Making Sense

Sensors Quick Referent

The table below list the sensors by name, the engine types it is found on, the class of sensor it is, mounting characteristics, and very briefly what it does. It is intended as a quick reference only. For more detailed information read the related article(s).

WARNING! Over torque of NPT threaded sensors can cause major engine damage. NPT, National Pipe Thread, is a tapered threading used for plumbing. Sensors with this thread can crack block and manifold mounting ports if over tightened. Do not treat this like a regular bolt. NPT sensors will normally "go tight" about 66% of the threaded area.

Long name	Short name	Engine	Type	Mount	Ground?	Operation	Notes
Idle Air Control	IAC	V6 , 4C3	Stepper motor	Threaded w/ gasket	No	Stepper rotates screw valve.	1
Idle Air Control	IAC	4C7	Stepper motor	2 Torx Screws w/ O ring	No	Stepper rotates screw valve.	1
Throttle Position Sensor	TPS	All	Rheostat	2 Screws	No	Voltage Divider. Output voltage increases with throttle angle.	7, 12
Engine Coolant, ECM	ECT	All	NTC Thermistor	NPT	No	Resistance lowers with Temp. Used to measure coolant temp	2, 3, 12, 13
Fan Switch		V6, 4C3	Thermo Switch	NPT	Yes	Switch closes path to ground at 235F (Single speed)	2,11
Oil sender		All	Multi	NPT	Yes	Closes fuel pump switch above 4 PSI. Variable resistance to ground for Gauge.	2
Temp Gauge/Alert		All	Multi	NPT	Yes	Alert light on at 265F. Resistance to ground for gauge.	2
Air Charge Temp	ACT	V6	NTC Thermistor	NPT	No	Measure air temp in filter can.	2, 3, 4, 12
Manifold Air Temp	MAT	4C7	NTC Thermistor	NPT	No	Measure air temp in intake manifold.	2, 3, 4, 12
Oxygen Sensor	O2	All	Air Battery	Threaded w/ gasket	Yes	Generates Voltage according to amount of O2 in exhaust gas.	6, 12
Manifold Absolute Pressure	MAP	All	Barometric	Snaps onto bracket	No	Manifold pressure/vacuum produces variable voltage	8, 12
Crank Position	CPS	4C7	Magnetic	Under Coil Pack	No	Notches in crank wheel generate pulses.	10
Distributor Pickup		V6, 4C3	Magnetic	Screws	No	Generates pulses as distributor "lobes" pass.	9
Primary Ignition Module	PIM/ICM	All	Amp/Driver	Varies by type	Yes	Provides ignition coil drive current and amplification for the crank or distributor pickup.	5

Engine Codes
4C3	VIN R motor, 2.5l, 151CID, 300 TBI, HEI distributor ignition
4C7	VIN R motor, 2.5l, 151CID, 700 TBI, DIS distributorless ignition
V6	VIN 9 motor, 2.8l, 173CID, MPEFI, HEI distributor ignition
Ground?
Yes	Open loop sensor that requires ground path via sensor shell and engine block. Open loop sensors are susceptible to errors and failures caused by bad engine ground straps.
No	Closed loop sensor with all connections directly to ECM. The sensor element is electrically isolated from the sensor shell.
In this case open and closed loop refers only to the sensor current path and has nothing to do with ECM modes.

Notes:

These are electrically pretty much the same. The difference is mostly in the method of mounting.
Don't use Teflon thread sealing tape on grounded shell NPT sensors. It will likely interfere with getting a good electrical connection through the threads.
ECM coolant, MAT, and ACT, use the same value NTC thermistor internally. The test table below applies to all of them. (An NTC Thermistor is a temperature sensitive resistor that drops resistance as temperature rises.)
MAT and ACT are essentially the same sensor with different names based on location of the sensor. ACT sensors often tend to use epoxy-coated thermistors while MAT is not coated. Sensors without the coating are a little faster responding than coated ones. This is rather important for MAT where it can see 20+F temperature changes in a couple seconds. An ACT sensor, because of its location in the air cleaner of the V6 can be a little slower and still function correctly. Uncoated sensors will always have a plastic guard cage.
On HEI systems, the PIM/ICM is normally screwed to the distributor and uses mounting screws as part of its ground path. The PIM on DIS motors is sandwiched in between the coils and the aluminum mounting plate. The crank position sensor is under the DIS mounting plate.
Always check engine grounds whenever there is a problem with the O2 sensor. This sensor generates only 1VDC max. Grounding problems can cause the ECM to read the wrong voltage.
The TPS is a mechanical sensor and is subject to wear contamination and breakage. The TPS is the only sensor in Fiero with moving parts.
This sensor measures pressure with a solid-state transducer. The top or bottom of this sensor may have vent holes. Do not allow solvents to enter the vent holes as you could ruin the transducer.
Lobes, vanes, flaps, whatever favorite name for them is...
No. You can't convert a distributor motor to DIS or the other way around. The crank isn't the only issue.
Early model 4 cylinder applications may use a 2 speed fan. According to GM schematics this fan has 2 senders. One sender is a single switch that activates at 235F. The other sender has 2 switches that activate at 221F and 246F. The low speed will activate when the 221 or the 235 contacts close or when the air conditioning is on. High speed will activate only when the 241 contacts close.
All repairs to the wiring of this sensor MUST be soldered and water proofed. Corrosion of improperly repaired joints will result in bogus ECM errors.
The original round connector ECT sensor used by the ECM in early Fiero and other GM cars is no longer available. The new sensor requires a new connector. This connector is included with Borg Warner packages but will need to be purchased separately with many others.

