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Discussion Starter · #1 ·
Hi everyone,

Ive been having this "fan stuck on full speed" issue with my car since I bought it. Didn't really bother me until it came to the emissions test (Which the car has failed miserably several times now) - One of the issues is that the car never gets up to operating temperature (halfway mark on the coolant guage) - it only gets to about 1/3

Car has new thermostat, cooling system has been bled properly, - other things for emissions have been done such as spark plugs, cap, rotor, leads, fuel additive, injector cleaner etc

Also - car has had the ac system removed (badly) by a previous owner so we are going to disregard the with A/C cooling fan circuit and just focus on the w/o AC fan circuit - ive already tried the simple stuff like jumping the pressure switch and it just doesn't work


SO - I have decided I will be converting the car over to the w/o AC fan circuit below:
Rectangle Font Slope Parallel Schematic

The issue im having is with the following piece of the circuit highlighted below:
Font Parallel Rectangle Slope Diagram

I have ordered a w/o AC water temp switch - my problem is that this switch seems to operate as a Normally Closed switch - from what I can research this is correct and they are supposed to be Normally Closed.

My issue is that - since the switch is Normally Closed - The fans are going to still be on full speed?? - I thought it would be a Normally Open switch - and when it gets to 85-90 degrees - the water temp switch would close - activate the relay - turn on the fan.

In this state, the fan will technically turn off when it gets to operating temperature? This relay operates that when 1 and 2 are grounded - 3 and 4 are closed - fan turns on. Since the switch is Normally closed - it will run until it gets to operating temperature... then turn off
Gesture Font Smile Parallel Circle


The only other condition here would be if the relay was reversed (So that when 1 and 2 are closed - 3 and 4 are open / 1 and 2 are open - 3 and 4 are closed) - Again this is only a compensation for what in my opinion is the wrong operation of the water temp switch.

Please let me know if I'm completely missing something here - This diagram is super simple yet I cant figure out a way in which it would work correctly without modifying the components (getting a Normally Open temp switch or reversed relay)
 

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'91 MR2 Turbo, '20 BRZ
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So with an original A/C system, there are two fans (Radiator(passenger) and AC Condenser(driver side)). Many, when removing the A/C system, do NOT remove the electrical system and cooling ECU (A/C Amplifier) that controls these fan relays. The temp sensor is either switch (non-AC) or thermistor (A/C). If you put in a switch instead, when the A/C amp is used, without all the other non-AC jumpers, it's likely to cause the A/C system to turn on the fans (both) full time as you describe (low resistance).

I ran into this once, where a customer bought a replacement radiator, and it had the wrong sensor in it vs. what the car was built for. Changing to the proper sensor resolved the problem. So verify if the electronics was indeed removed and re-wired to non A/C style

Schematic Font Rectangle Engineering Parallel


As you can see below, w/A/C there is a temp sensor per spec below...

Font Parallel Rectangle Number Line art


When it's W/O A/C then it's a switch:
Rectangle Font Parallel Number Circle


Secondly I believe you are reading the schematic wrong on the relay. Now it states that at low temps, the temp switch is CLOSED. When looking at the relay in the schematic you posted, which is the non A/C system, it shows AT REST, closed (Normally Closed type). Meaning, if the relay coil is NOT powered, it's contacts will be closed. (It's not a Normally Open relay...it's literal as Toyota draws it). Thus when the temp switch is CLOSED, it energizes the relay, which OPENS the fan power. Once the temp switch OPENS at >=93C, the relay then de-energizes and closes...thus powering the fan.

Hope this helps & clarifies!
 

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Discussion Starter · #3 ·
So with an original A/C system, there are two fans (Radiator(passenger) and AC Condenser(driver side)). Many, when removing the A/C system, do NOT remove the electrical system and cooling ECU (A/C Amplifier) that controls these fan relays. The temp sensor is either switch (non-AC) or thermistor (A/C). If you put in a switch instead, when the A/C amp is used, without all the other non-AC jumpers, it's likely to cause the A/C system to turn on the fans (both) full time as you describe (low resistance).

I ran into this once, where a customer bought a replacement radiator, and it had the wrong sensor in it vs. what the car was built for. Changing to the proper sensor resolved the problem. So verify if the electronics was indeed removed and re-wired to non A/C style

View attachment 82298

As you can see below, w/A/C there is a temp sensor per spec below...

