Thanks for all the help everyone. I am pretty sure I was hooking up to the right wire.
So, I think, somehow, my tachometer died between pulling the 302 out and putting the 300 in. No idea how.

I pulled it out of the dash and hooked it up directly. Made sure the B and G prongs had 12v between them, then put the tach wire on the C. Nothing. Nothing I did even made the needle wiggle.

Got out the bad tachometer from the other gauge cluster (it only works over 2000 rpms) and hooked it up the same way. As soon as I touched the tach wire to the C, it'd jump.

So, again, I have no clue how, but it seems it's dead. Maybe I put power to it wrong and burned it up? I opened it up and didn't see any char or damaged components, but it doesn't do anything.

I saw a tach in a truck last weekend out at the junk yard, maybe it's still there.

 

This is the plug on the back of my 85 300 bronco WITHOUT a tach. Might be able to eliminate some wires this way. Green powers the small gauges. Brown is the voltmeter. Blue is indicators. Red is ground. Colors not specific to hot/ground



This is my diagram, showing which inputs go where. Sorry it's a mess. Start at the dual row of blocks in the upper right. The hard to see silver line is the fuel level sender wire.



plug, just for reference I guess

top view


bottom view

 

Thanks for the diagram and pics, nice work on tracing all of that out!

In this case, it looks like my tachometer really did die.
I just so happened to find another one in the junk yard (first one I've seen in a while now, so I definitely lucked out). I hooked it up in the engine bay and it worked immediately.



On the V8 wiring, it is indeed the light green wire with the white. So, I have a stock tachometer in my dash again, and everything's working perfectly.

As for my original tachometer.... I have no idea why it died. It's worked flawlessly for the last 15 years, and it dies while sitting in the garage during an engine swap? Seriously, what are the chances??
On that note, is there any way to repair them? Who knows how long the one I have will last, and I would like to repair my old one. The stock tachometers seem to be getting much harder to find, and getting much more expensive.
As I mentioned earlier, I opened it up and there looked to be nothing wrong with it. No burned circuits, no damaged parts. The needle moves nice and free, and has a healthy "spring" to it, and returns to zero when let go. Everything about it seems to check out. Just.... dead. Any ideas on how to diagnose what happened to it?

 

https://www.rccinnovations.com/index...enu-trucks-all

 

Thanks! Expensive, but really not too bad. Especially with updated circuitry that'll most likely last a long time.

Also, he says that he can only do the enclosed tachometers, not the open circuit board style.
Fortunately, the one that died is the enclosed style. The one I got from the junk yard is the open style.

This did bring up another question. What is the difference between these and why are there two designs? Even more interesting is, when I compared them, they both had the exact same part number, all the way down to even the sub-model number.

 

I recall finding a thread on those tachs that had instructions for cleaning the PC. There was some sort of film or something that would develop on the PC & keep the tach from working. Do some searching on Google. It may work on yours.

 

By PC, do you mean Printed Circuit?

If so, the "new" tach I got out of the junk yard works with the printed circuit. Also, I hooked up the previous one in the engine bay with a set of alligator clips/wires and it had no signal (the new one fired right up as in the above pic).

 

Yes, PC = printed circuit.

https://www.ford-trucks.com/forums/7...dywhompus.html

 

From what I have heard here, Ford changed suppliers and updated the technology at the same time. That tech of that old variant is from the 1970s, looking at makes me think of vacuum tubes in TVs.

Compare the replacement, it's got an actual IC chip on it. At least late 1970s technology, the serious beginnings of the electronics/computer revolution of the 80s & beyond.

I haven't seen the guts of a modern one (that's set up to plug into a MSD ignition) but I imagine it's little more than a matchbook-sized circuit board.

 

Oh!! I was thinking you meant the printed circuit board on the back of the instrument cluster, not inside the tach.

I'll definitely check that and clean it up. I'll let you know if it works!

 

Definitely interesting stuff.
Do you know which is which though? As in, which style is the new and which is the old?

I'm guessing the enclosed style is the updated version.

 

Yes, the enclosed one, with the smaller parts:

 

The one you have pictured looks like the non-enclosed one.
Unless, we're thinking "enclosed" in different ways. I'm thinking this one is enclosed because it has a removable cover that it's "enclosed" in.

This is the one I removed that died:



Also, it looks as if there is NO deposits or build up on the terminals (as per that link above about the corroded PC). I was hoping there was so I could clean it up and get it working again.

 

Here is you some reading material, Justin: https://www.ford-trucks.com/forums/1...g-numbers.html

 

Thank you David, looks like you all had quite a discussion on this already!
That led me to this page, which looks like they got from your discussion. It summarizes it all quite nicely.

Tachometers - Gary's Garagemahal (the Bullnose bible)

Looks like the "enclosed" style, with the back cover that goes on, is the older style. And the open one is the newer one with the integrated circuit.
My old one that died was the older enclosed one, and the new one I got out of the junk yard is the integrated circuit one.

However, one thing that doesn't align with what is documented on that thread is the part number.

BOTH my older enclosed style and my newer open style tachometer have the same part number printed on them. And it's printed on the back (a part that does not interchange between the two, like the faceplate).
E0TF-17360-AA

I'm certain the newer one was not an E6, but an E0.