# Radiator fan stays on even when truck is cold



## Nicka (Jan 7, 2020)

I’ve owned my 94 Hardbody for about six months now, and it’s always had a problem where the temp gauge reads that the engine is cold. It’s winter now and the needle barely gets above the cold mark. Even when the truck is ice cold in the morning, in below freezing weather, the radiator fan comes on as soon as I start it and it stays on. Is this normal? I’ve checked the fan when the engine is off and it spins freely, which from what I’ve heard, means that the fan clutch isn’t locked. I haven’t replaced the thermostat, but I think it was replaced before I bought the truck. It just doesn’t seem right that the fan would turn on as soon as I started it.
Any help would be much appreciated!


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## jp2code (Jun 2, 2011)

First thing, you are describing an electric fan. The Nissan D21 Hardbody trucks did not come with electric fans, so whoever installed it didn't do a good job. They may have hard wired the fan to turn on when the engine comes on or they may not have known how to set the ON setting for the electric fan.


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## Nicka (Jan 7, 2020)

jp2code said:


> First thing, you are describing an electric fan. The Nissan D21 Hardbody trucks did not come with electric fans, so whoever installed it didn't do a good job. They may have hard wired the fan to turn on when the engine comes on or they may not have known how to set the ON setting for the electric fan.


No it has the stock mechanical fan! Sorry I didn’t know how to differentiate. I guess I thought they were all just called radiator fans. My bad for the confusion.


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## jp2code (Jun 2, 2011)

Ok, so I misunderstood this piece below 



Nicka said:


> Even when the truck is ice cold in the morning, in below freezing weather, the radiator fan comes on as soon as I start it and it stays on.


You should have a fan clutch then.

With a fan clutch, the fan will always spin a little, but the clutch should lock the fan ON whenever the engine starts to pull hot air through the radiator and gets the clutch warm.

When you crank a cold engine, it is normal for a fan clutch to grab the fan and make a lot of noise under the hood for 30 seconds or so. 

Is that what is going on?


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## Nicka (Jan 7, 2020)

jp2code said:


> Ok, so I misunderstood this piece below
> 
> 
> 
> ...


That would make sense for it to grab the fan when I first start the engine and keep it on for a little. But I checked this morning and after about 10 minutes the fan still didn’t slow down or anything. I’m worried that if the fan doesn’t turn off, that could be why the engine isn’t getting up to operating temp.


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## jp2code (Jun 2, 2011)

The fan would not effect the engine not getting up to temperature unless the thermostat was missing or the wrong temperature thermostat. Until the engine coolant gets to the thermostat temperature, the thermostat stays closed and keeps the coolant in the engine. That helps the engine get to operating temperature quickly. Once that temperature is reached, the thermostat opens to let coolant flow into the radiator where it cools down. 

It sounds like your thermostat is bad or you got one that is designed to open too low.

I think the Hardbody trucks are supposed to be a 180 degree thermostat.


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## Nicka (Jan 7, 2020)

jp2code said:


> The fan would not effect the engine not getting up to temperature unless the thermostat was missing or the wrong temperature thermostat. Until the engine coolant gets to the thermostat temperature, the thermostat stays closed and keeps the coolant in the engine. That helps the engine get to operating temperature quickly. Once that temperature is reached, the thermostat opens to let coolant flow into the radiator where it cools down.
> 
> It sounds like your thermostat is bad or you got one that is designed to open too low.
> 
> I think the Hardbody trucks are supposed to be a 180 degree thermostat.





jp2code said:


> The fan would not effect the engine not getting up to temperature unless the thermostat was missing or the wrong temperature thermostat. Until the engine coolant gets to the thermostat temperature, the thermostat stays closed and keeps the coolant in the engine. That helps the engine get to operating temperature quickly. Once that temperature is reached, the thermostat opens to let coolant flow into the radiator where it cools down.
> 
> It sounds like your thermostat is bad or you got one that is designed to open too low.
> 
> I think the Hardbody trucks are supposed to be a 180 degree thermostat.


Okay. I think the previous owner replaced the thermostat. He thought that the problem was the temp gauge, so before I replace the thermostat again, I want to check the coolant temp to make sure it isn’t just the gauge reading wrong.
Can I take off the radiator cap while the engine is warm and check the coolant temp with an infrared thermometer? I’ve heard of people doing this, but I always thought taking the radiator cap off would cause coolant to spray out


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## jp2code (Jun 2, 2011)

Nicka said:


> Can I take off the radiator cap while the engine is warm and check the coolant temp with an infrared thermometer?


You can remove the radiator cap while the engine is cold, crank the engine and let it warm up.