Temperature Sensors Vs. Errors in Reporting

A problem with any Temperature sensor is more sensitive to error as the temperature increases.

Poor wiring, Carbon build up or cooking can alter the sensor resistance by a few ohms. At low temperature, a few ohms of error are a small percentage of the sensor resistance, however as temperature rises the percentage of error increases as the sensor resistance drops.

Air/Coolant Temperature	Normal Sensor Resistance	Error	% Error
70F (20C)	3400 Ohms	20 Ohms	0.59
160F (70C)	450 Ohms	20 Ohms	4.44
210F (100C)	185 Ohms	20 Ohms	10.81

The sensor is supposed to be accurate within plus/minus 1 degree C. Any error will affect fuel economy and performance, even with no codes. If we figure the high temperature as 160F(70C) then a 4.44% error is plus/minus 7.104F(3.11C) of error. This can drive the engine rich or lean depending on if the ECM thinks the air is hotter or cooler than it really is.

Note that a bad sensor or wire/plug for that circuit can have a much higher error that 20 Ohms and still not set DTCs that says "Lookout me, I am a bad sensor!" Examples, code 14 coolant sensor low-voltage will trip at 270F for 3 seconds and code 25 MAT needs a totally dead sensor.

Another Example: The MAT sensor on 87 and up Duke Engine. When this sensor has an error and intake air temp is up around 130-150F, common during idle, the sensor is lying to the ECM. The ECM adjusts the fuel mix based on this and if the lie is big enough you'll get DTC 45 errors before you trip DTC 25...

This is the reason why soldering any connection is required. Most Crimper tools can't make good joints for sensitive sensors. Soldering and Weather Proof heat shrink is good idea for TPS and any other sensors that uses by the ECM.

Temp Sensors Values

ECT and MAT Sensors
Temperature to Resistance Values
(APPROXIMATE)
°F	°C	Ohms
210	100	185
160	70	450
100	38	1,800
70	20	3,400
40	4	7,500
20	-7	13,500
0	-18	25,000
-40	-40	100,700
Source: 86 Service Manual, Code 14 and 25 charts.

MAP

The MAP, Manifold Absolute Pressure, sensor is used for both fuel mixture and ignition timing. It is also used briefly during startup to measure the local barometric pressure, which helps the engine compensate for altitude and weather. If this sensor is batty you will have trouble and it often will not set an error code.

One typical symptom is when you punch the gas and the car backfires or stumbles.

This first thing you have to do is make sure the sensor vacuum line is good. It has to be clean inside, have all joints sealed tightly, and be connected to the correct vacuum port. Even a tiny leak will screw up this sensor.

Keep the vacuum line short so that the MAP sensor will respond instantly to manifold pressure changes. A long vacuum line will take longer to pass the change in pressure to the MAP. This lag can be enough to cause a problem. The exact effect depends on the inside diameter of the line and how long it is.

Don't count on new sensors being good. I've had the stumbling problem and when I did it was a new MAP sensor that caused it. I changed the sensor again, this time to Borg Warner product, and the problem went away.

Map Test

This is a basic test described in many manuals...

Measure voltage of the MAP sensor's output terminal (Center terminal of most GM ones) with ignition on and engine off. Start engine and apply 10 in.(250 mm) Hg to unit, voltage should be 1.2 to 2.3 volts less.

What is Absolute Pressure?

Absolute Pressure means that Zero PSI is referenced to an Absolute Vacuum, instead of Atmospheric Pressure like your Tire Gauge. Absolute Pressure is used because it simplifies the engine computer and sensors, especially with Turbo/Super charging setups.

Absolute Pressure is often marked as PSIA, short for "Pounds per Square Inch, Absolute."
Atmospheric Pressure is about 14.7 PSIA depending on altitude and weather.