1jz thermo fan switch

**triptek** · 05-04-2013, 11:22 AM

I'm running a jzx81 radiator in a 1jz cressy. I'v removed the hydro setup and will be running twin thermo fans. Was planning on removing the factory hydro fan sensor in the bottom rad tank and replacing it with a thermo temp switch. In the past I've used a 92 on 87 off switch but this was located in the top rad hose, which worked great. Should i get a lower switch since it's in the bottom tank or would this be fine.

**jondee86** · 05-04-2013, 08:59 PM

Your thermostat is most likely regulating the engine temp at around 85 deg. By the time the bottom tank gets to 92 deg, the top tank could easily be 20 deg higher i.e. at 112 deg, which is getting into danger territory. Reading the temp off the upper hose will give much better control, but for sure, if you use the bottom tank position, you should try and get a lower temp switch. Any idea at what temp the factory hydro sensor switches ?

Cheers... jondee86

**Z2TT** · 05-04-2013, 11:17 PM

The Factory Hydro one does not switch, it is variable resistance to the Hydro ECU which controls the fan.

Your better off to tap a Switch into your thermostat housing or use a Thermostat with probe in the radiator hose.

Or what you could do is measure the temp difference from top to bottom of radiator while car is running and then based on that see if you can get an appropriate switch that will compensate for the temp drop.

**dnegative** · 05-04-2013, 11:57 PM

Tap into the existing temp sensor in the thermostat housing, Jaycar make a kit.

**hosking1991** · 06-04-2013, 08:22 PM

On my 2jz mx83 I run a sensor In the top hose , fans switch on at 95c and off at 87c

Cheers Luke

**Shakotan_Aaron** · 06-04-2013, 08:51 PM

Originally Posted by jondee86

Your thermostat is most likely regulating the engine temp at around 85 deg. By the time the bottom tank gets to 92 deg, the top tank could easily be 20 deg higher i.e. at 112 deg, which is getting into danger territory. Reading the temp off the upper hose will give much better control, but for sure, if you use the bottom tank position, you should try and get a lower temp switch. Any idea at what temp the factory hydro sensor switches ?

Cheers... jondee86

Not trying to be rude but your quite wrong here. there wont be 20degrees difference between top and bottom tanks once upto operating temp. radiators are no where near this effiecent! In this type of system with the thermostat in the lower hose there will only be around 1~2 degrees difference max as the coolant flows quite quickly.

i had a 95deg switch in the lower hose and a defi temp gauge in the top. The fan switched on when the gauge showed 95 and off when it showed 90 give or take ~1deg.

i can assure you that there wont be 20deg difference unless the thermostat isnt opening.

**Shakotan_Aaron** · 06-04-2013, 11:03 PM

Also it doesnt matter what the temp is at the top so much in the case of a fan switch. its only job is to aid in the cooling when the radiator isnt dissipating enough heat.

so for arguements sake if coolant is entering at 100 deg and exiting at 85 there would be no need for the fans to be on as the standard thermostat only starts to open at 82. Now the radiator isnt this efficent as ive already stated. thats just to make a point of what your actually trying to achive. Dont want your fans fighting your thermostat either. I might go for a 93/88 switch next. I think spal have some pretty awesome controllers with soft start to avoid pulling big start up amps.

**jondee86** · 07-04-2013, 07:51 AM

Originally Posted by Shakotan_Aaron

Not trying to be rude but your quite wrong here. there wont be 20degrees difference between top and bottom tanks once upto operating temp.

Yes, you are right... my bad

As I understand it, radiators are designed to have a temperature differential of 5 deg at full engine power and full design airflow. Despending on engine speed, load and road speed etc, this can vary, and may well be only a couple of degrees at 100 kph cruise.

So with a 92 deg switch in the bottom tank, the top tank could be around 95 deg before the fan starts; not a problem.

Interesting topic. I control my fan with my ECU which is sensing engine temp from the sensor on the back of the head. For normal driving the temp is controlled by the thermostat, and stays rock steady at 85 deg. The only time the fan comes on is when the engine comes back to idle after a run. Once the temp goes above 85 deg the stat is fully open and can no longer regulate. Using the example above with 15 deg temp differential, coolant is passing the wide open stat at 100 deg and the engine temp will be floating.

Ideally (and I'm making this sh*t up as I go) the fan(s) should bring the radiator leaving water temp back down to a point where the stat can start regulating the engine temp again. However, as pointed out, you don't want the fan switch to be set lower that the stat, to avoid a control conflict. I turn my fan on at 87 deg and off at 85 deg... seems to work OK.

Cheers... jondee86