellow wire.

 

On the back of the OEM tachometer, there are 4 lugs.


B = 12 volt
C = coil
G = ground
8 = 2nd ground required for V8 application

The original wiring harness had but 1 ground, when the engine was the 300/4.9L engine.

 

Thanks for the info guys.

The schematic on page 29 seems to help point me in the right direction. There's a Dark Green / Yellow wire that goes straight to the tachometer, and looks to come into the engine bay and splits to go to the coil and EEC. I'll look for that DG/Y wire.

As for the back of the tachometer, I did switch remove the second V8 ground.

I'll let you know what I find. Thanks!

 

https://www.ford-trucks.com/forums/1...l#post17391426

The above link has a photo of the 2 wire connector you are looking for, you will feed the tach signal via the green wire.
The extra ground for a V8 is also provided by the same connector.
Jim

 

Thanks for the help everyone.

I'm still having issues with this though. Not sure if I'm missing something.

So, I found the plug in question. It has a green wire with yellow dots on it, as well as a solid green wire.
To confirm, I looked at the 302 I pulled out, since it still has all the wires attached to it, and confirmed that the DG/Y wire goes all the way to the coil.

To make sure I was getting a signal, I put a volt meter on the TACH wire coming from the Sniper EFI kit, and at idle, it was putting out around 6.5 - 7v.
I connected it to the DG/Y wire and.... nothing. The tachometer still reads 0.
It worked perfectly until I pulled the 302 out, so I know the unit is good.
Also, when I start the engine, I can see the needle "kick" about 50 RPMs, so it's getting some kind of signal.

Lastly, to confirm, I pulled my dash out last night to be sure I had pulled off the correct nut, and hadn't accidentally removed the G instead of the 8.



I'll physically pull the tach out and hook it up in the engine bay (Ground, 12v, Coil) to make sure it's working.
If all else fails, I can run the wire all the way up to the back of the dash and hook it directly to the COIL wire, but I'd really rather not and use stock wiring.

Any ideas?

 

Did you try connecting the "tach" wire from the Sniper EFI kit to the green wire?

 

Isn't that just the ground?
I tried putting the wire into the hole in the plug, but I haven't actually cut and stripped the wire and connected them like I did with the Green/Yellow wire. I definitely could though.

 

On my 1986 with factory 4.9L, there is only the solid green wire for the tachometer. It did not work when I switched from Feedback to Duraspark, because I used a MSD 6 AL ignition box, which sent out multiple sparks at lower RPMs. The green wire was connected to a "tach" outlet on the ignition box and all worked beautifully.

 

I'll give it a try.

 

The tach goes to the ground of the coil in case that is where some confusion is coming from. The coil gets 12v from the ignition, and is turned on and off to create a spark from turning on and off the ground to the coil. This on-ff is what the tach uses to get a rpm signal.

You can test tach's on the bench. Power them up properly, and then take the wire that goes to the coil ground and drag it/brush it across the ground or negative on the bench setup. This intermittent grounding from the copper hairs on the wire, will create enough signal to make the needle jump up some.

 

That's good info.
With the EFI kit, the coil wire goes to the kit, and then there's a dedicated tach wire that comes off of it.

If this is the case though, does this sound like I can't use the stock tachometer with the Sniper EFI?
The "tach" wire coming from the unit is a steady voltage.

Does the stock tach work differently than aftermarket tachs?

 

It works the same as the aftermarket units. To really measure the square wave going to the tach, you need to use a old dwell meter or some other type of meter that can measure a frequency. A regular meter just averages the pulses together.

 

I am just having no luck with this.
I tried the other wire and nothing.

Pulled out my other gauge cluster that has a tach in it and hooked it up. B to the battery, G to the ground, and C to the wire from the EFI, and fired up the engine. Nothing happened. The tach in this cluster doesn't work very well but it should at least have flopped around.

I'll try pulling the good tach out of my actual gauge cluster and hook it straight up in the same way and see what happens.

So far though, I'm not sure what more to do.

 

Confirming the tach wire: There are two sets of dark green / yellow wires going into the cluster. One has DG/Y the length of the wire, unbroken. The other is a DG/Y with the yellow "dashed".

The tach is the dashed yellow on dark green.