Radiator Fans Won't Turn off

chadzeilenga · Sep 19th, 2009, 11:18 AM

I'm finishing up my 99 Sentra and when I drive it around the radiator fans stay on all the time. I do have a CEL for the rear O2, but don't think that could be causing it. I replaced the engine with another GA16DE engine and felt that I bled the coolant system well enough. Is there something I'm missing? I ran the car with a funnel in the rad to let all of the air bleed out.

zacward · Sep 19th, 2009, 09:06 PM

Coolant temp sensor?

pulsar86 · Sep 19th, 2009, 11:43 PM

I agree you most likely have a fault in your coolant temp sensor in the bottom tank of radiator that controls when the fan operates.

chadzeilenga · Sep 20th, 2009, 12:25 AM

The coolant temp sensor is in the lower tank of the radiator? I thought it was on the pass side of the engine back by where the heater hoses come off. Does any one have a pic?

IanH · Sep 20th, 2009, 12:36 PM

turn the AC off ??

pulsar86 · Sep 20th, 2009, 03:41 PM

The coolant temp sender is usually in the bottom radiator tank on most Nissans I have owned or worked on, the other sender on the engine itself is for the temp gauge. The sender for the fan will be easy to find just follow the wiring back from the fan itself.

chadzeilenga · Sep 20th, 2009, 10:21 PM

The AC is off, it hasn't been on because it's broken with a leak in the line.

I'll check the radiator for the sensor. You're saying that the one in the engine is just for the gauge and won't affect anything? The engine doesn't seem to run hot or need the fans on as much as they are.

chadzeilenga · Sep 21st, 2009, 11:00 AM

Last night I did the following items.

Opened up bleeder screw near heater hose with engine hot and running, few small bubbles came out. No change
Swapped the coolant temp sensor near this same plug. No change
Ran car for a while with full funnel in place of rad cap to help bleed air out, no bubbles, no change.
Squeezed upper & lower rad hoses a bit but noticed no bubbles.

The lower radiator hose is always very cool to the touch and the upper is very hot. What would cause this?

Beverly Stayart · Sep 21st, 2009, 11:16 AM

The coolant temp sensor should be clearly marked. If you can't locate it, check your manual or ask your dealer.

pulsar86 · Sep 22nd, 2009, 12:12 AM

It is normal for the top hose to be hot and the bottom cooler, it is because the hot coolant enters at the top from the engine block. It means your radiator is doing a good job. Have you located the sender in the bottom tank? Just follow the wiring from the radiator cooling fan and it should be connected to it. It sounds more and more like a faulty sender that controls the fan to me.

chadzeilenga · Sep 22nd, 2009, 01:40 AM

I looked at the wiring for both of the fans on the radiator and it doesn't seem to continue on to a sending unit. The wires to both fans end at the fan, maybe it is upstream a bit from the fan motors? Does anyone have a pic of where it's at?

IanH · Sep 22nd, 2009, 11:47 AM

IanH · Sep 22nd, 2009, 11:54 AM

you can see that the cooling fans are controlled by the ECU, the temp sensor ties into the ecu.

chadzeilenga · Sep 23rd, 2009, 02:38 AM

I figured it out. It was either the thermostat keeping the car too hot or it was the fan switch that I swapped out. The fan switch is located just below and rearward of the distributor on the engine and has a single prong to it. I did the thermostat last night and then fan switch today before I topped it off with fresh coolant. Now the fans come only only when needed. Thanks everyone!