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"Why is this engine so damn complicated??" Part 3: Cooling Fan Controls

3rd Generation Specific (1993-2002) 1993-2002 Discussion including performance modifications and Technical Support Sections.

"Why is this engine so damn complicated??" Part 3: Cooling Fan Controls

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Old 12-05-09, 08:57 PM
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"Why is this engine so damn complicated??" Part 3: Cooling Fan Controls

-- Part 1: Sequential Turbo Controls Demystified https://www.rx7club.com/3rd-generation-specific-1993-2002-16/why-engine-so-damn-complicated-part-1-sequential-turbos-demystified-841821/
-- Part 2: Emissions Controls https://www.rx7club.com/3rd-generation-specific-1993-2002-16/why-engine-so-damn-complicated-part-2-emissions-controls-841963/

-- Reliability Tuning on a Power FC using only the Commander hand controller: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/how-make-your-untuned-pfc-basemap-safer-idle-better-no-datalogit-needed-841706/
-- "Free" Power FC electronic boost control, updated diagram on page 2 https://www.rx7club.com/3rd-generation-specific-1993-2002-16/free-power-fc-single-turbo-probably-nonsequential-boost-control-846883/
-- Innovate LC-1 wiring with a Power FC and Datalogit https://www.rx7club.com/3rd-generation-specific-1993-2002-16/my-power-fc-lc-1-datalogit-wiring-diagram-707301/

Introduction

A lot of confusion and misinformation surrounds the FD fan control system, especially its stepped 3-speed control and associated wiring. In this article I will first give some background on Mazda's development of the cooling fan systems in rotaries and the advantages of the FD's design. I will discuss the stock FD control logic and wiring architecture during normal running. I will also present some educated guesses about how the cooling fan control module operates when it is running the cooling fans after shutdown. Emphasis will be placed on the overall advantages of the OEM designs, with an acknowledgment that the fan trigger temperatures do no favors for the FD's reliability.

Why an electric fan?

First I want to discuss clutch fans, which were used on rotaries up through the series 5 Rx-7. Clutch fans are driven directly by the engine, and viscous oil is used to control the fan's rotational speed relative to the engine's speed. The control of a clutch fan operates on the principle that different materials (fluids or metals) expand at different rates as they heat up.



Besides the engine drag issue, a clutch fan's speed is determined by engine bay temps, not by water temps in the engine itself. The diagram above depicts a pretty imprecise method of control by today's standards. It does show how the FC clutch fan improved on older designs, having a lot less of an on/off nature. Mazda also had a small auxiliary electric fan on some FC models that was controlled by a thermoswitch when temperatures got too high. You can see that even in 1985 Mazda was moving toward more progressive fan control that responded better to temperature changes and reduced load on the engine. The FD's 3 speed fan system modernized rotary cooling fan control.

Advantages of the 3 Speed Fan Control System


1) Less drag on the engine. The fan is driven electrically and in steps so current draw will be minimized.

2) The flexibility to create a more compact design.

3) More sophisticated control logic from the stock ECU, including a fault mode where the fans run constantly (as described in the service manual). Fan speed is controlled more precisely by actually measuring engine coolant temperature and monitoring for additional loads on the engine triggered by driver input.

4) Improved reliability due to the redundant dual fan design. My FC once hit over 125 C because the fan clutch failed and the single fan would not spin fast enough.

5) Ability to cool the engine when the engine isn't even running, which is the purpose of the Cooling Fan Control Module.

6) Less noise.

Fan Control Temps and Wiring






-- Relays 2 & 4 are connected to the ECU. Stock they are triggered at 105 C and bump up the speed by one level. This trigger is adjustable in the Power FC.

-- Relay #1 is the A/C trigger. This relay bumps the fan speed up one level.

-- Relay #3 is connected to the thermoswitch and is triggered at 108 C stock. This relay bumps the fan speed up one level.

You can see how this design can reduce the amount of time the fans run and reduce the effect the fans' operation will have on other vehicle systems. The most important thing to remember is: In stock configuration, the cooling fans will not run at their highest speed unless the ECU trigger (105C), the thermoswitch trigger (108 C), AND the A/C trigger are all engaged.

