My fan cycles often and loudly. sounds like the AC comes on but it is just the coolng fan.

 

The fan is triggered by either the A/C(so a/c OR defrost) or the coolant temp.
Are you running the A/C or defrost when it does this, or are you overheating?
If you're running ac/def it may be running normally but have a dying fan that's getting louder so you're noticing it more. If you're overheating (about 220-ish) and that's causing the fan to cycle frequently then you need to fix that problem since fan cycling is the least of the worries.

 

Whats the temp gauge read when it turns on? I think it turns on at 217* or so, or when the a/c is running. And/or if the temp sender in the thermostat housing is unplugged.

 

Mine does that when the refrigerant is low in the AC system. It's actually the compressor kicking on, realizing pressure is low, then kicking back off. Since that is one of the two things that will turn on the fan, the fan will also turn on and off frequently.

The AC compressor runs in AC but also in Defrost mode, so you might not necessarily notice low refrigerant when running the defroster.

 

It WAS in defrost mode. Also it does it in AC mode.
Is this an indication of impending problem?

It does not run hot and AC seems to work good.

 

If you're worried about it, have your A/C serviced - just to make sure the refrigerant/oil levels are good.
Otherwise I wouldn't worry about it. If the fan is loud just replace it. Like I said, I bet it's doing what it always has but you notice it more with the loud fan.

 

What triggers it when it is defrost mode?

 

Defrost uses the A/C, so you can look at it like it's another "A/C setting"

 

WOW
Now that is interesting.
Thanks

 

Think of it this way - what is "A/C" - it's basically removing humidity from the air - which has the effect of cooling the air. What is defrost trying to do? - Remove fog from the inside and melt snow/ice from the outside - so it also needs to remove humidity. So defrost is just air conditioned air pointed at the windsheild.
We tend to use it with the warm air selected, but you could use it with cold air too.

 

I didn't know def used the A/C until I stumbled upon it in the owners manual to my TJ... but it made sense when I thought about it.

 

Thanks for the explanation.
It was a big help and very interesting.

 

Anytime! I'm always happy to share my limited knowledge.

It's funny, in my Corolla it's a push button A/C, so you set it on whatever, like high only, high/low, etc and push the button for A/C or no A/C, but if you have it on A/C and switch it to either of the defrost settings the little light on the A/C switch goes off (since defrost is always A/C on) - but it's counter-intuitive. It looks like you can't run A/C on the defrost settings when in truth you can't turn A/C off on the defrost settings.

 

Go get you an AC recharge kit with UV dye in it. Follow the instructions. Wait a few weeks then look for leaks using the special light and glasses.