Yup. the thin red wire with the black stripe. 
It drives the dynamic oil pressure warning system as well, which is why you have one even though you don't have a tach at the moment.
Installing a diesel cluster with a tach should be plug-and-play.
Vince, my alternator does not have a w terminal (PO owner put in wrong alternator apparently) and the red/black wire is just hanging. So my dynamic oil pressure warning is not working? Can I add a W terminal to my alternator?
I have a CE2 diesel cluster with a tach for sale. It works off of 2 triggers that you epoxy to the crank pulley. Installation is very easy. Should take 1/2 hr to 45 min at most.
Here's what it looks like. Has glow plug LED. You even get a LCD clock! There's a segment missing though but another LCD clock will fix that and they're easy to find.
My tach in the 92 has been working perfectly. Here's a video of it. (click to play)
http://s574.photobucket.com/albums/ss184/EcoDiesel/?action=view¤t=Video_091009_002.flv
Why won't it work with the wire to the w-terminal? How much?
Steve just to get the right thread. He wants 15 plus shipping. I'll test it first.
Vince, my alternator does not have a w terminal (PO owner put in wrong alternator apparently) and the red/black wire is just hanging. So my dynamic oil pressure warning is not working? Can I add a W terminal to my alternator?
Yup... you just add a wire to the diode bridge assembly thus:
Steve just to get the right thread. He wants 15 plus shipping. I'll test it first.
Its just the tach right? Or is it the whole cluster? And does it have the digital clock? Either way, please test it and let me know. If its in good shape and it works, I'll take it.
PS I can mount the tach in place of the analoge clock in my cluster? right?! Good to be asking this now, huh?!!!
Steve just to get the right thread. He wants 15 plus shipping. I'll test it first.
Its just the tach right? Or is it the whole cluster? And does it have the digital clock? Either way, please test it and let me know. If its in good shape and it works, I'll take it.
PS I can mount the tach in place of the analoge clock in my cluster? right?! Good to be asking this now, huh?!!!
yeppers its plug and play