Automatic Fan Operation after Shutdown (Aftercool)



All Rx-7's with a fan control module are designed to run the cooling fans after shutdown when the engine is experiencing high water temperatures. The cooling fan module was added in a fuel hose recall, most cars should have had this done. I have not found a full wiring diagram for the module, but we do have some information on it from the 94 service manual being circulated.



There is no full description of the fan control module's logic. This is all we have. Recall that the FD thermoswitch trigger temp is 108C and the series 5 FC thermoswitch has a 95 C trigger. I think this is the basic logic for the module:

-- If the thermoswitch trigger temp had been reached for 2 minutes prior to shutdown, the fan control unit will first activate the #2, #4, and #3 fan relays to run the fans at MEDIUM speed for 10 minutes max.

-- If during that 10 minute period water temperatures fall below the thermoswitch trigger temp, the #3 relay is deactivated but the #2 and #4 relays continue to run the fans at LOW speed until the timer is up.

The implication of this logic is that I suspect engines with a fan control module and FC thermoswitch are much more likely to have the cooling fans run after shutdown during hot weather. I'm not sure if the ECU's fan trigger plays any role in whether the module comes on or not. I haven't gotten a chance to test this theory yet as my test FD is down with blown turbos, but I invite others to speak up.

How the Fan Control Module Works

Without a full diagram of the fan control module (preferably a 95 wiring diagram) we can't be exactly sure how the module runs these fans. But here is what I suspect based on the one diagram I posted above and the module pinout on page F-146 of the service manual.

The fan control module is hooked in-line with the ECU pin 3D ground signal for the #2 and #4 relays, we know that for sure based on the conceptual diagram in the service manual. The control module is also somehow hooked into one of the ignition switch circuits, but we don't know exactly how. I believe that the cooling fan control module has some way of switching the positive trigger for the fan relays even with the car off. Normally, the positive trigger comes from ignition power through the ignition switch, so this is the only thing that makes sense. As mentioned before, the ground trigger for the 4 relays come from various sources, and the fan control module is only connected to the ground side of the #2 and #4 relays. The only way the #3 relay could run is if the ground is supplied by the thermoswitch and the +12V side is supplied by the fan control module.

During aftercool operation the positive trigger for the fan relays is probably controlled by the timer inside the fan control module. The ground trigger for the #3 relay is controlled by the thermoswitch, so it would make sense that this relay would turn off when the temperature drops.

Conclusion

The 3 speed fan control system was a major advance for Mazda's rotary cooling systems, despite the high fan trigger temperatures. The fan shrouding is excellent and the fans pull plenty of air when running at max speed. Three speed fans reduce drag on the engine, are much more precise to control, and are more reliable. Many people wire up a manual switch to trigger the cooling fans, but the OEM aftercool fan control module accomplishes the same purpose without requiring driver input. There's no reason you can't have an effective cooling fan system using almost all OEM parts.

I have drawn up diagrams for a proposed hybrid 2/3 fan speed system that should increase fan speeds and improve aftercooling while retaining the advantages of stepped fan speeds. It carefully utilizes OEM parts and requires only one wire to be hooked up. I may post this up later.
Attached Thumbnails "Why is this engine so damn complicated??" Part 3: Cooling Fan Controls-fan_control_module.jpg   "Why is this engine so damn complicated??" Part 3: Cooling Fan Controls-fan_control_module2.jpg   "Why is this engine so damn complicated??" Part 3: Cooling Fan Controls-2_fc_cooling_fan.jpg   "Why is this engine so damn complicated??" Part 3: Cooling Fan Controls-fd_fan_control_temps_fsm.jpg   "Why is this engine so damn complicated??" Part 3: Cooling Fan Controls-fd_fan_relay_locations.jpg  

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Old 12-05-09, 10:25 PM
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The fan control module was added as a recall by Mazda. The components include the black box "brain" by the ECU with a short wiring harness and a short harness that goes between the fan control relays and the front wiring harness.

IMHO, the fan recall stuff was a hack to get around a problem without fixing the problem. FD's run VERY hot from the factory, and instead of changing the fan thermoswitch to start cooling the engine sooner they elected to rig up a method to run the fans after shutoff to cool the engine bay. Not a good decision, but I think Mazda tried to run the FD really hot to help with fuel economy and federal emissions.

