And keep in mind if you have a tinted rear window, overheating the defroster can blister the film. I can't believe Honda didn't put a timer on this.

 

Great. Looks like I'm going to have to buy another mod.

es

 

Those look pretty cool! Can you just add the timer to our defogger switch? Am at work so I don't have time to read the FAQ's. I'm sure I'll be guilty of forgetting to shut my defogger off ... 1st car I ever had with rear window defogger!!

 

I would presume yes, but would have to dig into it to know for sure. The price isn't that bad…

es

 

I too forgot to turn the rear defogger off. I was like man how much did Honda save on this by not installing a 10-15 minute timer?? $2?

Soon enough I will have to start doing a pre-flight checklist before take off. lol

 

As I stated before in this discussion you need to separate the operation of the rear and front defrost / defog operations. The front and rear have separate controls and can be turned on and off separately.

The front defog / defrost operation will consume additional engine driven power over just normal heater operation IF the OAT / intake temperature is over 40-45 degrees. Below that temperature the compressor does NOT run. It is automatically shut down to prevent the evaporator from freezing up. If it were to freeze up you would have no air flow through both the heater and the defroster and you would have to wait until the next thaw with warmer air temperatures for the ice to melt out of the evaporator.

The rear defrost / defogger is a completely separate operation from the front. It is totally electric and consumes 25 to 40 amps. In most vehicles if you have rear defogger on you also automatically have the heated mirrors operating for some additional electrical load.

In the picture posted by the OP, the rear defogger is on as indicated by the lighted indicator in the "rear" button.

So in the picture you have one and possibly two additional power drags on the engine.
1. You definitely have the additional electrical drag of the rear defroster and the higher voltage triggered by the ELD (Electrical Load Detector).
2. You also may have the additional drag of the AC compressor depending on the intake air temperature.

As a side note, if the fresh/recirculate lever were in recirculate (it is not in the picture) then the intake air would be coming from the cabin and would be above 45 degrees F. once the cabin (inside) air temperature warmed above 45 F. If it was in recirculate mode we would we would know for sure that the compressor was running.

 

One trick that usually works for me is to move the right airflow dial to the open space between the Floor and Defrost/Floor setting. You can hear the the A/C compressor come on as you approach the Defrost/floor setting, so just back the dial off a bit til the compressor goes off--your dial will be about 2/3's of the way to Defrost/Floor, but not all the way there. Air will still be directed out of the Defrost/floor vents, but you do not get the benefit of the A/C dehumidifying the air. it's a reasonable compromise when you don't have heavy fogging.

 

Except for the fan speed being way too low, your first picture is how you should have the settings for clearing fogged windows. But that's temporary.

Once they're cleared, you should cut the heat back to what's comfortable--about halfway, redirect the air to the dash and floor vents, and turn off the rear defroster. You might be able cut the fan speed back from 3 to 2, but leave the air source set to outside air. You need to have air exchange to get the moisture you generate out of the cabin.

 

It sounds like everyone is trying to scoot around the issue at hand here. I thank you all for your advice, I am taking it to Honda today. After a brief description of the issue they wanted to see it as soon as possible...this is not an issue with how my controls are set, its an issue with excessive fuel consumption under a certain setting...way more than I could mess up with running my AC or defrost too much.

I will update you all on the issue after I get back. Thanks again for the input.

If my settings are what is going to stick with you all for some reason then please keep in mind this summer I kept the AC full blast and was still getting 42-48 mpg, I know my car well enough to not be a "johnny dum dum..." or whatever else. This isn't an operator error.

 

I don't think any of us thought you were being a "johnny dum dum" or anything. We just wanted to make sure you were aware that you will always get lower MPG in the winter due to various reasons (colder air, lower tire pressure, using the rear defroster, heated mirrors, heated seats, etc). You never stated what your MPG was before you noticed the drop, just that it was lower. Now that I see you are talking about a very significant drop from 48 to 27, I agree something is wrong and this isn't a typical drop in MPG due to it being winter.

 

^This.

Yeah, that much of a drop is a problem.

 

Johnny Dum Dum not directed at the OP (JediPat) but directed at anyone who fails to read the owners manual. It certainly wasn't my intention to insult your intelligence. Wanted to give a "heads up" of warning in owner's manual about leaving rear window defogger on for extended periods of time.

My attempts at internet humor always backfire. I sincerely apologize.

Good luck with the dealership and I look forward to reading about your resolution

 

I appreciate what you wrote, as I haven't fully read my owner's manual and might just be called Johnny Dum Dum :-)

This is the first car I've ever owned, and I grew up in a very different climate... I don't even know if we had a button in the car to warm the back window? If we did we never used it.

I had no idea it shouldn't always be on... my usual routine is to switch on the front and rear defroster and leave it on there!!!!!!! So thank you & everyone else posting here for educating me! It also didn't occur to me that keeping it on re-circ would up the humidity (duh! why didn't I think of that??) and mean I would need to leave the front defrost on...

I, too, have had a drop in gas mileage, but it's no a huge bump, and due to things like me not driving as carefully for best fuel economy as when the car was brand new, "winter mix" (just learned about that from here... no one in person I know has heard about "winter mix" fuel), haven't checked tire pressure & have driven it almost 2,800 miles in about 6 weeks, weather has changed from 50s to 0F to 30F range. I don't even know if it matters which gas station I use to fuel up... I feel like gas quality should all be about the same but I have no idea. Also our state (Minnesota) requires ethanol to be in all gas :-/

Anyhow, I just wanted to say thank you to everyone who has posted here for educating me, and also to the OP--I hope you find out what is causing such a drastic change in fuel economy! And I hope it gets fixed quickly!

Happy Thanksgiving to all!

 

I took it in and they brushed it off as a cold weather thing. They check my fuel trim and everything was running well at the time. They wanted my car for a week to test but I have a hard time with that right now. My buddy from Honda r&d said I was 1 of 2 people in the country to have this issue...he is waiting on info from Canada as well.

Kind of bummed...but oh well.

 

Bummer! Well hopefully Honda can figure out what is causing this issue (sounds like they don't know since they wanted the car for a week) and get this fixed for you... or maybe they just trade out your car (lemon??) for a brand new one...? ;-D

Keep us posted & good luck!

 

I found out with my 2013 that in either summer or winter I had to keep my front defroster on all the time, so never noticed a drop in mileage as I never drove without it on. I have the 2015 front defroster on constantly too. Was curious how that would work as I have the dx trim model and it has no A/C. Seems to work fine so far.


Canada has had a quality control issue with NO rear defroster and no heater for side mirrors in the dx trim model - was just informed that the parts were in and will get it fixed this Friday. So, can't comment on that for lowering mileage, or km in Canada! Will see once they've installed the missing parts....wonder what else they've missed..

 

Most cars do it but I don't think the 3d gen fit shuts off the rear defrogger automatically. Odd that it turns the AC compressor on automatically when you turn the vent knob to defrost (though unfortunately it doesn't turn the green AC indicator LED on to inform you about what it's doing) most cars don't do that - yet it frogettes to turn the rear defrogger off automatically after its been on for 15 minutes. Oh well.

 

Had no idea the AC turns on with the defrost- and I was wondering why my mileage had been low lately.

 

I would think you could add a timer anywhere in the circuit. If it has a fixed, non-adjustable time, it could be very small and put in an out-of-reach spot.

 

It's pretty common, at least on the cars I've owned. The AC pulls the moisture out of the air.

If you search, there is a thread where you can pull one wire and disable this "feature". You'll have to manually push in the AC button if you want it on when running defrost.