i found this info for my 1975 plymouth trailduster with a V8 engine
theres another way to check the coolent temp sending unit on the  87-88 models

disconnect the wire from the coolant temperature sening unit
connct the lead of a 12 volt test light or the positive lead of a voltmeter to the gauge lead wire that was disconnected.
connect the other test lead to an engine gound.
with the ignition switch set to on or to accessory position a flashing light or fluctuating voltage should indicate that the instrument voltage regulator is operating and the gauge circuit is not grounded.
if the light stays on or the voltage reading is steady the instrument voltage regulator is bad.if no voltage is indicated by the voltmeter or test light check for an open circuit in the system.
if all of the above readings check out correctly yet the gauge does not work properly the gauge sender needs replacement.
.

cool, will make a good FAQ, thanks!

Any mods or site assistants, if I haven't moved this into the FAQ section in a week or so, please move it, thanks.