Fan Relay
water temperature problem
- Thread starter Martin_VTEC
- Start date
Hi, Martin_VTEC.
The most common cause for the cooling fan operation failure is the cooling fan control unit or the relay. The failure rate is not that high compared to the Main Relay but enough for the NSX specialists in Japan to always keep them in stock.
You seemed to have checked the cooling fan control sensor (brown) located at the thermostat housing (thermo cover) so we can eliminate this.
If you saw the engine bay cooling fan in operation while at idle rpm with normal coolant level, altitude and the weather was not that extremely hot (below 40degC), then you are very likely to have cooling fan control unit issue.
I never saw this engine bay cooling fan in operation at idle rpm even after tracking the car and getting back to the garage in hot summer above 40degC.
Even Honda removed this fan from later spec MT model.
I also remove this fan during my Engine Refresh service on any MT models.
If the coolant level is way too low, the sensor won’t be submerged in the coolant resulting in no cooling fan operation.
I have seen this when one owner had slow leakage and the coolant level was way too low.
The temperature gauge was bouncing up and down rapidly when the engine was blipped because the coolant was just enough to touch the sensor head (temperature sender) depending on the speed of the WP.
It lost about 2.7L of coolant so there was not enough coolant to touch the sensor head at the thermo cover.
Now that you installed the new radiator and presuming that it was bled properly, I believe it is very unlikely to be the coolant level related.
The cause of cooling fan control unit failure is the soldering crack.
Because of this, sometimes it works perfectly and other times, it could continuously run the radiator and even the engine bay fan when the engine was still cold or it won’t run any fans at all when it should be.
You can see my recent example with the video here.
http://www.nsxcb.co.uk/entry.php?981-Cooling-Fan-Control-Unit
If you have the multimeter, you can check the relay control line of the cooling fan control unit to see whether it creates the GND connection to activate the relay when the sensor at the thermo cover is at about 1Kohm or the coolant temperature has reached about 84degC for the first time and then deactivates the relay once it gets below 78degC.
As a quick check, you can re-solder all of the joints but ideally, you should replace it with a new one.
The soldering crack can happen at the base of the connector as well as anywhere on the board especially around the IC section.
Also, the relays can fail.
You can check them on the bench after removing them from the car or as you can guess from above, you can GND the wires at the cooling fan control unit to force the radiator fan On if you have the wiring diagram.
Of course, you could have other cause of the issue but just wanted to share my experience.
Kaz
The most common cause for the cooling fan operation failure is the cooling fan control unit or the relay. The failure rate is not that high compared to the Main Relay but enough for the NSX specialists in Japan to always keep them in stock.
You seemed to have checked the cooling fan control sensor (brown) located at the thermostat housing (thermo cover) so we can eliminate this.
If you saw the engine bay cooling fan in operation while at idle rpm with normal coolant level, altitude and the weather was not that extremely hot (below 40degC), then you are very likely to have cooling fan control unit issue.
I never saw this engine bay cooling fan in operation at idle rpm even after tracking the car and getting back to the garage in hot summer above 40degC.
Even Honda removed this fan from later spec MT model.
I also remove this fan during my Engine Refresh service on any MT models.
If the coolant level is way too low, the sensor won’t be submerged in the coolant resulting in no cooling fan operation.
I have seen this when one owner had slow leakage and the coolant level was way too low.
The temperature gauge was bouncing up and down rapidly when the engine was blipped because the coolant was just enough to touch the sensor head (temperature sender) depending on the speed of the WP.
It lost about 2.7L of coolant so there was not enough coolant to touch the sensor head at the thermo cover.
Now that you installed the new radiator and presuming that it was bled properly, I believe it is very unlikely to be the coolant level related.
The cause of cooling fan control unit failure is the soldering crack.
Because of this, sometimes it works perfectly and other times, it could continuously run the radiator and even the engine bay fan when the engine was still cold or it won’t run any fans at all when it should be.
You can see my recent example with the video here.
http://www.nsxcb.co.uk/entry.php?981-Cooling-Fan-Control-Unit
If you have the multimeter, you can check the relay control line of the cooling fan control unit to see whether it creates the GND connection to activate the relay when the sensor at the thermo cover is at about 1Kohm or the coolant temperature has reached about 84degC for the first time and then deactivates the relay once it gets below 78degC.
As a quick check, you can re-solder all of the joints but ideally, you should replace it with a new one.
The soldering crack can happen at the base of the connector as well as anywhere on the board especially around the IC section.
Also, the relays can fail.
You can check them on the bench after removing them from the car or as you can guess from above, you can GND the wires at the cooling fan control unit to force the radiator fan On if you have the wiring diagram.
Of course, you could have other cause of the issue but just wanted to share my experience.
