Rad fan comes on when heater is turned on

[Dragonslayer]

98 XJ 4.0L RWD

It's been cool and raining and the Temp gauge reads below 200º.

I noticed that whenever I turn on the heater/defroster - the radiator electric fan kicks in
and cycles on/off every 5 seconds. It does this even when I'm hurtling down the highway
at warp speeds, in cold outside air. All returns to normal when I turn off the
heater /defroster.

I know that the fan is supposed do this when the AC is engaged, but it seems
odd (and annoying) that the rad fan would come on to rob heat away from the
heater/defroster.

Is this supposed to be so?

 

[its an xj thing]

im not sure about the xj, but on most vehicles, when the defroster is turned on, the a/c compressor is turned on to supply drier air to the vents. makes getting the fogginess off the windows much easier. that may be why it is running the aux fan

 

[O-Gauge Steamer]

Two reasons. One is to help demist the windscreen and the other, prime reason, is to cycle the AC to keep all of the seals lubricated. In years past it was discovered that the AC would run better in the summer if it was cycled during the winter...

 

[TacticalFats]

There's an easy way to wire in a small switch to bypass this 'feature'. I generally use the floor/defrost setting all winter and hate to have the compressor, et al, accumulating wear and tear. I will cycle everything occasionally, though.

When it's *really* cold, I'll use the AC max/recirc setting to warm up the interior quicker w/o using the compressor, etc.

 

[WB9YZU]

Is this supposed to be so?

Yes, in any of the heat modes with defrost, the A/C and the Aux Fan will cycle to dry the incoming air. The Operation of the Aux fan is tied to the A/C and Engine heat. If either the A/C is engaged, or the engine is starting to heat, the Aux fan will kick on.

 

[Saudade]

that the rad fan would come on to rob heat away from the heater/defroster.

Not so. The thermostat will close and block any coolant from flowing through the rad.

 

[Dragonslayer]

Thanks all for the responses. I was worried that my XJ had a bit of bad wiring, a short,
or that a sensor was goofed. Seems all is "normal".

 

[98NWCherokee]

My 98 does the exact same thing.....it drives me crazy kicking on and off during the winter. I just pull the connector on the low pressure switch (passenger side near the firewall) and the aux fan won't cycle on and off. I did this all last winter and didn't suffer any ill effects.

 

[Dragonslayer]

I haven't had a chance to look under the hood to find that low pressure switch. If anybody
knows how to wire in a switch to defeat the constant Aux fan cycling I, (...er...we) would
appreciate you sharing the details. Besides the fan noise, the other annoying thing about
this is that the cabin doesn't ever get especially toasty when the heater is on (only luke-warm enough to take the edge off of the cold). I suspect the Aux fan is preventing
the heater from reaching a higher heat output potential.

 

[Saudade]

The aux fan *WILL NOT* reduce cabin heat with a properly rated and functioning themostat. Once the engine temp drops below 195 deg (the stock rating) the thermo closes and reduces the coolant flow through the rad. The elec fan will only engage with the AC in Defrost mode. So any other mode, like dash vents or floor keeps the ac/fan off.

You should try flushing the core. Do you know what thermostat you have in it? Try measure the temp and the dash vent at full hot.

 

[keep0njeepin0n]

When it's *really* cold, I'll use the AC max/recirc setting to warm up the interior quicker w/o using the compressor, etc.

Can you please explain how by turning the A/C on max/recirc setting, it will WARM the interior faster, AND how by putting it in that mode the compressor stays off? Did you do that custom or something?

I know in MY jeep, when I put the A/C in recirc, that shit gets COLD, especially in winter, and the compressor is ON.

 

[Begster]

I haven't had a chance to look under the hood to find that low pressure switch. If anybody
knows how to wire in a switch to defeat the constant Aux fan cycling I, (...er...we) would
appreciate you sharing the details. Besides the fan noise, the other annoying thing about
this is that the cabin doesn't ever get especially toasty when the heater is on (only luke-warm enough to take the edge off of the cold). I suspect the Aux fan is preventing
the heater from reaching a higher heat output potential.

The aux fan *WILL NOT* reduce cabin heat with a properly rated and functioning themostat. Once the engine temp drops below 195 deg (the stock rating) the thermo closes and reduces the coolant flow through the rad. The elec fan will only engage with the AC in Defrost mode. So any other mode, like dash vents or floor keeps the ac/fan off.

You should try flushing the core. Do you know what thermostat you have in it? Try measure the temp and the dash vent at full hot.

As Suadade said, the aux fan has no effect on the cabin heat. It sounds like your heater core is clogged, which is a cause for luke warm air instead of hot air.

 

[searchingforXJ]

Can you please explain how by turning the A/C on max/recirc setting, it will WARM the interior faster, AND how by putting it in that mode the compressor stays off? Did you do that custom or something?

I know in MY jeep, when I put the A/C in recirc, that shit gets COLD, especially in winter, and the compressor is ON.

I think he means put the one dial to the max/recirc setting, but have your cold/heat dial turned to max heat. You shouldn't be getting any cold air coming out if you have the temp dial turned to max heat.

No idea why the compressor would stay off, but maybe when you are requesting hot air it isn't signaled to run?

 

[keep0njeepin0n]

I think he means put the one dial to the max/recirc setting, but have your cold/heat dial turned to max heat. You shouldn't be getting any cold air coming out if you have the temp dial turned to max heat.

No idea why the compressor would stay off, but maybe when you are requesting hot air it isn't signaled to run?

Nope, the compressor does not get it's signal from the temperature selector....I have no clue what he's referring to, it confused me completely!