Microsquirt Coolant Temp Sensor setting

[Drscottsmith]

All -

Looking at the setup/calibration for my temp sensor setup. 1986 944T fully stock with Mustang Cobra injectors and LS COP ignition.

I have always felt like my temp gauge is reading a bit high, but wanted to see what everyone else is using/seeing. I have the temp II sensor (Bosch Jr timer connector) sending signal to the MS controller. The other temp sensor with spade plugs is just connected to the gauge as normal.

Using Clark's Garage testing page, it looks like the lower thin line is about 80c and the upper thin line is 100c. I have the calibration set to use the BMW 325 coolant temp sensor, but the values do not align with what is on Clarks page in terms of resistance.

Right now the gauge reads right at the 100c line when fully warmed up and driving. The cooling fans are set to come on at 195F but the gauge still goes to the 100c line or just a hair below it.

What do you guys see or are you using different calibration settings?

[Tom]

All -

Looking at the setup/calibration for my temp sensor setup. 1986 944T fully stock with Mustang Cobra injectors and LS COP ignition.

I have always felt like my temp gauge is reading a bit high, but wanted to see what everyone else is using/seeing. I have the temp II sensor (Bosch Jr timer connector) sending signal to the MS controller. The other temp sensor with spade plugs is just connected to the gauge as normal.

Using Clark's Garage testing page, it looks like the lower thin line is about 80c and the upper thin line is 100c. I have the calibration set to use the BMW 325 coolant temp sensor, but the values do not align with what is on Clarks page in terms of resistance.

Right now the gauge reads right at the 100c line when fully warmed up and driving. The cooling fans are set to come on at 195F but the gauge still goes to the 100c line or just a hair below it.

What do you guys see or are you using different calibration settings?

There is a bit to unpack there.... and I'm not 100% sure what you are trying to address, but if the issue is high temps on the dash, I doubt it has anything to do with Microsquirt settings. There are 3 sensors that seem relevant here: the DME temps sensor (blue with jr. timer connector); the coolant gauge sensor (with spade connectors); and the thermo-switch in the radiator.

If your car is running at 100c all the time, that's means something isn't right.

The thermo-switch in the radiator is designed to turn the fans on low speed at its lower set point, and then turn them on high speed if the temp gets up to the higher set point. If your fans don't kick on at all until the gauge is at 100c, then I'd be looking at the thermo switch in the radiator and/or the relay that controls the fans. Those are the most likely culprits when the fans are slow to turn on (or never do). Those thermo switches come in various temp ranges. Stock I believe is 92c (slow fans) and 102c (fast fans) although a lower temp version with 85/93c set points is popular too -- the exact set points should be stamped on the side of the sensor. Have you watched the car at idle to see at what temp the fans first turn on? If not until 100c or close to it, then you need to sort that out (bad thermo switch or bad relay most likely). On the other hand, if the car stays at 100c even when cruising down the highway at 65mph, that suggests it's more of a basic cooling system problem (blocked radiator, sticky thermostat, etc.).

I assume the calibration you have set to the BMW 325 is for the DME (blue jr. timer) sensor. If so, that's not relevant to your gauge per se.

To rule out faulty gauge readings... The gauge is driven by the sensor with spade connectors, and that sensor should line up with these reference points:

40c =287.4 ohms

80c = 69.1ohms

100c=38.5 ohms

If those values check out, then your gauge is probably accurate and you'd want to focus on making sure the fans turn on when they are supposed to and, if they do, then look at the efficiency of the cooling system itself (stuck thermostats, dirty radiator, etc.)

Does that help?

[Drscottsmith]

Tom -

Yes this is a multi part question for sure.

I guess step one is, in tunerstudio, I need to make sure the coolant temp sensor is calibrated correctly. Did you use the BMW E30 choice and its corresponding preassigned values or put the reference values you mentioned above in the boxes in the calibration screen (degrees c to ohms)?

[Tom]

Tom -

Yes this is a multi part question for sure.

I guess step one is, in tunerstudio, I need to make sure the coolant temp sensor is calibrated correctly. Did you use the BMW E30 choice and its corresponding preassigned values or put the reference values you mentioned above in the boxes in the calibration screen (degrees c to ohms)?

