I posted this Friday, but apparently the thread didn't work.

I have had an issue with my RPM being too high when the A/C is engaged. It has happened once before this past weekend, and this current tank has had pretty bad MPG because of it.

Since Friday, whenever my A/C is on (ECON or not), my RPM guage is showing about 500 RPM above normal at all speeds. It goes right up to 2000 at normal city speeds, and goes to about 2300 at 60 miles per hour. The worst part is that at highway speeds the RPM won't drop below 2000; normally, when going down hill the RPMs drop to around 1600, which really improves the MPG.

When I let off the gas in town, the RPMs go to 1100 (which is normal), but ANY pressure at all on the pedal causes the RPMs to fly right up to 2000.

I have discovered that it only happens when the A/C button is turned on, and apparently it only happens at high ambient temperature - it has been over 100 F this past weekend.

It doesn't affect EV mode, or shutting down the engine when at a stoplight.

The engine does feel like it lacks power when this situation occurs.

My mileage has been pretty poor this tank. It was at 24.5 for the first 60 miles (I had the family in the car and could not turn off the compressor). Since last night I have managed to get it back up to 29.4, by driving without the compressor. The temperature dropped to around 95 yesterday evening, so I did have about 10 miles of highway with the engine operating normally (going down to 1500 or so on the down hill), which helped. Mysteriously, last night I went shopping, with the external temperature again around 95F, and the problem re-occurred. This morning the RPMs were back to normal with A/C going on.

I don't yet have a scanguage, so I can't provide much more information.

Any ideas?

 

That is NOT normal, and I do not think it is just the A/C doing that.

It does sound like your HV battery is way too hot.
While I've never had my battery get too hot, ( living in Minnesota this past year ) what you describe sounds exactly like what happens when the battery is too cold.

I think because of the heat, your battery is going offline... then the gas engine needs to do all the work, not just 75% of the work like it is used to. This is making the RPM go higher, not directly the A/C.

I would almost bet this is the case.
Do you drive for long enough ( 30 minutes to 1 hour... ) to get the battery cool?
Is your A/C in the cabin cool?

You may want to check that your A/C is working correctly going to the rear.
If it is cold up front, but your battery is going off line, then maybe a valve is stuck or a tube plugged, ( or pinched under the car ) and you are not getting freon to the rear.

-John

 

Are you saying that the main A/C compressor runs the battery A/C? I would not think that the engine had to run higher to provide battery cooling - the compressor works at any engine speed.

I was also suspecting that the battery is hot. My trips were 45 minutes at most, and my normal trip is 10 - 15 minutes.

However, the "assist" meter indicates the battery is supplying power, the EV works, and the engine still shuts down at traffic lights. So the battery cannot be "off line" - it is still being used.

The A/C in the cabin is cool.

Would the scan guage provide any useful info in this case? I've been saving up...

Last, it DEFINITELY tracks with the A/C compressor. If I shut it off the engine returns to normal. The moment I turn the compressor on, the higher RPMs occur.

 

There is 1 A/C compressor and 1 freon reservoir. There are two loops of pipe. One for the front, and one for the battery in the rear. You can see the silver tubes for the rear A/C if you look under the rear driver side.

If your battery is not off line totally... which is rare... it may just be providing the bare minimum to get by, which happens frequently. In winter down to -15 degrees, mine is not 100% useless... just about 80% less power flows.

Or you may have a totally new, one of a kind problem!
I say the symptoms point to a hot battery.

 

I suppose that makes some sense, but why up the RPM, does that increase the compressor output?

And why does the battery / engine output / RPM seem to function normally when the A/C is off?

 

UPDATE.

The ambient temperature is now 90F, and the A/C is operating normally. Now all I have to do is get the MPG up past 29 within the next six gallons of gas...

 

The hot battery cannot provide enough current to assist the engine in moving the vehicle at times, therefore the engine has to work harder to get you up to speed. I believe thats what John is describing.

Incidentally.... There is a baffle or door in the back for the AC that is controlled by the computer. I had the motor on that door fail on mine. It gave me a check engine light on the dash but the light would reset every time I restarted the vehicle. Some genius at Ford decided an AC bypass door was a climate control and therefore "unimportant" to the engine and made it one of the simpler warning lights rather than one that gives a check engine light that STAYS ON. Well... that door cools your battery so its MOST DEFINITELY IMPORTANT. Is there any chance you've been getting a check engine light that goes off with the next restart of the key?

 

Might very well be SOP.

Most A/C compressors or of the positive displacement type. So your 2008 A/C compressor might be "downsized" for optimum FE for "average" conditions when engaged. GOOD compromise, really. But that also would mean that when under an unusually heavy load the engine would need to turn the compressor faster to keep up. With CVT available the engine speed can be pretty much independent of roadspeed.

Very good compromise solution vs electric compressor.

 

I don't think so, but I'll look next time. It is hard during the daytime. But I don't think I've seen it at night.

 

Just to confirm this issue, I asked for feedback from owners in Vegas or Phoenix, and a member from Vegas confirmed that his RPMs were about 500 higher when the ambient temperature is over 100F.

Not sure why Ford did this, though.