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What is the purpose of the engine coolant temperature switch?

Jason G.

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Oct 7, 2003
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Anderson, SC
Turns the fan on if it gets too hot I think. I don't remember at what temp that it turns on though. If the wire is grounded, the fan will come on with the ignition.
 

ktmrider

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Sep 10, 2007
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Tempe, AZ

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Which one, #4? This diagram is posted numerous places, like Jason mentioned it turns on the secondary/AC fan ( regardless of AC activity ).
 

turbowop

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Apr 29, 2001
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Yakima, WA
Sensor in bottom of radiator is what turns the puller fan on first under normal operation. Turns it on at roughly 206F and then back off at about 203F. The switch is actually set for a much lower temp since it's in the bottom of the radiator, but that is about what the coolant temperature is at the tstat when it kicks the fans on/off.

The #4 switch is a last resort to cool the car down during an overheating situation. It kicks on the AC pusher fan (even if AC is turned off) in an attempt to cool things down, but it doesn't kick on that fan until around 230F or so.

This is what is cool about ECMlink. You can run a wire to that #4 switch, and have ECMlink ground it to trigger the relay that controls both those fans, at whatever temp you want. I actually have mine setup to just trigger just the puller fan relay, but same idea. That way, when using a lower temp tstat like the OEM 180F version, you can have the fans come on sooner for proper operation. So now I have my puller fan set to kick on at about 194, much less than what it does under stock operation. Keeps things much cooler in traffic or while stuck in the staging lanes at a dragstrip.
 
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SleepinGVR4

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Feb 12, 2003
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2,483
Location
Danville, Pennsylvania
Thanks Mark that helped a lot. I had my wire to the coolant temp switch break so I had to solder in a new piece of wire and new connection to the switch.

But how did you used ecmlink to trigger the puller fan? I thought you couldn't do this on a 1st gen / gvr4 ecu?
 

turbowop

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You have to run a wire from one of the ECU's output pins that ECMlink can control, like the old FPS solenoid. Splice that wire into the ground wire on the relay that controls the puller fan and you can control it through ECMlink using any number of parameters. One of those parameters is coolant temp, so you can have it pull that relay to ground via the new wire that you ran at any coolant temp you choose, turning on the fans.

On the 2g cars, you can do all of this via the ECU without running wires since the fans were originally ECU controlled anyway. Those of us with 1g's or GVR4's have to run a wire and control it the way I explained.
 
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prove_it

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Jul 3, 2008
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Sioux Falls, SD
I always thought about rewiring my lower rad switch so I can install it in the thermostat housing. Hotter coolant there will kick the fan on faster. Not sure how feasible it would be though.
 

turbowop

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The fan would run nonstop once the car was warmed up if you did that. Might as well just hardwire it to the ignition at that point.
 

prove_it

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Quoting turbowop:
The fan would run nonstop once the car was warmed up if you did that. Might as well just hardwire it to the ignition at that point.



Yea, don't want that. would be too much when it gets colder. Damn non ecu controlled fans.
 
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