I have a 1996 Neon and when I run the defrost I can here what I am assuming is the a/c compressor kicking on and off. It only does it when the switch is in the defrost position and I can notice that it is drawing more power when this "noise" happens because the headlights actually dim ever so slightly and I can feel the engine power reduce just a bit. Is this normal or what should I check first, I just replaced the battery so I at least know those connections are good and clean.

 

My truck does this and read on another thread that this is normal. What they didn't explain was why it does this so maybe someone will tell us why this happens.

 

The compressor kicking on is a way to remove humidity (ie: water) from the air that causes windows to fog up and is the normal operation of the defogger for Neons. The one major difference to defog mode operation for the compressor vs. normal AC mode during summer is that the fan will still kick on/off as needed instead of coming on and staying on for normal AC mode.

Toyotas and Nissans (and some other cars) would automatically switch the air flow from internal (recirc) mode only to external mode to pull in outside air when the selector switch was set to defog but Neons (and a lot of other cars now) instead use the compressor to do the work.

One good thing about it, it exercises the compressor and clutch during the months when it would normally sit and do nothing, which may or may not be a bad thing. The only way to stop it (without changing belts) is to pull the AC fuse.

 

I do have a switch allowing me to bring in outside air or recirc the inside air. Either position seems to allow more fogging of the windows than I would have thought or would like to have but maybe that is just how the neons are. So if I pull the a/c fuse that will stop the compressor from kicking on/off but will I still get acceptable defog? And what do you think about the slight dimming of the lights when the compressor kicks in?
Thanks for your help.

 

The difference between what Toyos and Nisses are doing is that they mechanically force the switch from recirc to outside when the selection for defog is made and if you try to change it back to recirc mode, it moves the selector switch from defog to feet.

On the neon, you can keep the airflow switch where you want, but I recommend using the outside air on cold days (less humidity then inside), but on rainy days, use the recirc (less inside and especially so once the AC removes what is already there).

All I can say about removing the fuse is that you will just have to try it and see. You may live in an area where it will work just fine, but if you live where it is very humid outside, it may not. Removing the fuse gives you a free and easy chance to check it.

Do the lights dim and stay dimmer? If so, you may want to check the output of your alternator by putting a multimeter on the battery posts while it is running to see if you have a significant drop with lights on vs. off vs. AC on vs. AC off. A flicker when it kicks in is normal as any significant fast change in voltage load would cause, but if it very quickly returns to normal, then you likely don't have a problem.

 

The lights dim when the compressor kicks in and that is usually only for a short moment. It typically does a quick on/off and during that brief time and only during that time do the lights dim, as soon as the compressor stops the lights go back to normal. I will put a meter on it and see how much drop there is. I guess it makes sense that there would be a drop when there is a quick increase in the load, I just don't know what normal is. By the way, I live in MN and right now it is a bit chilly. Thanks again for your help.

 

Ok, so I guess I should have checked into this a little more before asking for help. Turns out it is not the compressor that I hear but is the radiator fan cycling on and off. There seems to be no rhyme or reason to why this is doing what it is. When I start the car from cold and put the switch to defog the fan begins turning on and off about every 5 seconds. Before the blades even stop it is turning back on again. Sorry for the misguidance but do you have any thoughts on this?

I did put a meter on the battery and the voltage is dropping from about 14.4 to 14.2 when the fan cycles.

 

Not a problem and exactly what you should be hearing. The compressor cycle is super short when in defog mode. You will likely hear a loud click (magnets on compressor clutch engaging) followed by your fans roaring to life for a very short time. Your system is working just fine. No worries mate!

Again, no worries mate! Drive on! Compressor engaged, lights on, fans running and still 14.2 volts? Excellent! Anything over 13.5 under a full load is great!