The fan recall module by the ECU is VERY simple. It watches the state of the fan thermoswitch at the back of the water pump, if it is tripped for more than a few minutes, it will run the fans for 10 minutes after the car is shut off. This is the reason for the harness going to the fan relays - the harness changes the wiring to the relays so they see constant battery power, enabling them to run with the ignition off.

In my opinion, keeping the car running at a sane and healthy temperature is FAR more beneficial than running the fans after the car is shut off.

Here's Dale's fan recall -

- Find and remove the fan recall stuff. Ditch the wiring to the relays, the module, etc.

- Install an FC thermoswitch, dropping the highest fan temp to 97 deg. C

- Get a PowerFC and set the ECU based fan control to 85 deg. C.

- Ditch the stock precat and install a ceramic coated downpipe.

I've actually gone further and use an HKS fan controller to control two of the fan inputs. My car routinely stays at 82-85 deg C even in hot Florida weather. On a real hot day with heatsoak the temps will spike when first starting the car, but the fans will come on full steam and get the temps back where they should be in VERY short order.

Keeping the car running around 85 deg. C is just a good idea. Heat is the big enemy on the FD, if you can control it, you have won a major battle towards having a healthy FD.

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Old 12-05-09, 10:57 PM
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To add to these excellent posts, removing the p/s and a/c improved the cooling on my FD quite a bit.
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Old 12-06-09, 09:19 AM
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Originally Posted by DaleClark View Post
IMHO, the fan recall stuff was a hack to get around a problem without fixing the problem.
I disagree that adding an aftercool function is a "hack." When you shut the car off in hot weather, temperatures are going to spike under the hood, whether you had been running at 85 C or 100C or whatever. Running the fans at 86 C (low speed), I can stop at a gas station and five minutes later water temps have hit over 100 C. If I manually leave the engine off and the fans running, water temps will climb a lot less. Then when I start the car I can just drive off and not have to be very concerned about waiting for that extra heat to dissipate. When used with an FC thermoswitch, the fan control module reduces the hassle of driving an Rx-7 by reducing the need for the driver to monitor heatsoak upon shutdown. It makes your car less of a science experiment that requires constant monitoring. You can let some else drive your car without having to tell them "oh btw, sit there and watch this water temp gauge for a couple minutes if you shut the car off at 7-Eleven."

Also remember that the aftercool function is stepped. The fans will run at medium speed and then slow down if the engine cools off below thermoswitch temps. So if it's very hot outside and you go into the 7-Eleven, when you come back your temps are likely to be around 95 C due to the aftercooling. That's a reasonable temperature because the thermostat doesn't even open fully until 95 C. So you can just drive off immediately and not be anywhere near as concerned about temperatures.

The Rx-8's have an aftercool function that is controlled by their ECU. It is triggered by an engine compartment temperature sensor (built into their underhood ECU) or by water temperature.



- Find and remove the fan recall stuff. Ditch the wiring to the relays, the module, etc.
I understand the desire to simplify, but again the aftercool function is important because it makes your FD less of a pain in the ***. I can shut down my daily driver, get back in 5 minutes later and just drive away without worrying about heatsoak. I should be able to do that in an FD as well, at least to an extent. My FC has a single speed GM fan without any kind of aftercooling, but I wish it had the FD's cooling fan control module.

- Get a PowerFC and set the ECU based fan control to 85 deg. C.
To each his own on this. You can set it as low as 85 C in the Datalogit (and it will actually come on at 84 C due to the PFC's logic), but then your fans will be running almost all the time. That increases power drain and wear on the fans. I prefer a little higher temps, closer to 90. In the winter the fans don't run much then. Lots of other cars with electric fans don't trigger them until the 90-95 C range. That's the rule rather than the exception. I don't think it's critical to run that cool, but I suppose it couldn't hurt.

