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So i just bought a new Koyorad for my fb with a 12a from racing beat. When I took everything off my old rad, I've got this sensor left over and I'm not too sure what it is/does. It seems to run and cool just fine, and my temp gauge is working currently. Anyone know what this does and if it's important? this is the place where I removed it
It's a temp sensor related to the air injection system (sensor #2 if you want to look it up). You don't really "need" it unless you're concerned with that system completely working properly.
I can't go look at mine right now but Im 99% sure that lower left sensor is for the sub-zero start system. If Im right (and someone chime in if Im not) you don't need it. The sub-zero start injected anti-freeze into the intake stream to raise compression a little to assist cold-starts below freezing temps. It was questionable and Mazda sent a service bulletin to its dealers to disable it back in the day. The antifreeze was held in a small plastic jug on the firewall, on the passenger side (you may or may not still have this).
It's a temp sensor related to the air injection system (sensor #2 if you want to look it up). You don't really "need" it unless you're concerned with that system completely working properly.
IIRC the sub-zero start system uses the oil temp sensor in the pan, not coolant temp.
I think you're right, my appologies. So what is the air-injection system you refer to? I have a new Koyorad sitting in the garage and Im going to have this same question soon.
Temp sensors I'm familiar with -
*temp guage connects next to oil pressure sensor
*choke temp sensor in back of WP
*cold-start sensor in oil pan next to level-sensor float
*water-level sensor on top-center radiator
When you say air-injection I think of the air pump. Does it cycle on/off with temp? I dont have the air pump /emissions anymore.
Oh Lordy Lordy, I thought this all sounded familiar so I looked through my own post history. Look up a thread titled
" Coolant Temp Sensor M10 x 1.25 what to buy " in the 1st gen Specific section. I was actually the one who answered this question in that thread a few months ago, and gave my references.
In short: This sensor is for early EFI cars (GSL-SE and S4 FC's). It controls the fast idle / fuel enrichment for cold starts. (Maybe thats what made me think sub-zero). On 12A cars it does nothing.... Its just there because all S3 cars ('84 & '85) share the same harness and radiator. The factory could wire and assemble all chassis the same before the motor was installed, with lesser wiring differences after that depending on 12A or 13B.
Ive been working 7/12's for 2 weeks with one more to go. My brain is fried.
No sweat, air injection isn’t something anyone really dives into unless they have to lol. I would always take the failed smog jobs - once you understood how the systems actually worked when working properly, it was relatively simple to figure out what was broken.
Correct regarding air injection = air pump/rats nest, all that stuff. The air pump itself doesn’t cycle on/off - if it’s turning, it’s pumping air. That constant air is then directed/diverted as necessary.
The sensor OP is asking about (#2) signals when coolant temp is greater than 59F. There’s also sensor #1, (the one located on the water pump), that signals when coolant temp is greater than 158F. According to the diag procedure, when engine temp is below 158, and the choke is activated at all, air injection is directed to the air cleaner. I can’t recall clearly (and don't have an FB to verify this), but I feel like once sensor #1 activates at 158F, the ECU considers that “having reached normal operating temp”, and sensor #2 is then ignored, so the incorrect function wouldn't cause issues during a smog/emissions check (which are done at operating temp, plus no OBD existed to log a problem like that).
FSM says #1 is also related to choke operation (as you already know) and the hot start assist motor (cable from the carb over to a little motor, yours is probably gone), which makes sense logically (>158F doesn’t need choke but is considered a “hot start”). I don’t think sensor #2 is used for anything else. I vaguely want to say it may have been used in relation to A/T solenoid pressure when cold ("if coolant is <59F, shift this way"), so a bad one could cause odd shifting when warm - but even if I’m right, I doubt anyone reading this is gonna have an A/T much less be troubleshooting it lol. Maybe I'll go back and check the FSM on that later.
tl;dr you won’t need to worry about sensor #2 at all, and OP only does if they still have the air injection system and want it to work properly during warm-up.
On 12A cars it does nothing.... Its just there because all S3 cars ('84 & '85) share the same harness and radiator.
Incorrect. Check my previous reply - it may not do anything noticeable in terms of driveability, or cause problems with state emissions tests, but it definitely is used by the ECM on 12A cars.
mRdna thank you for that information. I had gotten my info from an older thread from the 2nd gen section, which slso referenced the fsm's and diagrams. I should have known better than to trust those FC guys . I don't know about OP but all my emissions equipment and emissions harness wiring was removed and I doubt my computer does anything anymore. Thank you for correcting my misinformation - I dont want to be guilty of re-passing-on bad info. But I am happy that still does nothing in my application and I can leave it disconnected when I replace the radiator.
It's all good - even with that sensor bad/missing, I'm pretty sure it doesn't cause anything that anyone would actually notice unless they were specifically looking, so the sentiment of it "doing nothing" makes sense, just isn't actually correct in terms of function. I'm a stickler to understanding the function because there are sometimes items like that which appear unimportant, but then later on cause some weird effect or issue that becomes really difficult to nail down. That, and I really liked working with failed smogs because nobody else at my shop did, so I took 'em knowing the systems were complicated at first glance, but simple to diagnose once you knew how everything behaved when the system was working correctly. Which meant more money in my pocket lol.
Edited to add FSM references for anyone curious:
1985 RX-7 shop manual, fuel & emissions section:
4A-2 shows #2 temp switch in the System Diagram
4A-4 shows location of #2 switch (lower radiator)
4A-7 relief solenoid valve function description
4A-8 water temp switches (#1 and #2) function description
4A-(12-13) emission checking procedure for relief solenoid valve - steps 44-50 deal with the two temp switches specifically.
Last edited by mR.dnA; 09-13-21 at 08:28 PM.
Reason: added specifics regarding related FSM locations
09-13-21, 01:02 PM
. I don't know about OP but all my emissions equipment and emissions harness wiring was removed and I doubt my computer does anything anymore. Thank you for correcting my misinformation - I dont want to be guilty of re-passing-on bad info. But I am happy that still does nothing in my application and I can leave it disconnected when I replace the radiator.
