UPDATE: A friend's machine shop completed the drill and tap project for the oil drain plug temp sender Friday. Changed the oil and filter and used the new drain plug. Wired in the gauge using the above wiring "guess". Gauge red to positive connection on cluster, black to dash ground frame. Installed a yellow wire connected to the gauge yellow and connected it to the sender center terminal. So far, no indication that the gauge is alive, no temp reading. Will have to do further continuity checks, making sure my 12v source is real and there is electrical continuity at the sender. Only used thread lock on the sender threads as it will never be removed, only the drain plug. That shouldn't have affected the electrical continuity of the sender. Any suggestions from the group? My take, if the engine operates at the thermostat temp of 190, the oil temp should rise at least in the 160 degree range. Open to ideas.

Roger
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'85 240 DL B230F 4spdOD 330K; '87 740GLE B230F ZF22 340K