I don't know if this happens to you also rooster but when I turn on any of my climate control settings wether its a/c or heater my e-fan turns on also. Another possibility as to why it drops.

 

Anytime the AC REQ signals goes out (aircon, defog) to the ECU, part of its response is to turn on the e-fan to handle the extra thermal load and reduced heat rejection capacity of the system (the "reduced capacity" is because the condenser is in front of the radiator core, and the air flowing over the core is therefore that much warmer. It can accept that much less heat. Therefore, more of it is needed to accept the same amount of heat rejection as before. Basic thermodynamics...)

 

Seems like there should be 1 or 2 vent selector positions that do not engage the compressor (which engages the efan). In those positions, the efan should only be engaged by coolant temp.

 

That's what I thought the efan should only turn on when temp gets to like 218ish* iirc, which mine does. Then it should also turn on when the a/c has been turned on, as 5-90 described, because of the extra work the compressor puts on the engine, again something mine does. However, my efan also turns on in any of the heater settings.
These are the options I have for climate control. I usually have it on the vent or heat selection (I like the heat one because it comes out the floor boards and heat rises.



I'm not saying any of this is a problem just that its a possible reason for the sudden drop in temp.

 

I'm going to try a different setting and see what happens

 

The temp selector lever will mix in heat when moved to the right regardless of where the vent selector lever is set. The compressor should not engage in the vent and heat positions (no efan) thus no efan on in the vent/heat positions unless coolant temp rises high enough to trigger the efan.

 

That's what I want to see is if the e fan is making the temp drop happen...

 

My '91 fluctuates like this until it is good and warmed up, and it takes a while this time of year. When I serviced my cooling system, I removed the AC condenser so I get more airflow through the radiator. This is great during the warm summers here, not too bad during the winters. As a result, even on defrost, my aux fan doesn't kick on when I turn on the defrost-only when it gets to around 218 degrees. No AC in the summer, but I never use it anyway. Anywho, like was said before, running your heat will cause a fluctuation until the vehicle is fully warmed up. Nothing to worry about.

 

Anything that increases air flow thru the rad (the efan on for example) will drop coolant temp some but not 50 degrees. No need to drop coolant temp when a good/max heater is needed.

If ambient is cold enough to need the heater, one of the vent selector positions that does not engage the compressor should be used.

 

So out of curiosity why does the defrost engage the compressor? I figured it just put hot air through the vents on top of the dash, I don't see why the compressor would need to be engaged for that.

 

The heater is way more powerful than the a/c. Even though defrost is needed to remove ice from the outside of the windshield, the compressor is engaged.....but again, the heat is more powerful that the a/c so heat on the glass will melt ice. The a/c compressor is needed in warm and humid conditions to defog the inside of the windshield. To defrost the outside, the temp selector would be moved to the right.....to defog the inside, the temp selector would be moved to the left. The defrost/defog position on the vent selector is one in the same.

The XJ HVAC is no different than any other vehicle.....defrost (outside)/defog (inside) work the same.

 

Yeah - HEAT and VENT.

A/C actuating the compressor is obvious. DEFOG actuates the compressor to dry the air that gets blown over the windscreen.

And, natch - OFF.

 

....but some folks don't understand that 4 of the 6 vent selector positions (pic above) engage the compressor (efan on, '99 down XJs) and on later model XJs with 7 positions, 5 of the 7 vent selector positions engage the compressor. Likewise, some folks don't understand that the temp selector has nothing to do with compressor engagement (or efan on).