Kaz
Hi Kaz,
Thank u for sharing this with me
I definitely going to check this cooling fan control unit relais.
My car is a 1991 NSX with 198.000 km on it.
It had many electrical issues in the past 4 years: Main relais, Igniter unit, coil-packs, alternator, startermotor
So a faulty cooling fan relais could also be possible haha
I never had the symptoms that the radiator fan is going on when starting the engine.
Is it as easily to open as the main relais ? First of all I'll let this relais be checked and resoldered by a friend of mine who is an electrician.
Because at the local dealer the relais costs 310 euro, that's 410 dollar !!!!
If that's done I can rule that relais out.
I'll keep you guys posted !
Thank u for sharing this with me
I definitely going to check this cooling fan control unit relais.
My car is a 1991 NSX with 198.000 km on it.
It had many electrical issues in the past 4 years: Main relais, Igniter unit, coil-packs, alternator, startermotor
So a faulty cooling fan relais could also be possible haha
I never had the symptoms that the radiator fan is going on when starting the engine.
Is it as easily to open as the main relais ? First of all I'll let this relais be checked and resoldered by a friend of mine who is an electrician.
Because at the local dealer the relais costs 310 euro, that's 410 dollar !!!!
If that's done I can rule that relais out.
I'll keep you guys posted !
Hi, Martin_VTEC.
The basic of cooling fan control is very simple. However, in order to prevent frequent on/off situation, it is quite standard method to apply hysteresis on any on/off control circuit and hence, the presence of the cooling fan control unit.
The radiator fan control sensor at the thermo cover is connected to the Cooling Fan Control Unit (CFCU).
Based on the resistance reading, the CFCU will activate the radiator fan low mode relay by creating the GND circuit at the control terminal of the relay.
The amount of current passing through the radiator fan motor is reduced by that white square block resistor mounted at the top right side of the fan shroud just behind the radiator and hence, the fan operates in lower speed mode.
If the coolant temperature gets higher (above 90degC), then the CFCU will activate another relay (radiator fan high mode relay) and bypass the square block resistor to allow the fan to operate at max speed.
At the same time, it will switch on the engine bay fan (so this fan only operates when the coolant temperature is above 90degC).
By the way, it is easy to remove the lid from the CFCU.
Following link is from my NSX Technical and Service Information Index 02 thread at NSXCB.
For non-DBW model, it’s sitting next to the Main Relay.
When re-soldering, please ask your friend to carefully solder it because the coating behind the circuit board (parts side) tends to evaporate under heat through the hole preventing the solder to create good bonding.
So, as you can see, the whole control is just based on the resistor value of a single sensor.
It’s always handy to have several different resistors with a certain terminal attached when diagnosing resistance based control.
Ask your friend to get 1K ohm resistor with suitable terminal crimped.
Remove the sensor connector from the thermo cover and insert this resistor to the removed connector. Turn the IG switch to On position (no need to start the engine) and your rad fan should operate in low speed mode (engine bay fan stays Off). Please make sure to disarm your security device because some of them will not allow the power to certain electronics circuits on the car.
You don’t need to warm your engine up, in fact, you don’t even need to start the engine for a quick check by using this method although you need to remember that every time when you operate the cooling fan, you are draining the battery.
You can use the same method for testing the gauge as well.
Just get 50 ohm resistor and stick it into the single wire socket at the temperature sender unit and GND the other end.
Your temperature gauge needle would be around one marker below the horizontal line.
Probably, you are aware of this but for the clarification, the CFCU and the two relays (High and Low speed mode) are different things. CFCU is located behind the seat almost in the middle of two seats.
The two relays for your year model are located inside the black square box above the blower motor fan under the bonnet/hood in the front compartment.
Kaz
The basic of cooling fan control is very simple. However, in order to prevent frequent on/off situation, it is quite standard method to apply hysteresis on any on/off control circuit and hence, the presence of the cooling fan control unit.
The radiator fan control sensor at the thermo cover is connected to the Cooling Fan Control Unit (CFCU).
Based on the resistance reading, the CFCU will activate the radiator fan low mode relay by creating the GND circuit at the control terminal of the relay.
The amount of current passing through the radiator fan motor is reduced by that white square block resistor mounted at the top right side of the fan shroud just behind the radiator and hence, the fan operates in lower speed mode.
If the coolant temperature gets higher (above 90degC), then the CFCU will activate another relay (radiator fan high mode relay) and bypass the square block resistor to allow the fan to operate at max speed.
At the same time, it will switch on the engine bay fan (so this fan only operates when the coolant temperature is above 90degC).
By the way, it is easy to remove the lid from the CFCU.
Following link is from my NSX Technical and Service Information Index 02 thread at NSXCB.
