Hello,

I have a 2004 HCH and I've noticed during these cold months that my rear defroster seems to turn on by itself. It goes like this:

1. It's a frosty morning and the car has a light coating of frost.
2. I start the car and begin scrapping. Maybe I get most of the material but then I get in the car.
3. I DO NOT TURN on the rear defrost switch. I drive to work.
4. Very soon I begin to notice that the places I've missed in the rear window are beginning to thaw out around the resistive heat wires. I look down, the switch for the rear defrost is STILL OFF! Not lit.

What gives? Why is the rear defroster on? Is there some sort of auto mode that turns this thing on for me? Can I shut it off? I know it is impacting my hybrid battery performance because the 12v accessory battery needs to be replenished everytime it heats the rear window. Please help.

Nate

 

You've assumed it's on. It's likely not on. The wires through the window have much higher thermal conductivity than the glass, a poor thermal conductor.

As the interior warms, the wires increase in temperature faster than the glass between the wires, so you will see signs of melting at the defrost wires even when they are off.

Test this by pulling the fuse for the defroster. Be aware that you may lose other functions.

Consider that the power consumed by the rear defroster is still very small compared to the capacity of the IMA system. Even under very light accel, the IMA system uses about 3000W. The defroster is using 120-240W tops.

The 12V accessory battery needs to be replenished all the time. Every normal car system (plugs, condenser fans, electronics, heater blower, etc.) still uses the 12V in the HCH.

If you're running the heater/defroster while idling and scraping, that's a big performance hit. When fully warm, the engine produces a LOT of excess heat and that can be used for interior heating without penalty. If you're running the heater during warm-up, you're creating a secondary path for the heat to go rather than into the engine thus making it run longer, colder and at lower efficiencies.

Defroster is double the hit as it's running the A/C compressor to dry the air.

If efficiency is a big concern, then consider all aspects you can optimize.

 

Right, I understand all the concerns about running the defroster and the compressor. I didn't know that the rear defroster was only pulling that many watts. I like your idea about thermal conduction but I have to say that this "auto defrosting" is happening immediately. The cabin is still pretty much stone cold when I take off. In fact it is more noticeable when I don't scrape at all. It certainly looks like the heating elements are on to me. Water melts and runs off the window. Again, the rear defrosting switch is off when this occurs.

But I also like your idea of pulling the 30 amp fuse. Is that fuse labeled pretty clearly?

Thanks.

 

I don't have an HCH1, so I can't say for sure, but I would expect that you can decode the labeling.

Steve

 

I am nowhere near my car at the moment, so cannot look at a similar car, but the defrost wires in the rear window of many cars come out to a sort of tab that sticks out some place on one side, and the wires providing power to it attach there via a spade. If you are lucky this will be somewhere above the plastic of the back deck, so that you may be able to unplug it by pulling the spade off, and not have to pull the fuse. Be gentle though, the tabs can break off.

 

Update. I located the fuse and pulled it and the behavoir goes away. The rear defog does NOT come on. Put it back in and the problem comes back! So I'm thinking at this point its a stuck relay? I'll investigate further and repost. Here is the location of the 30 amp fuse, it's in the main fuse panel, under the hood, right side.

 

I stand corrected. Good work.