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I recently installed a non-OEM Gentex EC/Comp/Temp rearview mirror from a salvage GM; eBay find if you couldn't already guess. The custom fit wiring job was fairly simple and I'll explain it all once I get this one problem resolved.

Scouring through the Tundra's EWD for the back-up wire and routing everything through a scorching hot headliner and windshield, I finally got it nailed down and it functions almost perfectly, of course. I'm only having one problem, the temp probe is giving me a trouble code reading "SC" (Short Circuit) or 140F. The 140F reading is the highest the mirror goes until it reads "SC".

The manufacturer's troubleshooting guide says the SC code represents two possibilities: 1) a grounded temp probe wire, 2) bent or crimped temp probe wire. But neither of those are possible since I have run a single set of pvc shielded thermistor wires straight from the probe harness directly to the mirror harness, with both end connections are properly soldered. And there are no kinks in the wire; there is actually some slack so it arcs around any bends (firewall to A-pillar, A-pillar to headliner). And the temp probe is not near the radiator, so that's out of the question.

The only thing I can think of that could be another area to troubleshoot would be the sensor probe itself. SO I did that and tested it with an Ohm meter, which it passed. There is resistance in the sensor and when I warm or cool the sensor probe end, the resistance increases or decreases. I then plugged the harness into the sensor and checked the resistance on the other end of the 15' of wire, got the same result. However, I have no baseline to go by since this thing is used. SO I don't know what the proper resistance should read.

The mirror supposedly puts out 5V to the sensor probe through a "Low Reference", per the drawing schematic. SO I removed the Low Ref wire from the mirror harness and connected a low voltage to the wire going to the probe. This sent a low voltage signal to the mirror probe, so the probe can measure the resistance and send the mirror a temperature reading. Well, that worked and I got a somewhat normal reading of 83F. Remember though, this is a false reading because the low ref signal I'm sending is not a normalized signal that the mirror is constructed to read. I can get the same reading on the mirror if I by-pass the probe and just send that same voltage straight to the return line (from the temp probe).

One more thing to note, probably an "Ah-ha" moment here, there was some grease in the sensor probe connection. I cleaned it out as best I could using MAF cleaner because with it in there, the probe registered a trouble code "OC" (Open Circuit). Meaning the mirror wasn't getting any signal return from the low reference. Could this be the root cause of my problem?

Do I just need a new sensor?
I think AC Delco makes these specific probes for about $12.
 
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