Anyone know why my engine idle drops whenever i switch on the rear defroster?

 

Is the engine actually droping speed or is the tach just showing a lower RPM?

 

I have a similar problem when I turn on my rear defrost, the rpm only changes for a second, but the volt gage drops from 14v to about 11v.

 

Resistive heating elements take a lot of juice to run. Usually, your dropping idle is going to be because of that higher demand on the alternator. That demand makes the alternator harder to turn so the idle drops until the computer notices and opens the IAC a bit to compensate.

There's not a whole lot you can do about it, really. It may or may not indicate your alternator (or more likely your battery) is getting on the old side, though by no means "bad".

 

yeah, i hear and feel the drop and then just noticed it by looking at the tach one day. as long as the heater is active the enging idles lower.

 

As stated above, the alt is having to work harder and drags the engine down a few RPM's. Its kind of like turning on all your lights at once (hi/lo beam & driving), the engine idle will drop noticably.

 

Thanks for the reply Jza, I was wondering if that drop was normal. Both my Alt and battery are only 2 months old and test great. Thanks again.