well I pulled the instrument cluster to change light bulbs and now is the time to make the tachometer work correctly with the 5.0 eng. it is a 1999 4 cyl truck with a 1999 Mountaineer 5.0. what do I need to do???

 

Here is what you need

Only Pin 16 of the 16-pin grounded ==> V8
Only Pin 8 of the 10-pin grounded ==> V6
Neither grounded ==> 4cyl

A 4cyl wouldn't have a ground wire on either pin, so you will need to make a ground wire for pin 16, on 16 pin connector

1999 cluster wiring below
The tan/yellow wire on pin 15 is the Tach signal, so ground the next one pin 16

There are a couple of ground wires you can splice into for this, which is why I posted the diagram

 

Are pins available to put into this connector?

 

Think I just figured out that this was originally a 6 cyl car and not a 4 cyl like the last guy that had it had told me. pin 8 has a blue/red wire in it like a 6 cyl truck would have. Now I just need to figure out how to extract the pin so I can insert it in to the pin 16 slot.

 

Most cluster connectors use pins crimped onto the end of the wire, each pin has a tab that will spring out when inserted into a connector, holding pin/wire in place
So you use a small screwdriver to push tab in, on pin side not wire side of connector, and when tab is pushed in the wire with pin can be pulled out

 

well I was wrong about this being a 6 cyl truck, it was a 4 cyl. I just removed pin 8 ( blu/red) of the 16 pin and installed it on pin 16 location, (WRONG) nothing worked.
I put pin 8 back in it's original position and am still looking for a new pin to install the ground in pin 16 location. any ideas??

 

Only blue/red wire I see is on pin 8 of 16pin connector, its for dash lights, it would only have 12v with Park lights on, so no harm done
Pin 8 of the 10 pin is the one you wanted but not there on a 4cyl

pin 16 should be open and have the tach wire next to it a tan/yellow

In any case, wrecking yard would be where you could pull a wire from cluster connector and cut it off, long engine to ground it
Ford Used Black/white wires as grounds on the cluster if you want to match that up
If you can find a 1995 to 2003 with cluster already pulled out that would be easier to pull a wire from

An auto parts store will probably have "connector pin" kits
Pull out the blue/red wire and take a good picture of the pin
Then hit autoparts store
Dorman sells this type of stuff, sample here: https://www.summitracing.com/parts/r...SABEgKWjfD_BwE
You will also need some wire to add the ground