Temperature Sensors

(current as of Oct 04)


Automotive temp sensors

Tracy Crook's EC-2 temp sensor

Homemade temp sensors




Automotive Sensors

(this is an extract of MegaSquirt EFI Computer web page: http://www.megasquirt.info/manual/mwire.htm#clt )

MegaSquirt uses coolant and air temperature sensors to determine the warm-up characteristics of the engine and the density of the intake air. They are essential to proper functioning of MegaSquirt. Both sensors are Negative Temperature Coefficient (NTC) thermistors. This means that they are resistors whose resistance decreases as their temperature goes up.

Naturally aspirated engines using MegaSquirt can use the same sensors for coolant and air temperature. These sensors are inexpensive (roughly US$9.00) GM units readily available from any parts store (GM part number 12146312, may have been replaced by #15326386). They have a 3/4" hex.

However, you will save some money if you can source these from a salvage yard, with the mating connectors (which are GM #12162193). If you are unable to get them this way, consider using a “spade-type” connector or reusing your existing sensors (with EasyTherm and/or resistor calibration adjustments).

Turbocharged or supercharged engines should use an open-element air temperature sensor for a faster response time. Here are some reported part number equivalents for both the coolant and air temperature sensors (verify before ordering):

Coolant temperature sensor (CLT)

Air temperature sensor (IAT)

GM #12146312
(may have been replaced by #15326386)
Wells SU109
Standard TX3
AC DELCO 213-928

GM #25036751
Wells SU107
Standard AX1
GP SORENSEN 779-19001
AC DELCO 213-190

Connector Pigtail (CLT)
(mushroom key way)

Connector Pigtail (IAT)
(square key way)

Wells PN 254
Conductite/Dorman 85100
(~$9 @ Autozone (PN 047131))

Wells PN 235
Niehoff PN PS77421 (~$15)
Conductite/Dorman PN 85110
(~$12 @ Kragen partsamerica.com)

The coolant temperature sensors were apparently found in the following applications:

Note: A few early installations using the open-cage MAT sensor experienced vibration induced failure of the sensor. The thermistor bulb is supported only by two thin wire legs.  These can apparently fatigue and break when installed in high vibration environments, such as occurs when you screw it directly into an intake manifold. Several people solved the problem by "potting" the legs of the thermistor with O2-sensor-safe silicone (most silicone sealer/adhesives destroy O2 sensors, so pay attention!), squeezing it down inside the sensor body but leaving the bulb exposed.

Note that these sensors have different connectors. The coolant temperature sensor uses a “mushroom” shaped key way where it inserts into the sensors, while the open element intake air temperature sensor uses a “rectangular” connector key way.

The wiring schematic for DB37 shows only one input for all of the sensors (except for the two for the TPS). The recommended GM sensors all have two wire connectors. The missing connection is a ground wire for the sensor. Sensor grounds should be brought to the same grounding point on the engine block as the MegaSquirt ground, unless they are grounded through the body of the sensor.

If you are looking for sensors with a standard “spade” type connector and a ground through the body of the sensor, GM part number 25036135 is what you need - see the illustration below:

The resistance curves for the MegaSquirt/General Motors coolant and air temperature sensors, as well as various part number cross-references, are listed in this section.

GM Temperature Sensor Resistance

Degrees F

Degrees C

























The thread for the recommended General Motors (and equivalent replacement) coolant and air temperature sensors for MegaSquirt is 3/8 inch National Pipe Thread [NPT]. A 9/16 inch pilot hole is required for the tap. Recall that pipe sizes are based on nominal inside diameters, not outside diameters as for standard National Coarse [NC] and National Fine [NF] threads. The sensors are designed to be tightened to 20 N-m (15 ft-lbs).

Approximate sizes

Nominal Pipe Size
- actual ID is slightly bigger

Approx. Outside
Thread Diameter

Drill Size










These sensors were been used on practically all GM cars in the 1980s and are easy to find - the same is true for the correct connectors. However, other sensors can be used if the EasyTherm software is used to recalibrate your MegaSquirt.


Tracy Crooks Temp Sensor (white)



Tracy Crook's EC-2 Temp Sensor


If you purchase Tracy Crook's EC-2 ignition and injector control unit, you'll receive a thermistor-style sensor to place for incoming air temperature.  I chose to make the sensor easily removable by embedding the thermistor within a screw-in fitting, then tapping a hole in the throttle body to accept the fitting.


How to mount the sensor:  This fitting is from a brake caliper bleed port.  They're used to bleed hydraulic fluid from a car's disc or drum caliper.  I cut off each end then drilled out the center hole so Tracy's thermistor-style sensor would slide through.









Where to put the sensor:  Any place in the incoming airflow will work.  i chose a place on the throttle body that I could tap for 3/8 x 24 to accept the above fitting.











Prior to potting the sensor:  Tracy's sensor is a little over 1/8" in diameter.  I used a 1/4" hole to allow some JB Weld to fill in around the sensor.  The black part of the sensor is about 1 and 1/4" long.










Potted thermistor:  Mix about an erasers worth of JB Weld or other heat tolerant epoxy and use a toothpick or matchstick to push it into the fitting.  It's useful to start the sensor out to the left about 1/4" in this picture, then fill the gap and slowly pull the sensor to the right.  Fill both ends until full.









Sensor in place:  The sensor hole is drilled first with a 3/16" drill, then drill the hole to 5/16" coming close to the inside throttle body opening.  Because this sensor has a grounding wire, there's no need for contact between the sensor and ground (throttle body).  Use Teflon tape or liquid sealant if necessary.  The sensor is placed well into the incoming airflow.











