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Apologies if this had been talked about before, I did some searching and read both Ray's and Dale's excellent cooling system threads, but couldn't find any mention. I'm sorting through the cooling system on my 92 JDM FD, and was wanting to test the thermo-switch, as it looks particularly well used.
Both the USDM 93 and 94 FSM's have this test data listed in the cooling section:
Which, to my logic means its a normally closed switch (<0.5 Ohm between the terminal and switch body), that opens when the temp gets above 108 degC... Easy to test; my switch measures open circuit at room temp so I assumed it is buggered and didn't go any further.
I then read a bit more about the cooling system and looked through the wiring of it, and this being a normally closed switch didn't make a lot of sense. With the right resistor locations on the fans, it could still work, but it just seemed odd. I then looked through the JDM FSM's with some helpful google translate action, but unfortunately this test isn't outlined in the cooling section, it's alluded to in the coolant fan operation section later on, but it's just plain missing from the cooling section. Frustrating.
However, the on the JDM models, the state of the thermoswitch is monitored by the ECU, on pin 4F. This is the split air bypass solenoid pin on the USDM models, as I assume they needed to repurpose it to get those crazy emissions systems on the USDM cars working. Looking further through the JDM FSM, you get to the trouble shooting section that tells you what signals you would be seeing at the ECU pins under various circumstances, and Pin 4F:
Excuse the screen-shottyness of google translate.
But that spells out that when the coolant temp is below 108 degC, you'd be expecting to see battery voltage at pin 4F on the ECU. This is the output of the switching coil of the relay, so to see battery voltage here the thermoswitch must be open, as in the terminal is not connected to the switch body (ground). When the temp gets above 108 degC, the thermoswitch switches, and connects the output of the relay switching coil, and pin 4F to ground. This means the thermoswitch must be a normally open type.
I haven't seen multiple part numbers for the thermoswitches, and I don't imagine there are two identical types out there that switch in different directions. Bloody weird. Anyone got any ideas what's going on here?
I've read that its common to swap in an FC thermoswitch due to their lower switching temperature, anyone have any solid FSM or similar data on which direction they switch?
Early JDM cars had a thermoswitch but after December 95 when the ECU's went 16 bit they changed the fan system. From then onwards there was no thermoswitch and that fourth input was controlled by the ECU.
The thermoswitch is normally open. When it hits the thermoswitch set temperature it closes and grounds the coolant fan relay, triggering the fan relay and upping the fan speed by one level.
To test you'd want to see no continuity until the set temp.
I remember my old 98 RS didn't have the thermoswitch.
So, yeah, the USDM 93 and 94 FSMs on the interwebs are wrong with their specified test. You should be expecting to see the opposite results. Good to know.
Yep, the switch is a basic circuit. It's normally open until the temperature switch point then it grounds out. That switch is quite reliable, the only problems I've ever seen have been them physically broken.
I figured out the use of the FC thermo switch a LONG time ago. I actually found out about it looking through one of the Japanese RX-7 magazines, they showed doing that to drop the fan temp. In other words it works fine on a JDM car.
As stated, the fan switch was only on 92-95 cars. In 96 when they did the ECU revision and went to the "black box" rat's nest under the hood they got rid of the thermo switch and just had the ECU turn on 2 fan inputs instead of just 1. Since the ECU already has a coolant temp sensor it was easy to add on. I think they only did it then due to a move to a faster/more capable ECU (I think that's when it went from 8-bit to 16-bit) and the ECU could handle that task.
FYI, the input to the ECU on fan temp from the fan switch is just for idle control. That lets the ECU know there's a higher alternator load and it adjusts idle accordingly. The fans themselves just have their relay grounded off that switch and kicks them up a fan speed.
The resistances and FSM procedure is correct for all thermosensor equipped cars, it's just a switched ground really.
Yup yup, I understand how the whole system works, and that there were changes to it over the years, that's not what I'm really trying to get at; What I'm saying is that the testing procedure outlined in the 93 and 94 USDM FSM's seems to be wrong. It's saying that the thermoswitch is switched to ground up until 108degC, where it then opens. In reality I'm pretty sure its around the other way, the switch is open up until 108degC, where it then closes. It looks like the two lines in the FSM are around the wrong way. Means if you go to test one, it'll immediately appear as stuffed where it might not actually be.
Forgot to address that in my last post, but yeah, I do seem to remember that was an error in the shop manual. As in Mazda had a TSB or note or something that addressed that.
06-08-21, 02:10 AM
