My t-stat went south a couple weeks ago. While debating over the OEM or aftermarket replacement it got cold here. So I started blocking off the radiator to get to operating temps. Also discovered my block heater doesn't work...

What's weird is it still seemed slow to warm up although temps were/are in the upper teens. One reason I discovered is that as soon as it started up the electric fan came on (even though the AC has been unplugged until I recharge the system). Once it "warms" it turns off. This is opposite of what it should do right?

The fan and ambient air sensor is new. Sensor and fan from NAPA, I remember there was a two prong or three prong option when I ordered the fan and I just matched prongs. Is there anything else that controls the fan cycle? The block temp seems to work properly. Everything appeared to function correctly over the summer but I didn't pay much attention to the fan operation.

Hope to finish the t-stat R&R this weekend and button it all back up and see what it does. Waiting on a new turbo intake hose as I discovered mine was ripped when I took it apart.

Many thanks,

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2005 Liberty CRD. New rockers, exhaust valves; Provent, EGR block & butterfly delete & V6 air box at 100K

1998 XJ 4.0L, 2" lift

First Jeep: 1962 Willys CJ-3B

There are two ways for the fan to turn on.
1. coolant temp above 205F. Not likely in winter at first startup.
2. AC on. Again, not likely at first startup in winter. But...

If you have the vent controls set to any of the defrost settings, that will cause the AC to cycle on as a way of drying the air to prevent fogging up of the windows. Even though your AC system might be disabled, the ECU doesn't know this and still tries to turn on the AC.
So that might be one reason for the fan to turn on.

The other possibility is that there a loose connection in the fan control circuit.
For example, if you removed one of the fan control relays, the fan will turn on at low speed and stay on for as long as you have the ignition turned on.
So, possibly one of the fan relays has a loose connection when cold and then once warmed up it closes the circuit.

Still another possibility is a faulty AC pressure switch sending a signal to the ECU to turn the fan on.

If you want to fully disable the fan, pull both fan relays.

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U.S. Army Retired

Thanks for the leads. I got it back together last night so I'll fool around with it and see what's going on.

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2005 Liberty CRD. New rockers, exhaust valves; Provent, EGR block & butterfly delete & V6 air box at 100K

1998 XJ 4.0L, 2" lift

First Jeep: 1962 Willys CJ-3B