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So here I have an 86 cluster without tach from the truck compared to an 82 cluster with tach. The yellow circle is one difference, I don't know what that 3-prong plug does. And in red, I assume this would be the wire I attach from the coil ground side? Could I just run that wire to the screw that the pin runs to? And that switch on the back of the 86, anyone know what that is all about?
https://imgur.com/q0yrXPL
I know little about the Gen 1 cluster, but try looking in the Manuals section. There's a complete 1980 service manual which might help with the 82 cluster, and a wiring diagram of the 86 truck. Together they might answer your questions.
http://www.mightyram50.net/vbulletin...ory-manual-PDF
http://www.mightyram50.net/vbulletin...Wiring-Diagram
I wonder if that switch might serve the same purpose as the one at the bottom of '87-on 2nd-gen clusters -- flipping that one just resets the Check EGR light in the cluster that comes on every 40k(?) miles.
What an odd function.
I assume that the only pin that is different between the two is the tach. The three-wire connector may just be for warning lights that nobody needs? Gauges FTW.
If you follow the traces on the board they actually identify what circuit they operate (IGN/ignition,FU/fuel etc) The 3 pin spade connection on the '86 cluster you have circled powers the instrument lights for high beam and brake warning display. The threaded bolts on the '82 are circuits for the fuel gauge and live power from the ignition (shows charge light and oil pressure etc) You are right that the multi pin socket carries one extra connection for the tach feed. Also take note that your '86 cluster is marked "4WD" so there will probably be an added circuit to activate the 4WD engaged light