why are impreza fans 3 wire and not 2 ??
Scooby Newbie
Joined: Apr 2009
Posts: 3
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From: Vermont, USA
fans with 4 wires
Subaru calls one of the fans the "main" fan and the other is called "subFan". The ECU sends signals to several relays in order to manipulate and control the fan operation. (one/both/slow/fast....etc) The ECU 'knows' the engine temparture and if the AC-compressor is running. These signals are used to determine fan-settings.
Relays are used so the ECU can control high-current devices (fans) using low-current signals. Also, a fan is an inductive load which could fry the ECU if connected directly to ECU.
The first answer is correct, There are 2 indipendant coils in each motor which allow for 2-speeds. However, in some subies, the 2 coils are wired in parallel which effectivly make it a single-speed fan.
My car used to be wired like that from factory. A half- hour with a DVM (Digital Volt Meter) allowed me to draw up a wiring diagram of my system and re-design it.
I have rewired my car to take the 3 signals from the ECU and run BOTH fans at low speed at initial kick-in temparture. This way, my cooling-system is essentially silent except when max. cooling is needed.
Here in Vermont, I do not expect my fans to EVER need to run at max. speed..... we just dont get the hot weather.
Last edited by brucep; Apr 21, 2009 at 02:44 AM. Reason: Some folks felt original append was abrasive - Do not want to offend anyone.
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You dont know much about electronics - do you? I have engineering degree in electronics for over 20 years and work in semiconductor manufacturing plant. (Internal combustion engines is just an engineering hobby of mine.)
Subaru calls one of the fans the "main" fan and the other is called "subFan". The ECU sends signals to several relays in order to manipulate and control the fan operation. (one/both/slow/fast....etc) The ECU 'knows' the engine temparture and if the AC-compressor is running. These signals are used to determine fan-settings.
Relays are used so the ECU can control high-current devices (fans) using low-current signals. Also, a fan is an inductive load which could fry the ECU if connected directly to ECU.
The first answer is correct, There are 2 indipendant coils in each motor which allow for 2-speeds. However, in some subies, the 2 coils are wired in parallel which effectivly make it a single-speed fan.
My car used to be wired like that from factory. A half- hour with a DVM (Digital Volt Meter) allowed me to draw up a wiring diagram of my system and re-design it.
I have rewired my car to take the 3 signals from the ECU and run BOTH fans at low speed at initial kick-in temparture. This way, my cooling-system is essentially silent except when max. cooling is needed.
Here in Vermont, I do not expect my fans to EVER need to run at max. speed..... we just dont get the hot weather.
Subaru calls one of the fans the "main" fan and the other is called "subFan". The ECU sends signals to several relays in order to manipulate and control the fan operation. (one/both/slow/fast....etc) The ECU 'knows' the engine temparture and if the AC-compressor is running. These signals are used to determine fan-settings.
Relays are used so the ECU can control high-current devices (fans) using low-current signals. Also, a fan is an inductive load which could fry the ECU if connected directly to ECU.
The first answer is correct, There are 2 indipendant coils in each motor which allow for 2-speeds. However, in some subies, the 2 coils are wired in parallel which effectivly make it a single-speed fan.
My car used to be wired like that from factory. A half- hour with a DVM (Digital Volt Meter) allowed me to draw up a wiring diagram of my system and re-design it.
I have rewired my car to take the 3 signals from the ECU and run BOTH fans at low speed at initial kick-in temparture. This way, my cooling-system is essentially silent except when max. cooling is needed.
Here in Vermont, I do not expect my fans to EVER need to run at max. speed..... we just dont get the hot weather.
and what you are saying here is plain bullsh*thigh and low current, i think you mean high and low voltage here my friend, there is a significant difference, and reading that i doubt you have an electronics degre in anything, i might even say you have no degree at all, perhaps a degree in stupidity
perhaps an electronics degree from mars 
this is a forum where ppl help each other understand about their subarus and a simple question like the above requires a simple answer, KISS remember.. keep it simple stupid
sub fan ? lets just call it the fan for the aircon which would not be present if the car didnt have aircon, and the fans run only one speed and thats full speed until engine has cooled down and and yes the ecu will signal the relays located in the engine bay fuse box to start the fan when it has reached a certain degree, around 96 degrees celcious on top of my head
Scoobyboy : alternators today have a voltage regulator built in to prevent power surge when fans and other power consuming gadgets turns on
So crawl back under the stone where you came from bruce and dont try to impress anyone here with you non sence bullsh*t
reg
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Joined: Nov 2001
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From: uk
reg i didn't say they do it to prevent a power surge it's there to relieve the load on the alternator so it doesn't have to work as much hence why subaru went over to a smart charge system where it only charges if it needs to,
if you have a problem with why they do it that way take it up with subaru!
if you have a problem with why they do it that way take it up with subaru!
