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...to consider besides the warning light circuit. An Ignition Switch problem might also account for the Fuel and Temp gauge problem and the "OD light" that's On when it should be Off.
Interruption of power (red wire) provides the ground for the idiot lights to illuminate. Check.
[Part of the ground path. See blue wire.]
Interruption of the ground (blue wire) interrupts the power to the lights. Check.
[No, it opens ground path from the lights, same as red wire.]
The overdrive light coming on suggests there is power to that lamp. Yes.
[YES, but that power is separately hard-wired to the panel at push-on terminal 34, Yellow-Red wire.]
The overdrive light coming on suggests the power has been cut to the overdrive solenoid. Yes? Actually "No".
The Upshift Arrow light ON means no power to the OD relay, which should be automatically energized when the Key is turned on. The Arrow bulb voltage is hard-wired (see above). The ground side is thru the (de-energized) OD relay.
1) The source of this missing power at the OD relay is a Blue-Black wire from Fuse 11. Fuses 11-12-13 get their power via Blue-Yellow wire from Ignition Switch terminal 15I.
[Check for +12V at the LEFT (common) side 11-12-13 fuse contacts with the Key On.]
2) The power for failing Warning lights, as well as the Voltage Regulator for the Fuel and Temp gauges, comes directly from Ignition Switch terminal 15 via a Blue wire to Instrument Cluster terminal 233/3 (see 3-pin push-on connector at the back of the speedometer).
Ignition Switch terminal 15 circuit also powers Fuse 5 for Turn Signals and Backup Lights.
[Check for +12V at the LEFT side fuse 5 contact with the Key On.]
--
Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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