View attachment 82301

When it's W/O A/C then it's a switch:
View attachment 82302

Secondly I believe you are reading the schematic wrong on the relay. Now it states that at low temps, the temp switch is CLOSED. When looking at the relay in the schematic you posted, which is the non A/C system, it shows AT REST, closed (Normally Closed type). Meaning, if the relay coil is NOT powered, it's contacts will be closed. (It's not a Normally Open relay...it's literal as Toyota draws it). Thus when the temp switch is CLOSED, it energizes the relay, which OPENS the fan power. Once the temp switch OPENS at >=93C, the relay then de-energizes and closes...thus powering the fan.

Hope this helps & clarifies!
Thank you very much for such a comprehensive reply.

Let me elaborate on the whole AC vs non AC circuit thing - All of the wiring is present and of course only AC cars have 2 fans. For ease here I am ignoring this second fan, I just removed the relay for it for the time being and im gonna pretend its got 1 fan.

I believe that the AC Amplifier is probably fried as when I first got the car - when the AC pressure switch would be jumped - the fans would go into low speed mode (even with AC button turned off so I dont know what was wrong there at the time). Something changed somewhere along the way (im assuming AC amp blew) as now no matter what is jumpered in the system it will always run the fans at full speed from ignition power.

I will be the one converting the system over to non AC - I have owned the car coming on 2 years now and have driven it maybe 500 kilometers in that time - since I cant pass the road worthiness emissions test the car isn't road legal (even though it drives superbly with loads of power). I know that I probably have more issues here than just the fans but at the moment I'm just taking the issues one at a time, fans being the first one to finally get to the bottom of.

I have just brought in the relay to check out its operation and yes it seems to operate in this NC fashion! Which means its now down to the operation of the water switches.

I guess my next biggest issue is testing these switches I have to see if they will work, I've tried them in boiling water from a kettle but that doesn't seem to heat them up enough to make any changes.

Im going to boil one now and see if it goes open circuit (as all my previous tests have come up with closed circuit no matter what temp)
 

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Discussion Starter · #4 ·
Yeah, im still getting the same results. I have a brand new water temp switch and a used one from a 1992 w/o AC, both the switches have perfect continuity (closed circuit) no matter what temperature I put them at. Its unreal how difficult such a simple issue is turning out to be, I feel there's no point wiring in the whole circuit if I cant even get a temperature switch to function as per BGB on the bench.
 

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'91 MR2 Turbo, '20 BRZ
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Ah I understand, so you are going to convert the circuit since the A/C amp is likely faulty. Then by that your plan should work fine. Very odd that the new one, even past boiling (100C) isn't switching off. The only advice I could give, IF you think you are not getting them hot enough, is to actually install in the unit. Disable the fan (via relay or connector), where you should easily get above the threshold when the engine warms up and under load. If that doesn't switch it, it may indeed be faulty. Maybe the other board members have a better way to test w/out damaging it?
 
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Discussion Starter · #7 ·
IIRC the two relays are not for independent operation of the fans. The relays go in parallel in series to change the fan speed on both fans for hi-speed or lo-speed. Both fans are always operating. Removing one of the relays will have unpredictable results.
Yes, Im pretty sure this is right as I have read than in the Non A/C version - the single fan will turn at low speed at ALL times - then kick into high speed when at temp
 

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Discussion Starter · #8 ·
Ah I understand, so you are going to convert the circuit since the A/C amp is likely faulty. Then by that your plan should work fine. Very odd that the new one, even past boiling (100C) isn't switching off. The only advice I could give, IF you think you are not getting them hot enough, is to actually install in the unit. Disable the fan (via relay or connector), where you should easily get above the threshold when the engine warms up and under load. If that doesn't switch it, it may indeed be faulty. Maybe the other board members have a better way to test w/out damaging it?
Oh yeah, Im sure at this stage im after popping the A/C amp as it did have an effect by jumping the pressure switch last year. I have installed the sensor and bled the cooling system - just need to do the wiring when I get a chance. I am probably going to do what you're suggesting and rig it up first to see if the switch even functions..

Also - Sorry for being away for so long! I live in Ireland where it rains 330 days of the year - except this year where we have had extreme heat - my workshop is a steel shed an its hitting almost 40 degrees during the day so impossible to work on cars!
 
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