No, don't try taking the cap off of a hot radiator. It's under a lot of pressure.


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## rogoman (Dec 16, 2004)

Nicka said:


> I’ve owned my 94 Hardbody for about six months now, and it’s always had a problem where the temp gauge reads that the engine is cold. It’s winter now and the needle barely gets above the cold mark. Even when the truck is ice cold in the morning, in below freezing weather, the radiator fan comes on as soon as I start it and it stays on. Is this normal? I’ve checked the fan when the engine is off and it spins freely, which from what I’ve heard, means that the fan clutch isn’t locked. I haven’t replaced the thermostat, but I think it was replaced before I bought the truck. It just doesn’t seem right that the fan would turn on as soon as I started it.
> Any help would be much appreciated!


A fan clutch is a thermostatic engine cooling fan that can freewheel at low temperatures when cooling is not needed, allowing the engine to warm up faster, relieving unnecessary load on the engine. As temperatures increase, the clutch engages so that the fan is driven by engine power and moves air to cool the engine. 

Turn the fan by hand; grab the edge of one of the blades and give it a good push. While there should be a little bit of give, it shouldn’t spin more than two or three complete rotations. Too much free-wheeling is usually a sign that the clutch is prone to slipping. On the other hand, too much resistance means the clutch is binding and can’t turn freely. In either case, it will need to be replaced.


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## Nicka (Jan 7, 2020)

jp2code said:


> You can remove the radiator cap while the engine is cold, crank the engine and let it warm up.
> 
> No, don't try taking the cap off of a hot radiator. It's under a lot of pressure.


I started the engine and let it idle for about 15 minutes to get up to temp. The temp gauge needle got to where it stays normally, and my thermometer showed the coolant to be about 135. Coolant was coming out of the hole so I know it was flowing. I guess I’ll replace the thermostat and hopefully it does the trick.


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## Nicka (Jan 7, 2020)

Nicka said:


> I started the engine and let it idle for about 15 minutes to get up to temp. The temp gauge needle got to where it stays normally, and my thermometer showed the coolant to be about 135. Coolant was coming out of the hole so I know it was flowing. I guess I’ll replace the thermostat and hopefully it does the trick.


The thermostat being broken would also explain why I get such bad fuel economy and why my truck feels slower than it should be...


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## jp2code (Jun 2, 2011)

You can test a thermostat by placing it in a pot of water on your stove and bringing the water up to 180 degrees. But, thermostats are cheap. Most people would rather just buy a new one that go through the time to test one.


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## Nicka (Jan 7, 2020)

jp2code said:


> You can test a thermostat by placing it in a pot of water on your stove and bringing the water up to 180 degrees. But, thermostats are cheap. Most people would rather just buy a new one that go through the time to test one.


Alright, I will probably try that sometime this week. Much thanks for the info!


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## Nicka (Jan 7, 2020)

rogoman said:


> A fan clutch is a thermostatic engine cooling fan that can freewheel at low temperatures when cooling is not needed, allowing the engine to warm up faster, relieving unnecessary load on the engine. As temperatures increase, the clutch engages so that the fan is driven by engine power and moves air to cool the engine.
> 
> Turn the fan by hand; grab the edge of one of the blades and give it a good push. While there should be a little bit of give, it shouldn’t spin more than two or three complete rotations. Too much free-wheeling is usually a sign that the clutch is prone to slipping. On the other hand, too much resistance means the clutch is binding and can’t turn freely. In either case, it will need to be replaced.


I spun it as hard as I could and it barely turned past 1/2 of a rotation... Is that too much resistance?


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## rogoman (Dec 16, 2004)

Yes, too much resistance. That's why it's always spining hot or cold. Time to replace the unit.


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## Nicka (Jan 7, 2020)

rogoman said:


> Yes, too much resistance. That's why it's always spining hot or cold. Time to replace the unit.


Ahh alright. Thank you much for the info!


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## Nicka (Jan 7, 2020)

jp2code said:


> You can test a thermostat by placing it in a pot of water on your stove and bringing the water up to 180 degrees. But, thermostats are cheap. Most people would rather just buy a new one that go through the time





jp2code said:


> You can test a thermostat by placing it in a pot of water on your stove and bringing the water up to 180 degrees. But, thermostats are cheap. Most people would rather just buy a new one that go through the time to test one.


I tested the thermostat and it works. It opened while in the hot water and closed when it was out of the water. What else could the problem be? I’m going to replace the fan clutch while I have it out and hope it helps. Or it could be the temp gauge, and I could have just taken the coolant temp wrong or not let it warm up enough.


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