[87Bob]

First, thank you responders for your help now let me see if I can answer your questions. In doing a wire trace this is what I came up with:
As viewed from behind


Orginal cluster
Left Right
10 Fuel Gauge High Beam 1
11 Brake Warn Light Cluster Lights 2
12 Common for #2,#4 Volt Meter 3
13 Choke Warn Light Volt Meter 4
14 4 WD Warn Light Oil Pressure 5
15 Seat Belt Warn Lt Gauge Common 6
16 Rt Turn Signal Not used 7
17 Left Turn Signal Not used 8
18 Ground for #8,#7 Water Temp 9
Cluster Lights


Tach Cluster
Left Right
10 Not used High Beam 1
11 Brake Warn Light Cluster Lights 2
12 Common for #2, #4 Volt Meter 3
13 Choke Warn Light Volt Meter 4
14 4 WD Warn Light Oil Pressure 5
15 Seat Belt Warn Lt Gauge Common 6
16 Rt Turn Signal Fuel Gauge 7
17 Left Turn Signal Fuel Gauge 8
18 Ground for #8,#7 Water Temp 9
Cluster Lights

Phillatruck:
If I knew what year the Tach cluster was supposed to come from it would only answer half of the story. I believe that I traced down each wire on the circuit board and I know where they go except for the additional wires in the additional harness. I have an 82 shop manual which still shows a schematic for a tach equipped cluster. However the color codes are different then what is on the 85 truck and on the pigtail stub that I got with the gages. I did have the cluster checked by a guy the does custom gage repair he said all of them checked out as good. Also thanks for the tip on Crossy I’ll do a search.

BLE Burban:
*)Both Clusters have Volt gages and Elec. Oil pressure gages.
*) I looked for +- markings and found none on either cluster.
2) Does anyone have an 85 Truck apart that has a factory Tach that can shoot me some pictures? I believe G.M. stopped putting tachs into the trucks in either '82 or '83 Thanks for the info!4) Can I just move the wire in the loom to the new position for the gas gage or do I even need to? Yes you will need to as the sender circuit is on the opposite side of the plug from your original cluster Ok I’ll move the wire in the plug for the gas gage!80Sport:
Thanks for your input as well It would appear that I am closing in on getting this all figured out. Thanks for your help guys! Sorry the charts run together but when I edit them it looks right on the screen till I run preview then it bunches them together.