Factory gauge reading hot
 

I recently installed a megasquirt 2 as well as a new coolant temp sensor on my MSM. I have noticed now at 200 or so at idle the factory temp gauge reads about 3/4 to pegged.

the MS shows around 200 at idle and I can point my laser thermometer at the back housing and get a similar number.

the gauge is a different story it is constant showing pegged if I’m doing anything where the temps is over 205.

any suggestions? Maybe a bad sensor?

Did you use a new mazda coolant temp sensor?

no I did not. I used an autozone equivalent

Maybe I am misunderstanding. Did you replace the stock thermo sender that is on the back of the head with a not stock sender or did you add an inline sender to a separate gauge?

I replaced the factory sender (3 prong) on the back of the head. That is going to the ECU and factory gauge.

The factory gauge is calibrated to the factory sender. I would replace the aftermarket sender you bought with the oem sender. In addition, if you want to run your aftermarket gauge, you can run a splice connector to run your sender inline with a radiator hose like this.


I don't know the factory hose inner diameter off hand but I am sure you can figure out what the correct size is.

its supposed to be a factory sender. The megasquirt reads the same thing the factory gauge reads I can’t figure why it would be different. I’ll try a Mazda original sensor but a “oem” spec one should do the same

My understanding is that you want the aftermarket gauge to be on a spearate sender than the oem one. This is because the oem sender has a specific resistance curve and the process of introducing an additional circuit (the aftermarket gauge) can cause a resistance change to what the stock gauge reads. Your best bet is to make them 2 separate systems and check again.

I don't think he has an aftermarket gauge. He has the temp reading from his megasquirt and he has his factory temp gauge which shows hot at anything over a 205 reading in tunerstudio.

The NB sensors use two different elements in the same sensor housing. One is used for the ecu temp and utilizes 2 of the pins, the other is for the gauge and utilizes the remaining pin and grounds through the housing. If the gauge is reading high then the resistance reading through that element and it's associated wiring is to low. The most likely cause for that would be the wrong sensor. It looks like NB1 and NB2 sensors don't even have the same resistance curves for that element which means they can't be interchanged. Here's a thread with some info. https://www.miataturbo.net/megasquir...-values-74383/

If you have a multimeter you can confirm this very easily. Why did you replace the sensor in the first place?

Ahh, good catch @SpartanSV If you are correct, then I misunderstood what he was saying. Either way, I think the best course of action at the present moment is to get the correct oem sender for the back of the head as step 1 and move from there.
I also think it would be good to make sure the correct sensor curve is selected in tuner studio just as a double-check.

this is what I was trying to say^. I replaced the sensor due to me smashing it when I put the engine back in the car. I purchased the NGK sensor for a 2005. Fingers crossed!

the link provided was very helpful. I will continue to read through it.

I would guess it is either a grounding problem, or the new aftermarket sensor is crap and not reading correctly.

oh shit. You just reminded me I didn’t put that stupid steel ground strap back on

Ding ding ding, think we have a winner. Surprised it grounded well enough to turn the starter honestly, without melting one of the cables. Maybe give them a quick inspection...

That may be the issue, but it's the opposite of what I'd expect. The type of thermistor used in the sensor decreases resistance as temperature increases. If there is elevated resistance in the circuit I'd expect the gauge to be reading low.

Yeah but potentially the ground voltage of the engine is lifted compared to the ground voltage of the gauge. This means the gauge sees less resitance since the voltage drop across the thermistor is less. So very plausible the missing ground strap might cause the gauge to read hot.

so just got to this. I installed the factory ground. And then the sensor was still ready very high. Bought a NGK sensor and installed today. Reads perfect. And MS is reading the same as well. For some reason the shitty autozone sensor just had one circut(?) that was reading shitty.

thanks guys. All fixed.