Heading out this morning at dawn, I-10 west for South Bay, 3 stops/meetings. I noticed my indicated outdoor air temperature is erratic, up and down within a range from -40F to +179F, occasionally going to a double dash ( -- ) suggesting loss of signal. I began to wonder how many different sub-systems are multiplexed with this signal and what the effect might be. If the ECM believed the OAT, would the engine shut down?!?! I don't know. And I am headed away from home, like in WW2 when the red light came on, you were at the point of no return. I went into my climate screen and disabled the OAT display, like a total dork, as if that would do anything; still showed on the dash.
Three hours later, my first meeting is done and I returned to my car which was now well soaked in the brilliant sun. Turn it on, AC on, fan fast, vents up, -- "geez its HOT today". Punched the "MAX AC" button, minute later, still blowing hot air. DaFuq?! I glanced at the dash with foreboding... FML - Damned OAT showed a steady -40F. Climate Control ECM takes its cue from the OAT signal. Yeppers, at -40F, the compressor would grenade - can't let it turn. I power cycle the car a couple of times... no difference. I called Ford.
Apparently my favorite Service Advisor is "no longer with the company"... terrific. Some other young lady answered the phone, well-intentioned, but unable to help. I now had to decide if I would cancel my remaining appointments and haul ass home with ice water and windows down, or if I would man-up and suffer the heat. I had a sudden brain fart. From my limited knowledge of very simple programmable logic controls, I know that out-of-range signals can cause all kinds of hell. But a dead or missing signal, will typically cause a well designed system to revert to a limp-home or emergency operation mode. So I called back to Ford and asked for the young lady from earlier. She was unavailable and I spoke to a writer named Ken. I asked Ken if he would please go ask the EV tech, where this OAT is and if I un-plug it to null the signal, will my AC work temporarily.
At this point, I am 99% sure the guy will be rolling his eyes and telling me to just bring the car in, etc. But he didn't do that at all. He was taking notes and wanted to clarify a couple things. Really??! In 2014 America, tort lawyers on every billboard, someone is willing to use their brain, realize they aren't talking to an idiot, and do the right thing for the situation. I was amazed really. Ken said he would call back shortly. Then --- He DID!! This is what I learned...
C-Max energi does NOT have an OAT sensor, per se. It utilizes some kind of odd-tech pressure/temp delta sensor atop the evaporator (yes - inside the car), that some how or another talks to some other sensor somewhere and manages to figure out both the inside and outside temp via one signal. Great! The tech advises the sensor may be accessible behind the glove box, that it is a two-wire harness. And that if it were disconnected, indeed the AC would revert to a closed loop and blow cold air. WOW!! I also learned that there is an existing TSB on this failure mode so maybe you should ask your dealer about it next time you are in for service.
As Ken was talking I opened my glove box and realized I would need tools to pull it out. Then, with the deliberate hope of a petulant child, I slammed the glove box door closed fairly hard, half-hoping it might jar this thing in close proximity behind it. I can't believe it, but the sensor came off -40F, still stayed wacky all day, but above zero indicated, and my AC continued to blow cold the whole day. Which is fortunate because FWY 60 was shut down this afternoon, causing grid lock overflow across the region. I would not have attempted to come home until after dark.
Off to the dealer first thing in the morning.