Right, so let's break it down. I think there are three separate issues: 1) how to set up the temp scale for you MS engine management system; 2) why your temp gauge sits at the 100c mark while driving; and maybe 3) is the dash gauge showing an accurate reading for temp.

In Tunerstudio, I assume you are trying to set up the temp scale for engine management purposes (e.g., cold start enrichment, fuel and timing map trims, etc.). If so, you need to use the scale for the blue sensor with the junior timer connector. That sensor has a different scale than the dash gauge (spade connectors) I posted above. For the engine management (Blue jr. timer) sensor, the scale is detailed in this testing guide (not the scale I posted above, which are for the totally separate gauge sensor). Here are reference values from that guide for the blue sensor:

@32°F = 4.4 -6.8k Ω
@59°F - 86°F = 3.6 – 1.4k Ω
@104°F = 1 – 1.3k Ω
@176°F = 250 -390 Ω
@212°F = 160 – 210 Ω

I've never used Tunerstudio so can't give guidance there, but if possible I would manually enter those values into your system rather than picking a BWM sensor that may or may not have the same scale (and probably isn't right).

The other issues about your car running hot are unrelated to the blue jr timer sensor. They are most likely related to your fan operation (thermo switch in radiator, relay, etc.) if the car cools down while driving down the highway; or related to the basic cooling system capacity (thermostat, radiator efficiency, venting, etc.) if the car stays at 100c all the time.

[PSU_Crash]

@32°F = 4.4 -6.8k Ω
@59°F - 86°F = 3.6 – 1.4k Ω
@104°F = 1 – 1.3k Ω
@176°F = 250 -390 Ω
@212°F = 160 – 210 Ω

That's quite the range. I'd pull the sensor out and test it in an ice bath, then in boiling water and use those numbers for your specific sensor. Thermometer in the water to grab a few more points along the way.
Or just install a GM sensor and forget it. That's the route I have always used with megasquirt.

[Dare]

The "BMW E30 325i" preset in tunerstudio is what I'm using. The preset purportedly corresponds to the Bosch 0 280 130 026 sensor (datasheet here), which is the same as the 944 sensor.

The temperature map for the “BMW E30 325i” preset is:

- 9300Ω = -10C
- 2500Ω = 20C
- 335Ω = 80C

which is roughly the same as the values for the 944 sensor.

[Tom]

The "BMW E30 325i" preset in tunerstudio is what I'm using. The preset purportedly corresponds to the Bosch 0 280 130 026 sensor (datasheet here), which is the same as the 944 sensor.

The temperature map for the “BMW E30 325i” preset is:

- 9300Ω = -10C
- 2500Ω = 20C
- 335Ω = 80C

which is roughly the same as the values for the 944 sensor.

Agreed. I had chat calculate the best-fit curves based on those reference values and the two sensors have essentially the same curve for all practical purposes. @Drscottsmith -- looks like picking the BMW 325 sensor is a good choice, so you should be good to move on to why the motor is running so hot, which is mostly unrelated to microsquirt and its settings (unless you've made atypical efforts to have the microsquirt control the fans).

[Drscottsmith]

Ok - thanks all!

Looks like I have a gauge issue. I figured I should start with the safety factor and verify the actual engine temps at various spots and see how that corresponds with the gauge....

Screenshot 2026-02-09 at 2.14.49 PM.png (142.12 KiB) Viewed 459 times

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Here is the screenshot from clark's garage from the manual. My orange lines correspond to the needle positions in the table below (4 on top, 1 on bottom)

Screenshot 2026-02-09 at 2.08.56 PM.png (90.91 KiB) Viewed 459 times

So when the gauge pic shows 100, my digital thermometer is reading 90-95 on the head, just a bit below.

What is weird and really got me thinking too much was the fact that the thermostat does not fully open according to the manual until 90, and the factory spec for the cooling fans to come on low speed is 92 (198F) and high speed at 102 (215).

My fans are controlled by MS and right now I just have one on/off point set to come on at 196F.

Once the gauge gets to the 3/4 spot, it typically stays on 3 when moving and might get up to the line sitting at a light.

Thoughts?

[Tom]

Ok - thanks all!

Looks like I have a gauge issue. I figured I should start with the safety factor and verify the actual engine temps at various spots and see how that corresponds with the gauge....

