Hello friends, new owner of a 95 Maxima. Rescued from someone's back yard for $700 and been slowly getting it road worthy.

195K on the clock and the engine sounds great.

So far, 6 new injectors, spark plugs, water pump, knock sensor replaced.

Fresh coolant, oil, PS, brake fluid.

Been driving it around for a few days noticed the temp gauge would stay cool. Idling in traffic it would reach middle, but upon driving on the highway it would cool back down. Few cold days I barely had heat. Pulled out the t-stat and found a piece of gasket seal or something rubbery stuck in it. Oh well, so it was off the car, I replaced it anyway.

Topped everything off and started the car. Within a few minutes both cooling fans came on. I checked the temp with an infrared heat gun - barely at 100 degrees. I wanted to test my new t-stat, so on the right inside fender i found two fuses , rad fan 1 rad fan 2. I pulled rad fan 1 and both fans shut off. Cool.

Here's whats interesting...the car got upto 178 on my infrared and I saw the coolant start flowing in the radiator (i assumed the stat opened as it should). I let it run a little longer, about the time the infrared hit 192 both fans kicked on for about 30 seconds and then back off. Letting it idle, they continued to cycle keeping the car around 190 deg.

If i removed the rad fan 2 fuse they wouldn't come on at all. If i put the rad fan 1 fuse back they came on immediately again. If i turned on the a/c, both fans would kick on for about a minute then go off for a minute, then come on for about 30 seconds. The compressor wasn't cycling, but the fans were....

I've looked in the FSM, i'm lost on the operation of the cooling fans. Did I take out a primary circuit by removing that fuse and they now are on a backup circuit....? They seem to be working OK the car doesn't overheat.

I like to hobby with broken down cars, get them running so they can be useful to someone. Make a little money on the side. I've worked on quite a few different makes and models over the years doing this - but this perplexes me. I don't want to leave it with the fuse removed and pass a problem onto another person if it wasn't supposed to be operating that way.

Thanks in advance. Hope to be some help back to this forum as well.

 

4th Gen Nissan Maxima Cooling Fans 101

When you start the car and the engine is cold and the air conditioning is OFF, fans do not run. When the engine temperature reaches 200 degrees, both fans run at low speed. When engine temperature reaches 220 degrees, both fans run at high speed. The fans DO NOT ever run one at a time when the system is working properly

When you start the car cold and you turn the air conditioning ON, both fans run at low speed. When the engine temperature reaches 200 degrees, both fans run at high speed.

Pin 14 of the ECM goes to 0 volts, activating Fan Relay 1 which sends 12 volts to pin 1 of both fan motors, the low speed winding. Pin 4 of the motor is connected directly to ground.

Pin 13 of the ECM goes to 0 volts, activating Fan Relays 2 and 3. Fan Relay 2 controls fan motor 1, while Fan Relay 3 controls fan motor 2. The fan relays send 12 volts to pin 2 of the fan motor, the high speed winding. The fan relays also connect the fan motor pin 3 to ground.

On the fan motor connector:
pin 1 is a white with black stripe wire - low speed 12 volts
pin 4 is a black wire - low speed ground.
pin 2 is a white with blue stripe wire - high speed 12 volts.
pin 3 is a white with red stripe wire - high speed ground.

The ECM uses a water temperature sensor in the engine to determine when to turn on the fans. Also note that there are 2 water temperature sensors in the engine, one for the ECM and one for the dash gauge.

Dashboard fuse 17 powers the energizing coil of all 3 relays.
Underhood fuse D (RAD FAN 1) powers both fan motors at low speed.
Underhood fuse E (RAD FAN 2) powers both fan motors at high speed.

The ECTS (Engine Coolant Temperature Sensor) sends the temperature of the engine coolant to the ECU, which then energizes the appropriate relay(s).

-=*=-

because the cooling fans are 2 speed fans, this may be why you were thinking there was some sort of backup.

 

Well my dear Watson, perhaps we have solved this case.

If the coolant temp sensor is reading higher than actual coolant temp, I would assume this could kick on low speed (which i removed the fuse for) at 100degrees and then high speed at 190degrees (even though it should have been 220).

Is there a quick way to test the temp sensor?

Thanks for the super fast reply

 

Super fast reply was luck. Everyone wishes it would always be like that.

Anyway, you can test the ECTS two different ways. The easiest is to get an OBD code reader (not the real cheap type) and it will show you the coolant temperature as the ECTS reports it. Probably the best time would be in the morning before the car has been started as the reading should be about the same as the air temperature.

The other way is to remove the wire harness connector from the ECTS and measure the resistance. It should be about 3k ohms in the morning at 65 degrees, higher if it's colder.

 

Well thank you all for your replies. I want to update the thread, and perhaps alert those who may run into this same issue.

The problem was with the autozone / duralast thermostat.

Let's me explain what was happening. Many cars the thermostat is before the radiator, so as the engine heats up, it's a pretty simple process to open at set temperature and everything works as expected.

This car has the thermostat after the radiator. By leaving the cap off and getting a little more invasive with my infrared thermometer i discovered the following. The radiator inlet temperature and cooling fan operation was actually correct. At a radiator inlet of 200, the fans would kick on. Unfortunately, the outlet temp of the radiator was not yet high enough to open the t-stat. So with almost no coolant flow, the radiator fans would continue to cool the radiator and the engine would get hotter still. I saw as high as 222 inlet and 135 outlet temps.

By removing the fuse for the slow fan speed (as the above poster explained) my fans would wait until 220 degrees to kick on. By this time, the water was hot enough at the radiator outlet to keep the t-stat open, but apparently running the car too hot.

Driving down the road at highway speed the engine would run very hot as the airflow across the radiator would keep the thermostat closed. Engine performance seemed reduced, shifting harsh etc.

I didn't have this problem before replacing the thermostat (the old was stuck open) but I assumed a new part is good. Not in this case. Called the dealer, got price on an OEM Nissan T-stat and gasket for $24, dropped it in and it is perfect now. Dealer advised they had heard problems in cars with non OE t-stats.

I'm starting to feel that OEM t-stats are as requisite as OEM spark plugs and other things you just can't substitute after-market parts for.

I remember my parents had a subaru forester that the engine fans ran non-stop on also after an independent shop replaced the timing belt (and apparently did a coolant flush and installed a replacement thermostat from Orieley's). They thought the heads were gone, but $60 later from the Subaru dealer and it was fixed, again issue with non-OE t-stat. This car also had the t-stat after the radiator (on the bottom of the engine of all places).

Anyway - be aware. For $5.00 more than autozone, problem would have been fixed right the first time. Perhaps it should be noted since the How-To in this forum points to using an Autozone Thermostat....