On my '79, the coil needs 2 ignition switched hot leads to function. I am not sure if this is the case on the '81 coil, but it might still hold true. If that is the case, then the remaining plug is probably for the tach sender. What you need to do to see if you have factory installed tach wiring is:

1) Pull the Gauge Panel
2) Identify the pin for the Tach. It is most likely on the smaller of the two round plugs. I can identify the pin on mine, and post it for you later, though there may be variances from year to year. The best bet is a stare and compare between your gauge panel and the pinout in the back of that FSM posted earlier, wrong year FSM or not.
3) Tag, and then disconnect, the two plugs from your coil.
4) Get out your meter. If you have a continuity tester/toner setting on the meter, set to that. If not, set the meter to measure resistance in Ohms.
5) Stretch one lead into the passenger compartment of your truck. Plug it into the harness, on the circular gauge panel connector, on the wire that you have identified as your tach sender wire
6) Check for continuity between either of the two plugs at the coil and the pin on the harness. If one of them has continuity, you have working, factory installed tach wiring. If not, you don't.

If you don't have a working, factory installed Tach sender wire, remove your meter from the gauge plug, switch it to DC voltage, plug one probe into one of the coil plugs, and set your ignition to on. Check for DC voltage to ground (11-14v). If both of these leads are 12v hot, you don't have factory installed tach wiring.

If you identify which pin on the round harness plug for the gauge cluster should be your tach sender, and there is no wire in the plug, you probably don't have factory tach wiring.

If you don't have continuity to the gauge panel on either wire, and you don't have 12v hot on both of the wires, you most likely have a blown fusible link wire on the tach sender wire. Run a new length of primary wire (#12 AWG works great) from the tach sender plug to the correct position on the back of the circular gauge harness plug.

If you don't have factory installed wiring, you will need to run a new wire to the tach from the last plug available on the coil to the correct position on the back of the gauge cluster. You will need to get a pin that fits in the circular harness, and connect it to your new wire.

Good luck, post results!

 

well been busy but i pulled the dash out and ohmed teh larger wire, and got feed back from every wire behind the dash.

then i ohmed the smaller wire and only got feed back from pin 12 i think on the bug plug.

 

This link was given to me by aquayota here on these forums and you can download the FSM in a pdf format.

http://www.ncttora.com/fsm/

 

that is not all correct for the 81

 

i think the tach is burnt out.. and now on to my dome light not working,

new pics.