Scooby Newbie
Joined: Apr 2009
Posts: 3
Likes: 0
From: Vermont, USA
My goodness, 
It seems that I am not the one with some issues here. I was trying to provide a thorough answer to the question. I would think a fellow engineer would want to help in this reguard instead of nitpick about semantics.
You are correct that if we were not talking about an automobile electronic system here, the discussion about relays reguarding voltage might be different but I am trying to keep the facts simple (KISS)
But in this case - the relays are there to actuate HIGH CURRENT devices with a LOW CURRENT signal. (It is all 12 volts here) Semiconductors such as within the ECU would fry if they tried to drive a fan directly. (same with headlights and several other relay-controlled items.)
Also, the "subfan" is the one which is NOT for the AC. You may chose to prove this to yourself by pulling out your "subfan" relay and see which fan quits running.
To answer the question "why I would want my cooling-system to be silent".... I got tired of the subfan running ALL THE TIME even when -20F outside. (Fired ECU which the recall does not cover
) Also the constant load on my alternator was undesirable.
It seems that I am not the one with some issues here. I was trying to provide a thorough answer to the question. I would think a fellow engineer would want to help in this reguard instead of nitpick about semantics.You are correct that if we were not talking about an automobile electronic system here, the discussion about relays reguarding voltage might be different but I am trying to keep the facts simple (KISS)
But in this case - the relays are there to actuate HIGH CURRENT devices with a LOW CURRENT signal. (It is all 12 volts here) Semiconductors such as within the ECU would fry if they tried to drive a fan directly. (same with headlights and several other relay-controlled items.)
Also, the "subfan" is the one which is NOT for the AC. You may chose to prove this to yourself by pulling out your "subfan" relay and see which fan quits running.
To answer the question "why I would want my cooling-system to be silent".... I got tired of the subfan running ALL THE TIME even when -20F outside. (Fired ECU which the recall does not cover
) Also the constant load on my alternator was undesirable.
No its doing exactly what it should do. Dont be taken in by some of the bull above. They normally switch in at around 96c and switch off again at around 91c.
You dont know much about electronics - do you? I have engineering degree in electronics for over 20 years and work in semiconductor manufacturing plant. (Internal combustion engines is just an engineering hobby of mine.)
Subaru calls one of the fans the "main" fan and the other is called "subFan". The ECU sends signals to several relays in order to manipulate and control the fan operation. (one/both/slow/fast....etc) The ECU 'knows' the engine temparture and if the AC-compressor is running. These signals are used to determine fan-settings.
Relays are used so the ECU can control high-current devices (fans) using low-current signals. Also, a fan is an inductive load which could fry the ECU if connected directly to ECU.
The first answer is correct, There are 2 indipendant coils in each motor which allow for 2-speeds. However, in some subies, the 2 coils are wired in parallel which effectivly make it a single-speed fan.
My car used to be wired like that from factory. A half- hour with a DVM (Digital Volt Meter) allowed me to draw up a wiring diagram of my system and re-design it.
I have rewired my car to take the 3 signals from the ECU and run BOTH fans at low speed at initial kick-in temparture. This way, my cooling-system is essentially silent except when max. cooling is needed.
Here in Vermont, I do not expect my fans to EVER need to run at max. speed..... we just dont get the hot weather.
Subaru calls one of the fans the "main" fan and the other is called "subFan". The ECU sends signals to several relays in order to manipulate and control the fan operation. (one/both/slow/fast....etc) The ECU 'knows' the engine temparture and if the AC-compressor is running. These signals are used to determine fan-settings.
Relays are used so the ECU can control high-current devices (fans) using low-current signals. Also, a fan is an inductive load which could fry the ECU if connected directly to ECU.
The first answer is correct, There are 2 indipendant coils in each motor which allow for 2-speeds. However, in some subies, the 2 coils are wired in parallel which effectivly make it a single-speed fan.
My car used to be wired like that from factory. A half- hour with a DVM (Digital Volt Meter) allowed me to draw up a wiring diagram of my system and re-design it.
I have rewired my car to take the 3 signals from the ECU and run BOTH fans at low speed at initial kick-in temparture. This way, my cooling-system is essentially silent except when max. cooling is needed.
Here in Vermont, I do not expect my fans to EVER need to run at max. speed..... we just dont get the hot weather.
I too have a qualification in Electronic Engineering, even though 'only a HNC'. Not quite as good as your degree, but I can spell INDEPENDANT!
It's not nice to come on here with your SECOND post and rant on. I know you are trying to give 'technical advice' but chill out a bit.
Last edited by Proby521; Apr 20, 2009 at 10:52 AM.
Scooby Newbie
Joined: Apr 2009
Posts: 3
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From: Vermont, USA
You did not read carefully enough... my fan was running all the time because of ECU issue which was recalled for some vehicles. ( http://www.thel7.com/Recalls/WVB-12.pdf ) But my VIN is no on "their list" so I have to suffer with broken ECU. Thus, I took matters into my own hands and rewired my fans.
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