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Falsely overheated

pcb

The Diagram Dude
:
2002 MP5
Hmmm that could be your problem.

If it doesn't snap back into the plastic connector then it might be bent or something.

Try removing it from the plastic connector then remove the plastic connector from the ECU and plug it directly onto a pin on the ECU to see if its snug on the pin.

That's where you scavenge a pin from an unused wire from the big connector and use it for pin #91.

Or get a connector from the wrecker to remove a pin from it to use.
You cut the connector off with a few inches of wire to splice into it.
Then you'd have a whole bunch of pins to pick from.
 
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pcb

The Diagram Dude
:
2002 MP5
The problem is that you can't test for continuity between the little metal clip and the pin of the ECU once the connector is plugged in without taking the cover off the ECU and finding the backside of pin #91 to test the connection.

It's possible to solder the orange wire (white in your case) directly to the inside of the ECU and bypass the connector for that wire, but that's pretty involved.
 
I'm pretty sure that is the case.
Her car spontaneously died so the O2 sensors, throttle position sensor and transmission fluid temp sensor may all have failed.
Yep, either that or the main relay failed, fairly common in early Miatas as well. The INJ fuse powers the ECU and a host of other devices.

The "ground" return at pin 91 is not the same as chassis ground, BTW. For a quick test one can ground the orange wires to chassis ground to see if there is an open circuit but I wouldn't run it that way; the ground loop created can make the ECU do all kinds of funny things.

Loss of power to the ECU can make engine quit suddenly.
 

pcb

The Diagram Dude
:
2002 MP5
Yep, either that or the main relay failed, fairly common in early Miatas as well. The INJ fuse powers the ECU and a host of other devices.
She did have a mechanic look at it and he most likely checked fuses and relays and everything seems to be working on the dash except the temperature gauge reading max.



The "ground" return at pin 91 is not the same as chassis ground, BTW. For a quick test one can ground the orange wires to chassis ground to see if there is an open circuit but I wouldn't run it that way; the ground loop created can make the ECU do all kinds of funny things.
I was wondering about that but I didn't know if the sensor ground is even connected to chassis ground

Loss of power to the ECU can make engine quit suddenly.

The mechanic did check the voltage at the temperature sensor and I don't think it would have had any voltage at all if there was a blown fuse or bad relay?

The ECU probably needs to be powered up fully to send voltage down that purple/yellow wire to the temp. sensor.
 
I was wondering about that but I didn't know if the sensor ground is even connected to chassis ground
That's why they return all grounds to a single point. You could have several millivolts between sensors if each was grounded separately.
 

pcb

The Diagram Dude
:
2002 MP5
I wonder if there is continuity between pin #91 and the ECU ground?

Now I want to remove my ECU and check to see. Lol

Maybe I will, then I can offer mihkayla some comparative measurements.

If she gets a multimeter, we can compare notes. Lol
 

pcb

The Diagram Dude
:
2002 MP5



Fortunately, none of these systems are actually used so you don't have to worry about them. Lol






Ohhh...
Wrong Page...



 
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pcb

The Diagram Dude
:
2002 MP5
I found a couple of connectors under the hood that has an orange wire...

I removed half of the engine to pull a piston so I don't know what the connectors connect to.

But one has a violet/yellow wire so it's probably the temperature sensor.











 

pcb

The Diagram Dude
:
2002 MP5
.

The "ground" return at pin 91 is not the same as chassis ground, BTW. For a quick test one can ground the orange wires to chassis ground to see if there is an open circuit but I wouldn't run it that way; the ground loop created can make the ECU do all kinds of funny things.


I took the cover off my ECU and found this...






And this...












I found pin #91 on the backside of the circuit board.








I can trace the wires back to the ECU from the sensor connector.








Notice the Orange wire...




 

pcb

The Diagram Dude
:
2002 MP5
The "ground" return at pin 91 is not the same as chassis ground, BTW. For a quick test one can ground the orange wires to chassis ground to see if there is an open circuit but I wouldn't run it that way; the ground loop created can make the ECU do all kinds of funny things.
I tested for continuity between pin #91 and the four B/W pins and there is continuity between pins # 24, 51, 76, and 77.

If the OP were to tap the orange wire into one of those ground wires, would that create a ground loop?

They are all grounded together within the ECU.

I guess if it didn't matter then that orange wire would just be another B/W wire and grounded with the others?





I found this too.
There are more sensors connected to the orange wire.











 
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