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Discussion Starter · #1 · (Edited)
For the most part I know how this little project is going to go. I just need a little confirmation that the wiring path for the two switches that Toyota originally used to control the fans will be correct.

I have an SC300 with a 2JZ-GTE swap and a stock 1997+ MKIV TT Auto cooling fan setup (main engine clutch driven fan + OEM dual radiator pancake helper fans + big OEM TT cooling fan on the A/C condenser).

I had this setup fully working and wired up with a separate relay to provide power triggered by a +12V Switched signal and an aftermarket Mishimoto ground-switching fan controller w/NPT coolant sensor and A/C clutch trigger override for the control of all three fans. Combined amperage draw from all three OEM auxiliary cooling fans is about 19A and the controller is supposed to be rated for a continuous 25A MAX.

Unfortunately the Mishimoto controller or its included wiring coming off from the controller unit itself seemed to have an issue handling the 19A continuous from the fans so I want to revise my wiring setup to eliminate it and instead fully replicate Toyota's dual switch control scheme.

I had not tried this before because I had thought the Supra TT had a "special" type of A/C pressure switch (the MKIV TT electrical TSRM makes it seem as if this is the case in its description when compared to the Supra MKIV NA and SC300 electrical TSRMs) but as it turns out the SC300/400 and all MKIV's seem to use the same four-pin A/C pressure switch. It's just that only two of the pins are used on the non-turbo 2JZ-GE vehicles.

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So that being the case I am just going to add two more OEM terminal wires to the A/C pressure switch connector in my SC300 and use those two wires along with the MKIV TT's OEM radiator fan switch part to control a heavy duty 30/40A relay and some properly gauged wires for the fans in place of the Mishimoto controller. I have no need of any custom adjustment of the cooling fan trigger points. I just feel that doing it 100% Toyota's way will be more reliable in the long term.

First I'll post my research references and below that I will post my question.

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--My three OEM aux fans are wired up already as depicted in the above factory wiring diagram.

--I have already recreated the "Fan Relay #2" portion of the circuit and made that its own safely fused 30A circuit.

--Wire sizing for all of this was calculated correctly for the distance and Toyota specified 30A fuse load that they intended.

--I have new OEM terminal repair wires on order to populate my SC300's existing four-pin A/C pressure switch connector pins #2 and #3 for use with the fan circuit.

--I have an MKIV TT screw-in radiator pressure switch and its connector and matching terminal wires.

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My remaining question to ask is simply if there is anything I am missing regarding how the MKIV TT wiring schematic is telling me to order the wires from the two pressure switches to provide the correct factory original fan control operation:

Fan Relay #2 Pin 1 (Energizing current) --> Fan Relay #1 Pin 4 (energizing current) internally flows to its Pin 3 --> Fan Relay #1 Pin 3 to the A/C Pressure Switch Pin 3 wire --> A/C Pressure Switch Pin 2 Wire to MKIV TT radiator coolant fan "switch" Pin 2 --> MKIV TT radiator coolant fan "switch" Pin 1 goes to chassis ground point to complete the circuit.

And now the three OEM aux fans will have their combined ground (a good size 10GA wire) to a main chassis ground point.

This reverts the three fans to a Positive-switching relay scheme rather than Mishimoto's Ground-switching relay scheme.


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Does that plan sound correct and in accordance with the Toyota fan control setup?

It seems VERY simple to me other than being a tedious wiring job (which I don't mind doing). I just want to be sure that I will be completing their original style circuit correctly so that it operates exactly as it is supposed to before moving ahead with it.

I know a mini project done this way isn't for everyone's setup. I appreciate any advice you all may have-- especially if I am missing a crucial part of the puzzle.

Thanks!
 

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Your current flow (in bold in your OP) is correct, but keep in mind that the way this works is that the fans will be forced OFF by Fan Relay #1 when the E5 "Engine Coolant Temp Switch" is cold/closed. E5 opens when hot, killing the ground to Fan Relay #1, which closes Fan Relay #1's relay contacts (pins 2-1), which turns on all the fans. The way Toyota has drawn Fan Relay #1, it's showing "normally closed" contacts (at pins 2-1). So activating the relay with a complete circuit along pins 4-3 and all of the downstream switches (which lead to ground) will "open" the relay and turn the fans OFF.
 

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Discussion Starter · #4 · (Edited)
Your current flow (in bold in your OP) is correct, but keep in mind that the way this works is that the fans will be forced OFF by Fan Relay #1 when the E5 "Engine Coolant Temp Switch" is cold/closed. E5 opens when hot, killing the ground to Fan Relay #1, which closes Fan Relay #1's relay contacts (pins 2-1), which turns on all the fans. The way Toyota has drawn Fan Relay #1, it's showing "normally closed" contacts (at pins 2-1). So activating the relay with a complete circuit along pins 4-3 and all of the downstream switches (which lead to ground) will "open" the relay and turn the fans OFF.
Thank you for the confirmation, bitshftr!

I may have worded something inaccurately in my original post but that's what I had thought: when the TT radiator temperature switch (E5) is cold/closed the overall energizing circuit for Fan Relay #1 is not completed and thus the fan(s) do not turn on yet. Once sufficient temperature has been reached (fixed parameters of the switch design) it opens and completes the circuit allowing energizing current to flow to the chassis ground which turns the fans on. And then it (E5) doesn't turn off (closed) again until the coolant temperature gets back to 197F or so. Have I got it right?

The part of the control circuit design that looks straightforward to wire in but which still perplexes me a little is that within the same switching circuit that the E5 coolant switch is a part of the A/C Pressure Switch's Pin 2 & Pin 3 being in line should also have an effect on the current flow being completed or not completed as well.... even if you do not have the A/C turned on.

