Michael Filippello

ok, I posted this in response to a thread but figured you guys in this section could shed some light on what I have deciphered in the fan wiring circuit.

I am doing some cooling mods to my car while I have the engine out. I plan to install a small electric water pump in the heater line that will come on after the car shuts off along with the fans(maybe just one) for a certain amaount of time or till the temp falls below a certain point. While deciding how to wire this up, I have looked into the cooling circuit on the scuderia website as well as post here on the forum.

I am considering changing the stock thermoswitch for the miata one as it actiavtes at 205 instead of 226. This will turn the system on if heat soak rises above 205 and turn it off when it falls below. In my attempt to understand the way this interfaces with the fans and the ecu, I have come upon discrepancies with the way people think the thermoswitch works.

DamonB, if you see this post please reply as I find things different from you.

1st) the low speed setting is controlled by the ecu. This is universally accepted. If you look at the wiring daigram you see that relays 2 and 4 are powered on both the coil side and the latch side by constant 12v. It is the ground side of the coil side in both relays that is connected to the ECU as well as the TFA port. The output side of the latch goes to the fan motor which is then gronded. When the ecu is powered and the thermosensor(not thermoswitch) reaches the preset temp point set in the ecu (adjustable in the PFC), the ecu grounds this leg (coil side) activating the latch and powering the fans. You do the same thing when you ground the TFA port. This is low speed.

2nd) Relay #3 is the medium speed relay, since it is operated by the termoswitch (not thermosensor) it is a dumb circuit. It will only activate after relays 2 and 4 are already on as the termoswitch is set to activate at a higher temp than the lowspeed setting controlled by the ECU, unless you set them higher by adjusting this setiing in the PFC. No one would do this! Futhermore, it is not connected to the ECU (at least in the 93 wiring diagram). It is connected to the fan control module which is simply a timer circuit. Further more, it does not supply power to the fans, it only closes an addition ground on the fan motor. If you look closely at the circuit for relay # 3, it has power from battery coiming into the coil side of the relay. the output side of the coil goes to the thermoswitch. when the thermoswitch closes (either 226 stock or 205 miata) it completes the circuit of the coil in the relay by grounding it. this activates the latch side. the latch side has no power going into it. it is gounded on one side and connects to the fan on the other. Therefore it simply connects an additional ground for the fan.

It would seem that you would have to have power coming from relays 2 and 4 for relay three to even do anything. I do not know the layout of the fan motors but it would seem that by suppling a ground on the additional leg, you allow the fan to spin faster. This means that the medium or second speed of the fans is not controlled in anyway by the ECU. The test connector for the fan mod is wired into the same line as the thermoswitch. by grounding it, you simualte the activation of the thermoswitch and if done for more than two minutes, activate the timer circuit in the fan control madule. The fan control module is also connected to the TFA line and must latch this at the same time to power the fans. Once the thermoswitch drops below its designated setting. relay three drops off and only 2 and 4 stay on for the remainder of the 10 min. This is exactly what happens when you flip the fan mod switch off after 30 sec. the fan speed drops to low.

I bet if you activate relay 3 without activating relays 2 and 4 at the same time you will not see the fans come on. The other relay, relay 1 is the A/C relay and is on a different circuit all together and is not important to this discussion.

If I have missed anything please point it out. According to this logic, the miata sensor is just as important to people with power FC's as it is to people with stock ecu's it also means that the ECU only can controll the low speed setting and not fan speed 2 as is implied many times. I do not know why it is even in the datalogit software.

That's all for my rambling.

Mike

spyfish007

Mike, you wrote quite a bit in that post. However, I am curious as to how you want your water pump to work. If you state how you want it to work, I'll tell you how to wire it. Also do you have a turbo timer?

Michael Filippello

I want it to come on for a certain amount of time if the temperature goes above a certain point. for example. I do not want it to run everytime I turn the car off as this might be uneccessary. only when the car is turned off and the temp goes above 205 lets say. this would be doable using the miata thermoswitch. As I am unsure how long the system would run I did not know wether to have it cut off at a predetermined time or after the water temp finally dropped below 205. I also figured it only needed to run one fan on low speed to be really effective.

I decided to learn how everything works and was getting confused by all the different posts of how it is supposed to work.

The only reason I posted all that was that I was really bored and have gotten alittle frustrated running accross all these posts that keep contradicting each other.

Sorry for the ramble, I need to get my post count up