My car routinely stays at 82-85 deg C even in hot Florida weather. On a real hot day with heatsoak the temps will spike when first starting the car, but the fans will come on full steam and get the temps back where they should be in VERY short order.
the whole point of the cooling fan control module is to mostly prevent you having to deal with this. I just don't see much benefit to removing it. It's there for a reason; it should be used more (by installing the FC thermoswitch) rather than removed.

Keeping the car running around 85 deg. C is just a good idea. Heat is the big enemy on the FD, if you can control it, you have won a major battle towards having a healthy FD.
Controlling heat is important, but IMO as long as you are running within the thermostat's temperature range you are fine. The thermostat opens at 82 C and opens fully at 95 C. It's been like that since the Rx-2, which had basically no emissions controls.
Attached Thumbnails "Why is this engine so damn complicated??" Part 3: Cooling Fan Controls-rx8_aftercool.jpg  

Last edited by GoodfellaFD3S; 10-13-11 at 02:55 PM. Reason: fixed typo--- 85 to 95.
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Old 12-06-09, 10:41 AM
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Well, let me back up and say the fan recall was a hack due to the fact that it didn't fix the key problem with the FD's stock cooling system and that is running the car so damn hot. I'd pull off the side of the road in a panic if my car was running 107 deg. C, and that's where it routinely ran stock from Mazda!

Also, for the fan after-run system to work you have to trip the OEM fan switch for a few minutes while the car is driving. My car doesn't run that hot; hell, I get nervous when the temps get into the upper '90's C!

Might be interesting to rig up an RX-8 engine temp sensor to an FD's fan recall, though .

Oh, one other beef with the fan recall - those are some way too big sloppy harnesses! The underhood harness could have been 1-2 inches long, instead of 8 inches!

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Old 12-06-09, 12:00 PM
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Great post and great information. I have seen more "bad" information about the RX7 fan cooling system than many other areas discussed on the forum. In summary, I believe 95% plus RX7owners would be perfectly well suited to use the PFC to kick the fans on somewhere between 85-90 degress C, then add the FC thermoswitch to trigger the highes speed at 97 degrees rather than the stock 108 degress. Other than that, the only thing that makes sense for the majority, might be to wire things such that high speed kicks in at 97 degrees whether the A/C is on or not (I haven't done this yet, but may -- arghx, is this one of the diagrams you are going to post?). The thing you need to emphasize is that you need the 97 degree thermoswitch even with the PFC. There are many posts on here that say that isn't the case.
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Old 12-06-09, 05:15 PM
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Originally Posted by arghx
The thermostat opens at 82 C and opens fully at 85 C
That was a typo. It cracks at 82 C and opens fully at 95--only OEM rotary thermostat I found that differed was the one that came on the Rotary Engine PickUp, which had a carb'd 4 port 13B.

Well, let me back up and say the fan recall was a hack due to the fact that it didn't fix the key problem with the FD's stock cooling system and that is running the car so damn hot.
Agreed. Interestingly enough, a lot of Mustangs run fan temps as high as the FD. I have the 98 Cobra ECU ROM for example and the low fan speeds come on at a higher temperature than you would expect. Maybe I'll dig it up and post the info later. I think the s5 and s4 turbo thermoswitch is actually 97 C as others have pointed out. Note that this is about the temperature where the Rx-8's switch the fans on at low speed. High fan speeds still come on at 108 C on the Rx-8.

The s4 automatic models had a 90 C thermoswitch, available from Mazdatrix: http://www.mazdatrix.com/86-92Electrical-Engine.htm . It uses a spade connector however.
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Old 12-06-09, 05:39 PM
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The Hybrid 2/3 Speed Fan System

I call it a hybrid system because

--During A/C operation the fans will be stepped 3 different speeds (low, medium, high).

--Without A/C, the fans will be stepped from low speed (PFC trigger) right to high speed (thermoswitch trigger), or from medium speed (PFC trigger) to high speed (thermoswitch trigger) depending on how you wire it.




