My 2008 over revs above 96 degrees OAT.

 

Wow Steve, interesting info. Does it go up to 2500?
Mine is either idle or 2500... There is no in between.
Our 08 definitely did not do this making it much more fuel efficient on hot days (normal, non-hypermiling driving.)

 

It seems to vary by speed. At around 35 it goes to 2000, which is almost 500 above normal (at cruising - not accelerating). At about 65 it is only about 200 or so above normal. I can tell immediately when it happens, and it is simple to confirm - just turn off the compressor and the Revs drop immediately. It seems a bit stupid to me, since my understanding is that the A/C compressor can run by itself to cool the battery, and presumably is running for that purpose. The behavior appears to be tied to the cabin A/C switch.

 

Yes, I agree. If I turn off the a/c the rpms instantly drop. There is no compromising for good mileage when in that higher rpm mode. It's a fuel sucking strategy. I'll bet it's no longer an issue on the newer models that no longer cool the battery pack other than to use interior cabin air for that purpose. That was a pretty smart move as the interior of most vehicles would most likely be around 76.

 

Right, no dedicated OAT sensor. I believe the first gen FEHs infer (predict rather than directly measure) a value for OAT from the measured IAT........for PCM purposes.

Some owners installed the Ford/GM/OEM review mirrors that display compass direction and ambient temperature....but that sensor is not connected to the PCM (I believe the instructions say to install that sensor low and forward in the front fender structure).

 

If you have a SG-II you can monitor the IAT. You'll clearly see that if idling at a traffic light the IAT will rise way above OAT but as your moving it drops closer. It can hardly calculate OAT from IAT easily if at all.

 

Right, but what does the Powertrain Control Module (PCM) really need Outside Air Temperature (OAT) for?

It measures what it needs,......the temperature of the air entering the engine (IAT).

 

I don't think that it does. Your the one that stated that first generation FEH infered it from IAT, I didn't. I'm just saying that infering OAT from IAT is next to impossible.

Other than the OAT location behind the grille, the OAT isn't even shown in the electrical diagrams.

 

Emailed my service rep about this and was told he's never heard of it and no longer has a certified hybrid tech, two things I didn't want to hear. He suggested a test drive.

 

Rick, it's over 300 bucks to get it done once you're out of warranty, so don't delay.

It is not coincidence that the feh over revs above 92 degrees. So it is monitoring air temp from somewhere.