For instance, if the A/C system is not being actively used but the vehicle's E5 coolant switch goes to the open position at sufficient hot coolant temperature, shouldn't the circuit's current flow remain interrupted in that specific scenario since there are two switches in-line on that circuit?

It's an incredibly simple circuit and I understand to wire it up as shown in the TSRM diagram and I know I am missing something extremely fundamental about this but it's just strange to me that the E5 should be able to regulate that energizing current flow for fan on/off operation in accordance with its hot temperature and cooler temperature thresholds while at the same time the A/C pressure switch that is a part of that circuit can seemingly interrupt the current flow if the A/C system is not in use.

I'm not confused about how to complete the wiring at this point. I'm just still confused about that one aspect (the two A/C pressure switch pins factoring in) of how the factory switching system works overall. And as soon as I have the "Aha!" moment I'm going to feel so foolish for having asked... but alas I'm not there yet.

Maybe I'm just too tired tonight overthinking the energizing control portion of how a very simple control circuit design works in total. The E5 operation I totally get. It's how the interplay of the E5 coolant temp switch + A/C pressure switch work together that I'm having a mental block with despite understanding the correct wiring path for both.
 

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Discussion Starter · #5 ·
I added the dual helper fans to my NA/TT swap. I just used this wiring harness from chasebays Chase Bays Dual Fan Relay Wiring Harness with 180º F Thermoswitch Unfortunately, its only for 2 fans, not three. I modified it to send a sensor ground when I want the ECU to trigger the fans (if the thermal circuit hasn't been triggered yet).
Thank you! I did come across that Chase bays fan controller kit while researching my old fan control wiring setup and it actually inspired me to identify and acquire the raw parts they made to create it (I bought most of the raw pieces from Mouser) so that I could create a custom mounted compact auxiliary twin relay and twin fuse box elsewhere in my tight engine bay. The main fuse box is by LittelFuse and the 30/40A relays are 280-style automotive units.

I may get another one of those for my fan controller circuit revisions or use another type of square mini relay setup. Both approaches offer great gasket sealing against the elements. Chase Bays chose a VERY good supplier for their kit!

Oddly the more I have tried to make a very clean and OEM-like installation of the fan control circuit I am now coming around to the same idea that Toyota's engineers had for the MKIV TT's and Soarer 1JZ's: mount the fan relays under the headlight with yet another custom bracket. There is just so little room ahead of the factory fuse box for a truly clean installation and I won't be relocating my battery.
 
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CinePhile, your confusion is justified. This is a weird circuit. It's kindof a double negative.

when the TT radiator temperature switch (E5) is cold/closed the overall energizing circuit for Fan Relay #1 is not completed and thus the fan(s) do not turn on yet. Once sufficient temperature has been reached (fixed parameters of the switch design) it opens and completes the circuit allowing energizing current to flow to the chassis ground which turns the fans on. And then it (E5) doesn't turn off (closed) again until the coolant temperature gets back to 197F or so. Have I got it right?
Here's how I would explain it (note that this is only valid for the GTE cars...not the GE cars):

When the engine is running normally, with A/C on or off (doesn't matter), the radiator aux fans don't run. This is because both E5 (Engine Coolant Temp Switch) and A3 (A/C Single Pressure Switch) are closed (connected), which allows power to flow through the coil (pins 4-3) of "Fan Relay #1", thus cutting the power supply to the fans, which comes from pins 2-1 of "Fan Relay #1" (the normally closed contact and the common contact, respectively). This is because "Fan Relay #1" is wired in a normally closed fashion (pins 2-1 are connected only when there's no power flowing through pins 4-3).

If either E5 (Engine Coolant Temp Switch) or A3 (A/C Single Pressure Switch) open up, the coil side of "Fan Relay #1" loses power and the relay "relaxes", contacting pins 2-1 and allowing power to flow to the aux fans. Note that if either E5 or A3 open up, there's a serious problem! (the car is starting to overheat or the A/C coolant pressure is abnormal.) Toyota designed this in a failsafe way...any failure (or abnormal situation) causes the aux fans to turn on to try to cool things down.

Said another way the aux fans are defaulted to always on, unless both E5 and A3 are happy. Fortunately, both E5 and A3 are happy most of the time (under normal operating conditions). The aux fans aren't triggered on in this design...they are triggered off when an abnormal situation occcurs.

I hope that helps with your "Aha!" moment. If not, let me know and I'll try a different way of describing how it works.
 
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Discussion Starter · #7 ·
CinePhile, your confusion is justified. This is a weird circuit. It's kindof a double negative.

Here's how I would explain it (note that this is only valid for the GTE cars...not the GE cars):
I hope that helps with your "Aha!" moment. If not, let me know and I'll try a different way of describing how it works.
Bitshftr, that explanation does help, thank you!!

It's no less bizarre but "double negative" sounds like a very good way to put it. I'll get into this soon once the OEM terminals and weatherproof aftermarket relay come in. As weird as the circuit is I'm going to just follow the wiring path exactly as Toyota has detailed it. Apparently it is an extremely well designed failsafe for the cooling system, especially with the TT Auto condenser fan added in (assuming one uses a factory side mount intercooler which I am).

I do think I'll remove my headlight and mock up a bracket template to approximate how it sits in the MKIV TT (space being at such a premium) intended for painted stainless steel.

Even finding pictures of the OEM TT fan relays in the bracket is difficult but it really does not seem to be that complicated a setup. And I've already got my equivalent Fan Relay #2 and its 30A fuse mounted up behind the driver's side shock tower in a weatherproof service box anyway.
 
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