So the new logic would be:

A/C ON, water temps below 90 C (or PFC trigger temp) -- low speed
A/C OFF, water temps below 90 C -- fans off

A/C ON, water temps at 90-96 C -- medium speed
A/C OFF, water temps at 90-96 C -- low speed

A/C ON, water temps at 97 C (FC thermoswitch temp) -- max speed
A/C OFF, water temps at 97 C (FC thermoswitch temp) -- max speed

Aftercool: engine runs at 97 C for 2+ minutes before shutdown, fans run at medium speed until temps drop below 97 C, when the fans will slow down to low speed. After ten minutes the fans will shut off. No major cutting or external switches are required with this wiring. It is idiot proof if installed correctly. You could also jumper the A/C trigger wire to the trigger for the #2 and #4 relays instead of the thermoswitch. That would result in medium speeds when the PFC kicks the fan on, and high speeds when the thermoswitch comes on. It depends on your preference.

The whole point of the mod is to make it so that the A/C does not have to be on for all 4 fan relays to be engaged (max fan speeds). A lot of cars don't even have A/C anymore, or you might be driving around with the A/C off and still want to be able to get max fan speed.
Attached Thumbnails "Why is this engine so damn complicated??" Part 3: Cooling Fan Controls-fd_fan_relay_rewire.jpg  
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Old 12-07-09, 06:51 PM
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Did this exact mod two years ago when it was a hot topic on this forum. Ordered the lower temp. thermoswitch from Malloy; Ray knows exactly what is needed and it will plug into the FD harness.

A couple of things noticed:
The fans are noticeably louder normal driving with AC on - minor inconvienance...
AC runs cooler when in stop and go traffic.
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Old 12-08-09, 01:18 PM
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Did this exact mod two years ago when it was a hot topic on this forum.
Did you jumper the #1 relay to the ECU or the thermoswitch trigger? Or just lower the PFC fan temps and install an FC thermoswitch?
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Old 12-08-09, 03:53 PM
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Originally Posted by arghx View Post
Did you jumper the #1 relay to the ECU or the thermoswitch trigger? Or just lower the PFC fan temps and install an FC thermoswitch?
I jumpered the negative coil side of relays 1 and 3 as well as the FC thermoswitch. In the factory wired condition the FC thermoswitch it's self really does nothing unless relays 1 and/or 2 and 4 are closed. So for high fan speed AC must be on (relay 1), normal cooling (2&4) must be on and the thermoswitch closed activating relay 3. I do not have a PFC only a stock ECM with a M3 stage III mod. With the jumper as you show in the drawing two things happen:
  • Should a failure happen in relays 2 or 4 the thermoswitch will close on high temp and activate both relay 1 and 3 thus causing medium speed.
  • AC "on" the fans step up to medium speed and promote better heat exchange and cooler AC when in stop and go traffic.
The down side is when normal cooling relays are on and AC is on the fans run at high speed and are noisy.

The only real thing the lower temperature thermoswitch does is activate relay 3 at a lower temperature; relays 1 and or 2 and 4 are required to run the fans in conjunction with 3 being closed.
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Old 12-08-09, 04:12 PM
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Originally Posted by ttmott View Post
The down side is when normal cooling relays are on and AC is on the fans run at high speed and are noisy.
Yes and that's partly why 108C is the normal trigger temp for the highest speed fan operation on both the FD and the Rx-8. You are understanding the challenges and tradeoffs that OEM engineers encounter. They seek to minimize NVH ("Noise, Vibration, Harshness") during normal driving, especially considering just how quiet and smooth a 100% stock rotary car is.

In hindsight the car would have been better served with a more aggressive cooling fan control strategy rather than quieter operation. But maybe they didn't want customers or the automotive press bitching about fan noise or battery voltage drops?
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Old 12-08-09, 06:16 PM
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Having said all of that, a far better cooling fan arrangement would be to have alternating current motors, DC to AC converter and variable frequency drive that could change fan speed using 0 to 5 volts from the PFC or other ECM to the VFD. The converters and VFD's are now packaged for automotive use. No relays or complicated sensor requirements.....

