Got back in the car a week ago and although it started and ran, radio, fan motor, turn signals etc. did not work. Had a couple of fuses in the panel that were not getting power so I figured it was the ignition switch. Found a NOS D1AZ-11572-C switch with repair harness on eBay. Out with the old and in with the new. Looks like this had been done before. All back together and now my Temperature and Oil Pressure gauges are reading way high. (Fuel gauge not yet working - needs a sender.) I know there is no way this old 302 has 70 lbs. of oil pressure (It’s never read that high) and with a 180 degree thermostat I know it’s not hot. (The temp has been reading below halfway normally with this thermostat.)
I haven’t found a wiring diagram for this '68 XR-7 and I am totally mystified why the gauges are now reading so high.
Any ideas?
Some how you have bypassed the instrument voltage regulator on the back of the dash.
@xr7g428 That makes sense so I pulled the cluster (again) and checked/redid the connections to the solid state regulator that I put in from WCCC some time ago. Even thought that maybe I pinched one of those wires. They look fine. No change on the gauges. Still high.
Further testing. Key in On position: Reading 5.3 V to the Temperature Sensor so it seems the solid state Instrument Voltage Regulator is fine. Using the GT-1 in the M position and the Temp Gauge reads in the center.
Car running: 6.3 V to the Temp Sensor. 14.8 V measured at the battery.
Gauges still reading high when running.
you have a bad IVR. Be sure it is grounded where it is screwed down to the metal part of the instrument cluster. 6.3 volts will definitely make the gauges read high. Should be really close to 5 volts.
Update: Put the original mechanical IVF back in. Cleaned the mating surfaces with fine steel wool, used dielectric grease and added a redundant ground wire to the ground spot on the support behind the cluster. I can’t read the new voltage at the Temp Sensor because the mechanical IVF puts out pulses but the Oil Pressure gauge seems back to its normal self. I’ll confirm the Temp Gauge tomorrow when I go out but I suspect it’ll be happy as well.
Still doesn’t answer the question of why the solid state IVF would suddenly take a dump with an ignition switch change though.
Thanks for the help, @xr7g428
Use the gauge tester to confirm the circuit and IVR output.
Possibly the ground failed. Usually that kills the solid state IVR