Almost all modern day HVAC, automatic climate control systems, have an OAT sensor as a control input.

 

Gary, the FEH speeds up the RPM by 500 when the A/C is switched on above 96 degrees. This is definitely tied into the OAT. That is one known purpose of the OAT tied into the FEH cpu.

 

True but it only controls the HVAC system operation.

 

Outside temperature effects all these sensors in some way but the computer uses these sensors to control all the functions of each system. Sure, when it gets around 96F OAT the sensors detect a need to increase compressor output and the PCM commands a higher engine idle. The computer does not rely on the OAT to regulate the engine idle with the A/C Off, only when it's On. Now if the Aux battery A/C is On which you have no control over, the sensors may cause the PCM to raise the idle also if needed. In other words the computer might require a combination of sensor inputs to the PCM before the PCM takes action changing fan speeds, increasing idle, changing timing or air/fuel mixtures etc. I don't think the PCM is effected by the outside temperature gauge the newer FEH's have but it certainly could be a source the PCM could consider with the other sensors if the engineers thought it was necessary. The PCM can't use OAT to regulate coolant temperatures, battery temperatures, engine temperatures, transmission temperatures, A/C compressors or evaporators on their own. For instance, if your driving on the highway with OAT at 100F at 70mph where there is no problem with the evaporator temperature or cabin A/C, you may find at a stop on the exit ramp the idle is normal.

Remember, the PCM is programmed to react on the MAF/IAT sensor readings together with all the other sensors. If the other sensors are out of normal range you may get a warning light. Same thing with vehicle speed, the engine and generator shaft, the eCVT (TCM) and ABS front wheel speed all have to be within range or you will get a warning light. This is why you can't just change to a tire/rim size much different than the stock size without a warning light. The PCM can be programmed to the new size tire/rim and gear ratio and stop the warning light but you see how sensors can work.

New for the '09 FEH is the "Enhanced Thermostat Monitor" which helps reduce the time it takes to identify a thermostat concern. This monitor uses intake air temperature, engine rpm, vehicle speed and engine load to predict engine coolant temperature. If the coolant temperature is to low, the thermostst may be stuck open and a DTC PO128 sets. Here again, the PCM is calibrating temperatures, speed, rpm and load all at the same time.

GaryG

 

Gary,
Lots of interesting info there.

I'm am not saying the OAT is used to "regulate coolant temperatures, battery temperatures, engine temperatures, transmission temperatures, A/C compressors or evaporators on their own"; I'm saying that OAT is fed into the CPU and used under certain conditions.

Nothing in your long quote explains the 96F shift in engine behavior. It appears to me that the engineers DID think it necessary to up the RPMS by 500 when the A/C was selected above certain temperatures, and knowing the outside temperature in the CPU is the only way this is possible.

 

I think Desertdog (CarlD) explained in a post on CleanMPG at what point and from where the high idle was signaled. I've read both the '05 and '09 FEH PCM/ED manuals and have never seen an OAT temperature sensor connected to the PCM other than the MAF/IAT sensor. In order for the PCM to recieve a signal to increase the idle an OAT sensor would have to be connected to it. The PCM/ED manual list all inputs and no separate OAT sensor is connected according to both the '05 and '09 manuals. The PCM could infer the OAT from the IAT sensor but why would it need to? The PCM could be calculating the reading of those sensors I mentioned to determine the need for a higher idle. I've read that the reason Ford eliminated a separate coolant sensor was to reduce cost of the sensor, labor, and wiring. Coolant temperature is inferred by the PCM from the CHT sensor in our FEH and it does not have direct contact with the coolant.

I have an a separate wireless OAT gauge for my '05 FEH and the placement of the sensor is very sensitive in getting a true OAT reading. The '09 FEH has a OAT sensor for the driver, but I see no connection to the PCM in the PCM/ED manual.

GaryG

 

Well Gary, what can I say. I don't think that any combination of other sensors would exactly mimic a specific temperature cut-in for the higher RPMs. It would happen at different times. Maybe, but difficult to imagine that it could happen so precisely without knowing the outside temperature.

 

The AC system has it's own evaporator temperature sensor. Maybe it's using the AC's system input somehow??

 

We all know the water coolent temperature is only inferred from the cylinder head temperature sensor. This much is in published Ford Manuals.

I find it very probable, and have data that indicates the OAT can be inferred from the IAT (intake air temperature) sensor.

It seems by design, the intake air is always warmed by 10 degrees from ambient, after the engine compartment has reached normal operating temperatures. It's 10 degrees hotter than ambient on a 95 degree day, and it's 10 degrees warmer than ambient on a zero degree day, IF THE CAR IS IN MOTION. ( hotter if the car is parked or standing still )

THUS... outside air can be assumed to be 10 degrees cooler than the IAT sensor. Of course, the IAT is an exact match of the OAT the moment you turn the key after a cold soak of the engine for several hours.

The software only needs to be sophisticated enough to count the IAT as actual on a cold engine, and IAT -10 on a "hot" engine, and ignore readings when the car isn't moving, and I don't see this as a hurdle.

 

I didn't think it was always going to be 10F all the time, so I tested that theory.

On a short shopping trip, with my FE idling while I connected my scanner with OAT showing 30F on the dash. IAT showed 42F. After thirty minutes or so of shopping, idling, it showed 39F with OAT still at 30F.

Driving is a different story! OAT at 30F all the time. IAT dropped to 32F driving at 45MPH but would climb at a traffic light stop. Got home, idling, IAT continued to climb, 42F, 44F, 48F, and 50F. I assume on the short drive the engine didn't warm up fully. It would be impossible to calculate OAT from IAT!

Where/how did you get your data???