BTW - I am an OEM Engineer but for spacecraft systems.
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Old 12-08-09, 10:33 PM
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^ Mitsubishi has been using stepless, pulsewidth modulated fans for years. For example, here is the fan control module on the Evo 8:



and SPAL makes a pulsewidth modulating fan controller for aftermarket use:

http://www.spalusa.com/store/main.as...tem=FAN-PWM-V3. Some FC owners use it. I know their older version had some problems but this new one is supposed to be a lot better. I'm not sure how it would work with the design of the FD fans though.
Attached Thumbnails "Why is this engine so damn complicated??" Part 3: Cooling Fan Controls-evo8_fan_thermostat.jpg  
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Old 12-11-09, 05:21 AM
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Good writeup. Searched this issue when I tested the rack solenoids as shown in Dgeesaman's writeup.
(Found two solenoids that failed to open when heated to 250 deg F. Replaced the solenoids, and added a thermal barrier to protect the solenoids, radiant heat soak during shutdown may shorten their life).
I have the stock ecu. I could order the FC thermoswitch, to force the cooling fans on at 97 deg for aftercool, engine off.
However, it would be nice to force the fans on for aftercool every time the car is shutdown to reduce heat soak/under hood temps (similar to the rx8 underhood temp sensor).

I don't like turbo timers, I always drive slowly the last 5 minutes, before shutdown.
A turbo timer could be wired to control the thermoswitch input to force aftercool, instead of using it to control the ignition switch input.

A better idea: wire an underhood temp switch in parallel with the existing thermoswitch?? other ideas???
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Old 12-11-09, 07:37 AM
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Originally Posted by arghx View Post
You could also jumper the A/C trigger wire to the trigger for the #2 and #4 relays instead of the thermoswitch. That would result in medium speeds when the PFC kicks the fan on, and high speeds when the thermoswitch comes on. It depends on your preference.

The whole point of the mod is to make it so that the A/C does not have to be on for all 4 fan relays to be engaged (max fan speeds). A lot of cars don't even have A/C anymore, or you might be driving around with the A/C off and still want to be able to get max fan speed.
I fall in this category. Let me see if I understand this comment. INSTEAD of jumping the A/C to the thermoswitch, I could have it run to the #2 & #4 Relays. So when the PFC kicks them in, it turns on the #1 relay AS WELL thus providing medium fan speed?

If thats the case, I think you should draw that up, and post it, bc I think it would need to be included on any A/C removal. Simply bc the control system, would still be able to give Max Speed should it be needed. Like you said, w/o the #1 relay jumpered, it would never exceed medium speed correct?

Interesting write-ups, but I hate when these come along. My brain is getting full as is.
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Old 12-11-09, 09:11 AM
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Originally Posted by cpnneeda View Post
Like you said, w/o the #1 relay jumpered, it would never exceed medium speed correct?
Exactly. All 4 relays must be triggered for max fan speed. Read this chart again, paying extra attention to the "cooling fan operation" column on the far right:



I think you should draw that up, and post it, bc I think it would need to be included on any A/C removal.
Merry Christmas then:



In stock configuration, ECU pin 3D is wired to the ground trigger on fan relays #2 and #4. The diagnostic box and the fan control module can also trigger the ground side of those two relays. By jumpering the ground trigger for the #2 relay to the #1 relay, the ECU will trigger MEDIUM speeds. Any time the #2 and #4 relays are engaged, the #1 relay is engaged and you will increase speeds by two steps. HIGH speed is still triggered by the thermoswitch engaging the #3 relay.

Side effects of Jumpering the #2 and #1 Fan Relays

Again, basically every fan operation that involves the #2 and #4 relay bumps up one speed. Full explanation:

If for seem reason you activate the fans with the diagnostic box, they will run at MEDIUM speed instead of low speed. This is because the #2, #4, and #1 relays will all be triggered instead of just the #2 and #4.

Aftercool operation is normally triggered if the thermoswitch has been running for at least two minutes prior to shutdown. With this mod, the fans will run at HIGH speed initially during aftercool. The fan control module will supply the +12V trigger for all 4 relays and supply ground for #2, #4, and now #1. The thermoswitch will ground relay #3. The result is HIGH fan speeds because all relays have been triggered.

During aftercool operation, if water temps fall below the thermoswitch temperature, the #3 relay will be disengaged. Fans will drop to MEDIUM speed, until the 10 minute timer is up. That would drain the battery a little more but since the timer is only for 10 minutes I can't see it being a big problem if your electrical system is healthy.

