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Old 04-18-2006, 07:55 PM   #1
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Radiator fan control

With summer coming up I am concerned about the heat. One of the upgrades I am doing for this is reworking the radiator fans. I figured some other people might also be interested in this.

I went out and measured the coolant temperature wire as the car warmed up. I found an almost linear dependency (as expected). The voltage was measured at the ECU. I then inserted a pot and altered the signal to the ecu and got some additional data points. Here are the results.
50 deg C = 1.8v
70 deg C = 1.27v
80 deg C = 1.08v
90 deg C = 0.88v
102 deg C = 0.67v
104 deg C = 0.66v 1 fan turns on
106 deg C = 0.62v
108 deg C = 0.57v 2 fans turn on
115 deg C = 0.5v

The stock water temp gauge did not move until 115 deg C. By 120 deg C it moved 4 marks. At 124 deg C it was in the red.

I'm now going to build a circuit that when it detects 90 deg C or 0.88v it trips on an external resistor to fool the ecu into thinking the temp is 104 deg C. At 95 deg C I wil trip on another resistor to make the car think it is at 108 deg C.

Any thoughts on my choosen temps to turn on the fans are appreciated.
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Last edited by rkostolni; 04-18-2006 at 07:58 PM.
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Old 04-18-2006, 08:25 PM   #2
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The only flaw to your logic is that the ECU will also think the car is hot and adjust EVERYTHING else accordingly...........

Wire in a switch to the fan relays and turn them on manually.......
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Old 04-18-2006, 08:46 PM   #3
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agreed, an external switching system is best, surely there is a way to wire it such that it doesn't cause issues with the OE fan power strategy
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Old 04-18-2006, 08:49 PM   #4
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I think if you trigger the relay that you should be OK if it is wired but not on. The ECU probably doesn't know that the fan is on or off......just that it triggered the relay to turn it on....and off. If you manually trigger it the ECU shouldn't be able to tell. Would need to wire a diode in there though to keep the circuit from feeding back to the ECU I think.
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Old 04-18-2006, 08:55 PM   #5
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I believe there are issues with the way the PCM alters the fans from parallel to series trying to do it like that, somebody mentioned this in another thread somewhere recently

edit: helps if I read your entire post first , yeah I was thinking diode or some other switching method too

Last edited by TeamRX8; 04-19-2006 at 06:25 AM.
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Old 04-18-2006, 10:01 PM   #6
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I'm using the Interceptor, so the stock ecu can't do much. I don't think the few things that it is still in control of should be affected by changing the temp from 90 to 104deg. Or am I missing something? The temperature won't be altered while the engine is cold, as I would think during this time it would be an issue. Even for those w/ the stock ecu, I wouldn't think its that large of a temperature adjustment, and 104 D isn't that hot that the ecu should start pulling timing or throwing fuel at the engine.

I don't believe you can manually control the fans while keeping the ecu connected to them. 1. The wiring diagrams show a feedback line from the fan to the ecu, I need to go back and look though to see if they could be diode isolated though. 2. Someone on here, I forget who, wired up a manual fan controller found that there were issues with keeping the ecu wired to the fans. He had problems with the fan fuse blowing.
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Last edited by rkostolni; 04-19-2006 at 07:58 AM.
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Old 04-21-2006, 01:19 AM   #7
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the fan turn on at 104 deg is because the thermostat fully opened at that point. if you want to lower the fan switch you need to lower your thermostat open temp to match your radiator fan, the easier way is to change your radiator to a aluminum one.
I got a aluminum radiator on my rx7 and once you hit 25 mph the temp gauge start to go down, but you need to go 55 mph with the stock radiator.
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Old 04-23-2006, 10:08 PM   #8
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Is the thermostat controlled by the ecu in our cars or is it a traditional wax based thermostat?
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Old 04-25-2006, 01:15 AM   #9
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the RX-8 Renesis does not have an electronic thermostat, I believe the fans work off of PCM programming based on a temperature probe and other conditions such as A/C activation, etc.:


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Old 04-25-2006, 02:14 AM   #10
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Quote:
Originally Posted by rkostolni
Is the thermostat controlled by the ecu in our cars or is it a traditional wax based thermostat?
wax based??? think spring controled.

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Old 04-25-2006, 02:14 AM
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2005, control, controller, controllers, ecu, fan, fool, gauge, interceptor, radiator, rx, rx7, rx8, switch, thermostat, thermostatic

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