For non-DBW model, it’s sitting next to the Main Relay.
When re-soldering, please ask your friend to carefully solder it because the coating behind the circuit board (parts side) tends to evaporate under heat through the hole preventing the solder to create good bonding.
So, as you can see, the whole control is just based on the resistor value of a single sensor.
It’s always handy to have several different resistors with a certain terminal attached when diagnosing resistance based control.
Ask your friend to get 1K ohm resistor with suitable terminal crimped.
Remove the sensor connector from the thermo cover and insert this resistor to the removed connector. Turn the IG switch to On position (no need to start the engine) and your rad fan should operate in low speed mode (engine bay fan stays Off). Please make sure to disarm your security device because some of them will not allow the power to certain electronics circuits on the car.
You don’t need to warm your engine up, in fact, you don’t even need to start the engine for a quick check by using this method although you need to remember that every time when you operate the cooling fan, you are draining the battery.
You can use the same method for testing the gauge as well.
Just get 50 ohm resistor and stick it into the single wire socket at the temperature sender unit and GND the other end.
Your temperature gauge needle would be around one marker below the horizontal line.
Probably, you are aware of this but for the clarification, the CFCU and the two relays (High and Low speed mode) are different things. CFCU is located behind the seat almost in the middle of two seats.
The two relays for your year model are located inside the black square box above the blower motor fan under the bonnet/hood in the front compartment.
Kaz
Thank u Kaz, I already removed the cooling fan control unit and brought it to my friend.
If I remember well, I checked te relays under the bonnet last year and they were OK.
I'll install the cfcu back on monday. I'll keep u informed.
If I remember well, I checked te relays under the bonnet last year and they were OK.
I'll install the cfcu back on monday. I'll keep u informed.
Ok, I got the cfcu back and there was definitely a few a solder cracks.
I've reinstalled the refurbished cfcu and took a ride and let it idle afterwards...
and....YES ! My fan is turning !!
I want to thank everyone who helped me try to figured it out and especially the guys who mentioned the cfcu issue
So I took her again for a half hour drive, and a few times drive her very hard....
Until...... My battery-light came on and i saw my volt-meter dropping from 14.5 to 12....Damm... I thought my alternator was broken again... It's only a year from new
From one problem to another problem....
I drove her back to my home, only 2 miles thank god, opened the hood en to my suprise the alternator belt was off the crankshaft pulley, but still on the alternator poullie.....
I took the belt off the alternator, twist the alternator pully and it didnt ran completely freely....a bit jerky.. I don't know how to describe it.
but after 10 minuts waiting, you could turn the pulley freely, nothing wrong with it....
So... My belt looks OK, It was also new when I installed the new (not OEM Honda ) alternator last year. I can offcourse reinstall the belt, but i've got a feeling that the bearing of the alternator doesn't run freely when
its really hot, and if I reinstall the belt it wil go off soon or later...
It could also be that with these old car doesn't have any automatic tensioners and it has a new belt that there was a little stretching and so it had slack ?
But I never had this problem with my other old Honda's..
The alternator is new, al lined up well with the crackshaft pulley .....Anyone had that same experience ??
Oh, the reason why a bought a new alternator last year was because the housing of the alternator was cracked.
Maybe I can switch the internals of the alternators ? or at least the bearings ?
I've reinstalled the refurbished cfcu and took a ride and let it idle afterwards...
and....YES ! My fan is turning !!
I want to thank everyone who helped me try to figured it out and especially the guys who mentioned the cfcu issue
So I took her again for a half hour drive, and a few times drive her very hard....
Until...... My battery-light came on and i saw my volt-meter dropping from 14.5 to 12....Damm... I thought my alternator was broken again... It's only a year from new
From one problem to another problem....
I drove her back to my home, only 2 miles thank god, opened the hood en to my suprise the alternator belt was off the crankshaft pulley, but still on the alternator poullie.....
I took the belt off the alternator, twist the alternator pully and it didnt ran completely freely....a bit jerky.. I don't know how to describe it.
but after 10 minuts waiting, you could turn the pulley freely, nothing wrong with it....
So... My belt looks OK, It was also new when I installed the new (not OEM Honda ) alternator last year. I can offcourse reinstall the belt, but i've got a feeling that the bearing of the alternator doesn't run freely when
its really hot, and if I reinstall the belt it wil go off soon or later...
It could also be that with these old car doesn't have any automatic tensioners and it has a new belt that there was a little stretching and so it had slack ?
But I never had this problem with my other old Honda's..
The alternator is new, al lined up well with the crackshaft pulley .....Anyone had that same experience ??
Oh, the reason why a bought a new alternator last year was because the housing of the alternator was cracked.
Maybe I can switch the internals of the alternators ? or at least the bearings ?
Last edited:
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