Hi Guys
Am I right in saying that the rad fans run all the time when the AC is switched on?

 

Yes, you are.

 

Hi
The fans on my XKR do not come on when I switch the AC on.I havent had any over heating problems and I have checked through some previous post and apparently you can short the connector on the temp sender with a 100r resistor.Do I do this when the engine has started ,or short it first then start it? If the fans come on does this mean the sender unit is at fault?

 

The temp sensor should be about 250 ohms at operating temp and a couple thousand cold. You can disconnect the connector from the sensor and jump the the connector pins with a suitable resistor. Doesn't much matter the sequence of events.

Fan operation per Jaguar technical reference:

. . . . . . . . . . . . . ON . . . . . . . . . . . . OFF
Slow (Series) . . . 90*C (194*F) . . . . . 86*C (187*F)
Fast (Parallel). . . 97.5*C (207.5*F) . . .93.5*C (200.5*F)

And the resistance vs temp:

Temp
. C/F . . . . Expected
70/158 . . . 435 ohms
80/176 . . . 320
90/194 . . . 250

Resolving my own fan operation issues I found that the sensors are not precision devices. It took a third new sensor to get close to the above resistances at an IR gauge's reported temperatures. Remember that you cannot work back from a Torque or scan tool reading of temperature. The resistance defines the sensor reported temperature, not the actual temperature. An IR temperature gauge measuring the brass body of the sensor should be the starting point.

And then there are the fan relays located on the left side of the rear of the radiator. The fans themselves are probably last on the list of things to replace.

 

Hi
I tried the 100R resistor across the sensor plug and the fans came on. Does this mean the temp sensor is at fault?

 

Measure the resistance of the disconnected sensor at operating temp. Should be around 250 ohms. If much above that your fans will not come on and the sensor is at fault . . . or your thermostat is stuck open. A suck open thermostat will not allow the engine to reach operating temp.

All these things are a single system and interact with each other.

 

I was having the same issue last year on my 97 and I recently swapped the climate control module as the display was shot and apparently the unit itself. Found one on EBay for $64.00 and replaced.

Fans no longer turn on when in Auto or AC until operating temp is reached. Something in the module must affect the fans.

My situation was when I started the car even before operating temp was reached, if I hit the Auto button, both fans would turn on. Turn off AC and fans would turn off and wouldn't come on until temp was reached. Then only the single fan would come on, not both.

 

Fans always run when AC is on.

 

Maybe I phrased that wrong and yes the low speed fan may run when AC or Auto is on but not The high speed fan. At least in the 8 yrs I drove the car the only time the high speed came on was during the summer when sitting at lights or in heavy slow moving traffic.

It's something you notice when you drive the car everyday. When you start the car in 0 degree temps the high speed fan shouldn't be kicking on right off the bat or at all for that matter on start-up unless engine is already at temp in hot weather.

Just a possibility if all tests else OK. My blue background stopped working a few years prior but my display was still fine as is displayed temp just not back lit. Then 9 months ago display became hard to read and that situation of the high speed fan coming on at start-up occured and I thought, well that's different .

Car was stored over the winter and this spring I replaced the Climate Control Module in the dash and behold, car starts like it use to without the high speed fan coming on at start-up.

May not even be his problem but thought it may prove to be a bit of useful information.

 

ok Im a bit confused now. Is there something in the AC circuit that switches the fans on?

 

The ECM has Outputs that energize the AC Clutch Relay and also has Outputs that turn the Cooling Fans on (High or Low).

The AC Control Module Requests the ECM to energize the AC Clutch.

This (Request) signal is a Hardwired signal (Not a Serial Buss Signal).

For 4.0 Liter Cars:

The ECM can turn BOTH Fans at Low Speed (Fans wired in Series - Both Fans receive 6 Volts)
OR
Both Fans at High Speed (Fans wired in Parallel, - Both Fans receive 12 Volts).

The ECM also monitors the Coolant Temp.

Based on these Signals, the ECM Controls Both Fans - Off or Low or High.

The ECM can not turn on ONE Fan Only.

The 4.2 Liter Fan Controls work differently.