Screenshot 2026-02-09 at 2.14.49 PM.png.
Here is the screenshot from clark's garage from the manual. My orange lines correspond to the needle positions in the table below (4 on top, 1 on bottom)

Screenshot 2026-02-09 at 2.08.56 PM.png


So when the gauge pic shows 100, my digital thermometer is reading 90-95 on the head, just a bit below.

What is weird and really got me thinking too much was the fact that the thermostat does not fully open according to the manual until 90, and the factory spec for the cooling fans to come on low speed is 92 (198F) and high speed at 102 (215).

My fans are controlled by MS and right now I just have one on/off point set to come on at 196F.

Once the gauge gets to the 3/4 spot, it typically stays on 3 when moving and might get up to the line sitting at a light.

Thoughts?

Unorganized thoughts:

[TL;DR version: I'd revert to factory control over the fans using an 83c/93c thermo switch in the radiator and an 80c t-stat. If you then still see high temps, consider radiator options...]

Just curious why you are controlling the fans with the microsquirt? The factory system works quite well when functioning as designed and does not rely on the DME/ECU, so you don't 'need' to control the fans with the microquirt.

The factory system has the fans come on in two stages (slow and fast) to help prevent the temps from getting as high as you are seeing. If you have just one set point, you are losing a level of control. You are either losing the slower fans coming on at a cooler temp (to help prevent it from getting up to line 4) or you are losing the fan coming on fast when it starts getting really hot. Either way is 'less good' than the standard factory set up. If your fans don't turn on at all until 196F, you are virtually guaranteeing the gauge will get up to line 4 at long stop lights.

Unless you have a specific reason to control the fans that way, I'd start by reverting to the factory system (using an 83c/93C thermo-switch in the radiator) and see if and how much better you do, or at least turn the set point down to 180F. Your car is currently working similar to a car with a bad thermo-switch that fails to trigger until the higher set point (and even worse if the fans only turn on in slow speed). In that situation, it's common to see the gauge bounce up near your 3 and 4 lines.

I'm not entire clear which sensor you are using to trigger the fans, but assuming it is either the DME temp sensor (blue connector) or the Gauge sensor (with spades), be aware that the factory system controls the fans based on the thermo switch in the radiator itself, rather than coolant temps in the block like you seem to be doing. All the fans can do is cool what's in the radiator, so the only temp that matters to the fans is the temp inside the radiator. There are circumstances where the block temps and radiator temps diverge, and the factory system protects the radiator from overheating (blowing end tanks, etc.) in those situations.

I wouldn't put much stock in the discrepancy between a point and shoot thermometer and the temp on the gauge. The sensor is submerged in the coolant itself, and the thermometer is pointing at the outer wall of an engine. I'd expect the other wall to be cooler...

If the fans were the ONLY issue, your car would run cooler than your line 3 on the highway, so agree it's worth looking at the t-stat and general health of the cooling system.

I think the factory liked these cars to run a bit hotter than most of us would like (opinions vary on that of course), so what I and many others have done is use a lower temp thermostat paired with a lower temp thermo switch. I have a 80c thermostat and a 83/93C thermo switch, and my needle will creep up to just under half way on the gauge at long stop lights, then the fans kick on and it settles back down to 80c (the t-stat setting). If you revert to factory fan controls, and a lower temp t-stat and thermo switch, chanced are your gauge will bounce between 80 and 90c. If not, it would be time to look at the radiator itself for internal clogging, etc.

Also be sure to remove all debris in front of the radiator. Leaves and bugs and dirt can significantly reduce the cooling efficiency.

[PSU_Crash]

The "BMW E30 325i" preset in tunerstudio is what I'm using. The preset purportedly corresponds to the Bosch 0 280 130 026 sensor (datasheet here), which is the same as the 944 sensor.

The temperature map for the “BMW E30 325i” preset is:

- 9300Ω = -10C
- 2500Ω = 20C
- 335Ω = 80C

which is roughly the same as the values for the 944 sensor.

Sounds like you are good to go there!
On my MS setups I have often run the factory fan control in parallel with MS control. That way it's redundant and I have the ability to run the fans constantly, or just a lower temp if I so choose. Like in the staging lane between autocross runs.