drastic MPG drop with heater/defroster on

I have to ask about the heated rear window - I drove for over half an hour the other day with the defroster on and it only cleared the bottom portion of the rear window. I don't think I've ever had any noticeable affect from the defroster on the upper portion of the rear window. Is this normal (i.e. does the Prius defroster just suck), or should I be trying to troubleshoot?

 

Yes it should heat upper and lower section. And (I think) it should eventually turn off by itself. I would first fix heating of upper rear window and then check that it turns off automatically.

 

Damage to the heating elements, maybe?

 

Which, unfortunately I have found out over the years, are easy to damage.

I had my windows tinted and specifically asked (begged) the place to be GENTLE on my back window. I swear that every car Ive ever had since I was 16 either had a rear defroster that didnt work, or only 1 line out of 10 did...lol. When I bought the Prius (I bought it used) I was pleasantly surprised that ALL grids worked!! I was as happy as a fat kid in a candy store!!

Wouldnt you know it?? After I got my windows tinted, 2 lines RIGHT IN THE MIDDLE of the window dont work now
Oh well, I guess its just my calling.

 

I'll be checking them tomorrow. Hopefully it will stay warm enough (50+) to do a repair if needed. No garage available.

 

Found the problem. Pictured is the connection for the upper window.

 

On the bright side, that can easily be fixed.

 

That's extreme, and I doubt it could all be explained as a consequence of using the heater and defroster. Interior heat is essentially waste-heat, diverted from the cooling system to the heater-core. The fan is a power user, but again, it doesn't need to be used at full speed all the time, and even then, it's using small amounts of current compared to the traction drive.

I'm not able to explain it, based on the information provided, but I think you can partially rule out that the entire loss of fuel economy is due to an energy-draw from using interior heat. I'll bet the eventual explanation will probably involve something more complicated than that.

I'm using my Prius in much colder conditions, and I've seen an undeniable reduction in fuel economy compared to the summer, but nothing close to what you describe.

My rear heating elements don't do much in very cold weather. I think they are deliberately under-rated to save power.

 

I'd heard that before and figured it was probably the issue - but I finally got fed up and checked. I went over every line of the elements carefully and was surprised to find no breaks. Almost gave up but decided to check the connections, and found the break.

I've ordered a kit with adhesive that got decent reviews for this type of repair. Everyone says the key is to follow the prep instructions well. I'll update this weekend when it should be warm enough for the adhesive to work.

 

With a bit of fiddling, I think I could boost the current through those elements with a power-converter/controller. I'd have to compensate it for temperature for it to work properly.

Something off the shelf might be a lot easier than building it from components:
Car DC converter: 12 volt to 12V, 15V, 18V, 21V, or 24V DC/DC
converter with 2.5 amp output.

 

2.5A? The defogger circuit is fused for 40A.

 

Good point. Something like that, once the current draw has been calculated.

 

I noticed a weak rear defroster today as well. 100% humidity and a mix or very wet snow and rain. had rear defrost on both ways for my trip and not sure it ever fully cleared up? and the little rear/side blindest window that has heater elements on it never really cleared either?

Does anyone know (I have the Persona) when the heated side view mirrors turn on? ie. is it when windshield defrost is on or does it kick on when you push rear defrost as well, as one would assume if rear defrost is on the side mirrors might be frosting over as well…


And finally I did notice that when I turned A/C on it did help defrost the windshield, BUT I always assumed they if on windshield mode the compressor is running thus defogging at its best, same with the feet/windshield mode is A/C needed as isn't the compressor running?????


EDIT: I didn't realize this was a Gen 2 thread, Do my questions even apply as I own a 2015 gen 3 Persona? If not sorry...

 

I believe for the Gen II the mirror defroster is linked to the rear window defroster, and not to the front window defogger. I've noticed the only chance I've got of defogging the rear window with my defroster currently inoperative is to use outside-air defogger on at least the middle fan setting.

For more Gen-specific comments you might want to post/search in the Gen III forums.