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Hey all.

Need help identifying some parts in my engine bay, it's a 1990 Australian delivered MX-5. The parts are in the vicinity of the temp sender located around thermostat housing.

The first two photos is of some weird electrical part with a resistor on the back of it. The third photo is of an unused connector, so not sure where it went or what it's for.

Any help is greatly appreciated.

the unused connector looks to be off the power steering pump, or lack of..

If it helps I'm pretty sure our NA6 has the unused electrical connector as well, right at the front of the engine, and we don't have power steering.

MizMac wrote:If it helps I'm pretty sure our NA6 has the unused electrical connector as well, right at the front of the engine, and we don't have power steering.

spikes wrote:the unused connector looks to be off the power steering pump, or lack of..

Thanks! Yeah no power steering here so that sounds right =)

Now gotta figure out that weird thing with the resistor.

looks like a thermistor of some sort

Trace where the wire goes to? that resistor doesn't look stock

Lose connection is normal, certainly on non-PS NAs. Other thing looks similar to some old knock sensors.

The thing has two wires, green black wires I think. They lead to the rear of the head, one wire is spliced with a green white wire, and the other wire goes into a connector that plugs into the back of the head.



The thing itself was mounted face down right next to the thermostat housing.

Just another photo showing how it was mounted before I removed to see what it was. It's defnitely not a factory install or part?

its not some stupid anti rust thing?? if so it hasnt really helped

is that blue wire a trigger for the aircon pump? thats the same colour wire on mY NB

So I've done some real detective work today... it's a 1.5kOhm resitors that is in series with the coolant temp sensor for the ECU. This temp sensor's function is tell the ECU when to use the cold map (rich fuel mixture), this resistor in series tricks the ECU into thinking that the engine is actually cold.

In parralel with the resitor is a thermoswitch which closes when temperature rises to a specific point (not sure what temp though), this then shorts out the resistor and everything is back to normal so the ECU now thinks that the engine is warm.

I believe whoever did this wanted the engine to run rich for longer because of Mazda's stupid coolant routing causes the rear of the engine to be hotter then the front. Meaning that the coolant temp sensor at the rear of the engine is reading warm when the engine is actually still a bit cold.

Could this explain it all?


The thermoswitch used - http://www.ngt.co.jp/en/Type03.html

It looks to be the same type of resistor that are in those cheap, fraudulent, "power chips".
Those things on fleabay for $10-$20 which promises to increase acceleration, power, blah-blah.
They apparently are wired into the AFM/MAF and changes the signal from the air temp sensor and tells the EMS that the car is sucking in cold air and to compensate thusly with more fuel.

mrpham wrote:So I've done some real detective work today... it's a 1.5kOhm resitors that is in series with the coolant temp sensor for the ECU. This temp sensor's function is tell the ECU when to use the cold map (rich fuel mixture), this resistor in series tricks the ECU into thinking that the engine is actually cold.

In parralel with the resitor is a thermoswitch which closes when temperature rises to a specific point (not sure what temp though), this then shorts out the resistor and everything is back to normal so the ECU now thinks that the engine is warm.

I believe whoever did this wanted the engine to run rich for longer because of Mazda's stupid coolant routing causes the rear of the engine to be hotter then the front. Meaning that the coolant temp sensor at the rear of the engine is reading warm when the engine is actually still a bit cold.

Could this explain it all?


The thermoswitch used - http://www.ngt.co.jp/en/Type03.html

This is probably it.

The resistance curve for a thermistor (sensor used to measure engine coolant temp) is usually a negative logarithmic one. When the resistor is reading a hot temp, the resistance is very low or almost zero (a short). Having a 1.5kilo-ohm resistor is just telling the ECU that the engine is really cold as you've said. This probably means the water temp sensor you have in there isn't connected to the ECU.