If you have a stock ECU and there is an open in the ECU's water temperature sensor circuit, fans will run at MEDIUM speed.

Originally Posted by xzl6b1
A turbo timer could be wired to control the thermoswitch input to force aftercool, instead of using it to control the ignition switch input.

A better idea: wire an underhood temp switch in parallel with the existing thermoswitch?? other ideas???
The fan wiring is pretty complicated. I would be careful with this idea. I'm not saying don't do it, just be careful. The Rx-8 system is OEM and is thus integrated into the rest of the car. The underhood temp sensor is actually built into the ECU (which is mounted under the hood). You would have to shoehorn something in.

Originally Posted by cpnneeda
Interesting write-ups, but I hate when these come along. My brain is getting full as is.
All I did for this particular series of writeups is read stuff that manufacturers and engineers wrote.
Attached Thumbnails "Why is this engine so damn complicated??" Part 3: Cooling Fan Controls-fd_fanmod_noac.jpg  
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Old 12-11-09, 03:34 PM
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I am glad you wrote this up, I was trying to find a neat way to use the A/C button on the dash panel to "turn up" the fans. That still may not be a bad idea though. Let them run in Slow and Medium (for the cool down, should it be needed) and the have the A/C switch for Medium and High speeds.
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Old 06-12-11, 04:59 PM
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So here's something interesting. It turns out that the Mark IV Supra has a thermoswitch that engages at 208F/98C. This is in addition to a clutch fan.



The thermoswitch is actually located at the bottom of the radiator.

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Old 10-12-11, 01:54 PM
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Rise, Chicken!

Can I just jump the grounds on all the relays to switches in the cockpit, and control the fan speeds as I see fit without ruining the stock control/monitoring system?
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Old 10-12-11, 02:51 PM
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I've never done it but I don't see why not, if the wiring is right. Personally I wouldn't do that on a street application but it's your car.
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Old 10-12-11, 07:16 PM
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The Supra electronic fan system is similar to the one on the FC. The FC's electric fan only runs when the AC is on and when the car is warm (97 deg. C). This is in addition to the clutch driven fan.

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Old 10-12-11, 08:03 PM
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The unique feature of the FD's cooling system are the three separate windings in each of the fan motors; uniquely configured so, as power is consecutively applied, the speed increases. Additionally, each winding alone will run the motor.
Also the inherent redundancy designed not only in the two fans but in the normal engine cooling relays (2 and 4).
Then add to that the thermoswitch and its dedicated operation of both fans.
In reality the factory designed system is very robust and fault tolerant.
The system is hard to improve upon. When in good repair there is no real need to modify.
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Old 10-12-11, 08:42 PM
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I was thinking I could splice into the ground wires in such a way as the system defaults to the stock control system. Regardless of if my switches are off the fans would come on as the factory intended, but that I could also turn them on with a toggle switch.

Is that making sense?
I want to turn them on, but not off.
Seems like it would be pretty simple.
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Old 10-13-11, 01:16 AM
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Originally Posted by Barban View Post
I was thinking I could splice into the ground wires in such a way as the system defaults to the stock control system. Regardless of if my switches are off the fans would come on as the factory intended, but that I could also turn them on with a toggle switch.

Is that making sense?
I want to turn them on, but not off.
Seems like it would be pretty simple.
So let's say you hooked your switch to the fan relay connected to the A/C control. If you flipped an in-line switch through the factory wires, nothing would happen unless the A/C were on. You are either enabling or disabling the factory control logic, that's all.

To override you need another way to supply ground. Some kind of printed circuit board would be simplest to control everything but you can set up a series of regular 5 pin relays like this:



85 pin - from switch (switch connects to ground)
86 pin - 12V source, could be constant or ignition powered
30 pin - original wire to the fan, harness side
87a pin - original wire to the fan, fan side
87 pin - ground

The way this works is that there would be continuity between the 30 pin, the common pin, and the 87a pin which is normally closed. With the switch off the fan functions like normal and receives ground when it's supposed to. When you flip the switch, it supplies ground to the coil in the relay. The relay now has continuity between the common pin (30) and the normally open pin, 87. Pin 87 is connected to ground and you will be overriding the control logic.
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