Electric fan does not shut off

installed electric fan. where do I hook up the temperature wire? presently it is on the thermostat. fan comes on at temp, but remains on. does not cycle on or off. also tends to run down battery while driving. amp gauge shows discharge. 13.5 at battery and alternator. Michael. 85 xjs

doesn’t anyone know how to set up an electric fan to cycle on and off?? or does it always stay on?

Have you replaced the thermostat, or removed it and tested it in water to see if the switch closes when it reaches temperature, then opens when the water cools?

I’m not sure what temperature that cooling fan thermostat should operate at, but I’m sure that others here will have that information.

Also, does then fan run-on after engine shut down, then the fan shuts off, or does the fan keeping running until you disconnect it?

If the fan is discharging the battery while the fan is running, I would check the alternator and charging system to verify that the charging system is working correctly.

Seems as though you may have more than just one issue?

new t’stats. fan shuts off with engine. aux fan comes on for a few min. and shuts off–new alt. puts out 13.5—may have other problems been chasing this problem for a while

Michael, check the XJS archives as well as Kirby’s “Experience in a Book”. Several different approaches have been taken by folks.

No the electric fan isn’t always on.

Michael, you need to provide more information. What engine do you have? “Installed electric fan”. Where? In place of the belt-driven fan, or in place of smaller electric fan beside it.
That fan senses temp through a thermal switch below and in front of the water pump.
“Temperature wire- presently it is on the thermostat” Please explain what you mean. Thermostat housing? Where?
With more info I think you can be helped.

I’m taking that as referring to the thermal switch, which is also a thermostat, albeit not like the one which goes into the water stream in the thermostat housing the radiator hose connects to.

took off factory fan off of 85 v12. replaced with a electric fan. the temperature wire is presently connected to the thermostat temp gauge for dash. should I hook temp wire to the thermal harness

the wire is is hooked into the thermostat housing. in the water stream–should it be connected to the thermal switch?

Thank you. Belt-driven fan is gone, replaced by electric. Remove your wire from temp gauge transmitter or wiring. You might have success connecting to thermal switch or wiring so that both fans run at the same time, but if it were my car, I would buy a thermostat with a probe that can be placed wherever you wish, and is adjustable to suit you.
I have a fan on my car that has a probe which senses air temp and is attached to the fuel rail.

What year is your XJ-S and what engine does it have? Are yiu asking about a stock fan or an aftermarket one?


did you replace the fan yourself, and what brand or kind?? have seen some setups where the probe is attached or clipped to the radiator. will reroute wire to the thermal harnesss

it is an 85 v12 [do u need engine #] took off factory oem fan and replaced with a aftermarket unit. was suggested that I hook sensor wire to thermal switch.

Neither of my fans have been replaced, but the small electric fan is triggered by-
Rise in water temp at thermal switch.
A/C turned on.
Rise in air temp underhood, mostly after shutdown.
Manual overide switch below dash.
None of which will help you!
I would get another adjustable thermostat if I were you.

Unless there is a thermalstatic/thermal swith at the thermostat housing which the radiator hose connects to … yes, needs to be connected to the thermal switch to function.

Lots of correct advice here, but it is somewhat convoluted. Let’s try to sort it out. @michael_bucklew2 this is a long post, please read it carefully several times, I think it will address your questions.

  1. The thermal switch at the front of the engine near the bottom radiator hose triggers the small stock electric fan.

  2. The trigger circuit for the small stock electric fan includes a diode pack that provides a latching function. This means that if the fan is running when the key is shut off, the fan will continue to run. Once the fan turns off, it will not restart unless the key is on, even if the temperature is high enough to activate the thermal switch. This is to prevent the fan cycling on and off and draining the battery.

  3. The sensor for the dash temperature gauge is a different sensor than the thermal switch and should not have anything to do with any of this. You should not be connecting anything to do with fans to the gauge sensor.

  4. Since it seems like you are new to fans and wiring, I would suggest the simple approach: Get a new thermostatic switch with a probe that goes in the radiator fins and an adjustable trigger point. Like this:
    Perma-Cool 18905 Perma-Cool Adjustable Fan Wiring Kits | Summit Racing

  5. The fan motor should be wired to a good 12v source (like the bus bar on the firewall) via a relay and not through any existing wiring. It should have a quality in-line fuse close to the 12v source.

  6. The trigger side of the fan relay is controlled by the temp probe. The 12v supply for the trigger side can be wired two ways: 1) To a constant 12v source which means your fan will come on whenever the temp probe tells it to, key on or off OR 2) To a switched 12v source which means the fan will come on only when the temp probe tells it to AND the key is on.

  7. The instructions with the temp probe kit should describe all of this, and will likely include other options, such as adding a manual switch on the dash so you can turn the fan on whenever you want, and having the fan come on with the AC to improve airflow over the AC condenser at low vehicle speeds. These items are extra, add complication and are not necessary for basic cooling. You can add them later if you want.

Here is my setup, just for perspective, I am not recommending you do this:
My personal setup is two large electric fans which are wired via independent relays and fuses to the a lower-value thermal switch in the stock location. My small electric fan and all of the latching nonsense is long gone. My stock belt driven fan is long gone. My fans run whenever they want to, the key does not have to be on. I have no issues with battery drain. My fans also run whenever the AC is on. I have a manual switch on the dash that I never use. I have a light on the dash that tells me when the fans are on.


Bob, shouldn’t he just replace the existing thermostatic switch first (if still not working properly after moving the from the water temperature sensor for the temperature gauge to the thermostatic switch)?

If new to fans and wiring, replacing the existing switch would be simpler.

As I understand it there is a new primary fan in place of the original belt driven fan. Based on this, my reasoning is:

  • For simplicity, I would keep the new fan and thermo sensor totally separate from the stock system with the latching circuit and old wiring. Who knows what shape any of that is in.
  • Also, and I would have to dig into the wiring diagrams to confirm this, having the existing switch activate the new fan might cause other strangeness due to the latching circuit, back feeding voltage, etc.
  • The existing stock switch is likely a higher temp value than you want on your primary fan.
  • Having a separate adjustable set up for the primary fan means that any issues you encounter with the primary fan are isolated to the new set up, and not due to old stock components which may be failing or have been improperly modified.
  • Having the new setup separate from the old gives you redundancy. If one temp sensor or fan or relay fails the other will still work. Although if the primary fails and you are relying solely on the Aux you are likely on borrowed time for an overheat.

My .02-

Ahh, I am reading it as the new fan replaced the existing electric fan (which went bad?). I may have read it incorrectly?

This can also be done to turn the electric cooling fan on after a short engine warm-up time for those who would like the electric cooling fan to run more, but not run immediately when ignition is turned on.

Oops, wrong drawing - correct drawing now … I think … I’ll find out when I see it.