Homemade Temperature sensor(white)


Homemade Temperature Sensor


A homemade sensor reduces cost and can provide a dozen or more reliable sensors for a fraction of what you'd pay for ready-made sensors.  From start to finish this project will take about 3 hours for 25 sensors.  


Concept:  A TO-92 type of transistor housing made of plastic with a flat face on one side allows you to glue the sensor to a thin piece of aluminum sheet.  The aluminum provides a mounting surface and transmits the temp quickly to the chip.  Inexpensive sensors can be used for many places you wouldn't otherwise consider monitoring.


The aluminum piece allows mounting almost anywhere.  This will work for radiators, coils, engine blocks, inside/outside cockpit, alternators, batteries, but not exhausts.  Some chips have lower heat ranges than others, so ensure your application doesn't exceed the chip's range.  You'll have to decide if you want Fahrenheit or Centigrade for the readout, as most chips are calibrated for one or the other.  At first I didn't see why you'd care if the chip measured either Fahrenheit or Centigrade, as you could calibrate the readout through the display electronics.  But with the LM34/35 series, you can send the signal from the sensor directly to a multimeter and read a very accurate temperature with no calibration required.  What a great deal!  More on temperature sensor chips.  A link to more than you want to know about temp sensors.


Parts: (total cost about $3-4 per sensor. Click on pic for larger view)

- Temperature "Sensor-on-a-chip" such as an LM34, LM35, LM75 or LM335Z.  Don't spend too much, as the 50-90 cent chips work.  The "Z" on the end of some of these chips means it comes in the TO-92 model, which is a plastic cylinder with a flat face.  The flat face allows gluing to the aluminum.  eBay is a great source if you want 10 or more sensors/chips.

- 24 gauge, multi-strand wire.  I used 22759/16-24.  26 gauge would probably work fine.

- 3/8"x1.5"  "Double-wall" heat shrink tubing.  Inner wall melts to seal sensor and wiring.

- Thin aluminum sheet.  Choose a thickness that is strong enough to support the chip, yet allows fast heat transfer.  Scuff before gluing chip.

- 1/16" heat shrink tubing.  I prefer "polyolefin" which remains flexible, but normal heat shrink will work just as well for this sensor.

- 15-40 watt soldering gun and rosin-core solder.  20 watts is plenty of heat.





- cut wire to length and strip 1/4-1/2" of jacket

- cut 1/16" heat shrink tubing in 1" lengths

- cut 3/8" heat shrink to 1.5" lengths

- trim aluminum pieces to size and scuff for glue adhesion.  3/8" wide x 1" long works well. 

- glue chip to aluminum using "super glue."  The idea is to keep the film of glue as thin as possible to improve heat transfer.

- carefully bend the outer two legs of the chip slightly outward to allow soldering of wires.  You'll bend back inwards later.  Be careful not to exert too much force, or the wire can break where it meets the chip.  A technique is to use two needle nose pliers, one right next to the chip, and the other right next to it to do the bending.

- pre-tin ends of wires and legs of chip.  A link to soldering techniques.

- solder one wire to each leg.

- heat shrink each leg with a 1" long piece of 1/16" heat shrink tubing.

- bend the legs back.

- heat shrink using 3/8" tubing.  Ensure you have enough tubing to seal both ends as in pic below.

- Crimp or solder a 3-pin Amp or Molex connector to the sensor wiring.  When wiring the aircraft, you may need to use a capacitor to reduce noise in any long length of wiring (<2').  Attach one end of the capacitor to the signal wire, close to the readout, and the other end of the capacitor to ground.  If you have 3-wire shielded cable (Mil Spec 22759 is available) you'll get a clean signal without the capacitor.  Attach the shielding to a ground point near the readout, and not to the sensor end (and don't connect it to grounds at both ends, as you may get a grounding loop, which is hard to troubleshoot).


Note: Here's a quote from National Semiconductor's datasheet for the LM335Z chip on using the double-wall heat shrink tubing:



Meltable inner core heat shrinkable tubing such as manufactured by Raychem can be used to make low-cost waterproof sensors. The LM335 is inserted into the tubing about 12" from the end and the tubing heated above the melting point of the core. The unfilled 12" end melts and provides a seal over the device."  


(I don't know where they came up with inserting it 12".  Instead, just insert it 3/8").


Close-up:  The double-wall heat shrink works well to seal the sensor.  It "mushs" out each end as if it's liquid asphalt.  Typical temp ranges for these chips are at least -20 deg F to +250 deg F.  That should cover water, oil and air temps, as well as many engine components such as coils, alternators, and regulators.









Which chip to use?

I recommend the LM34 for Fahrenheit readings, and the LM35 for Centigrade.  Both are precision sensors, have temperature compensation, and only require a stable 5 volt reference voltage.  This means for about $1 for each chip, you can make many very accurate sensors.  Other types, such as the LM335Z that I used, require additional electronics to complete the system (the LM335Z requires a trimming potentiometer to bring the temperature reading from -273 Centigrade, to zero (when you calibrate the sensor in a bowl of ice water).  Not a big deal, but the LM34/35 series are calibrated right out of the packaging.












Suppliers (white)



Temperature sensors: 

Mouser sensors, Digikey sensors, eBay for "Thermistors."

Heat shrink tubing:

eBay for "Heat shrink tubing." "heat shrink tubing wall" (looking for double-wall or dual-wall tubing)

Wire, aircraft:

eBay for "22759"  "mil spec wire"  "Teflon wire"