I'm sure some of these have been answered in various threads, prolly when something has gone wrong, but this is more of a learning post for me.

I'm trying to find out what potential problems various sensors going bad could cause. I'm replacing my EGR valve and noticed a sensor under the plenum, which got me thinking. I'll list some of the sensors I know of. Could someone please give me a short rundown of what to expect should one of these go flaky or die completely? I'm looking for real life answers/experiences, not what this factory service manual is telling me(yeah, the 90 dollar shop one, lol) I'm pondering yanking out the majority of them and replacing them with nice GM originals, new of course. I'm also hoping I'm listing ones my car has as opposed to the other models.

Also, if it's not asking too much... what these sensors monitor specifically so I know where to look if the sensor itself isn't the problem. Yeah, I'm having a few minor issues with the car and need to get a handle on what in the world all these do. It's all well and good that I can yank out anything and replace it, but I'd like to know just WHAT I'm replacing and why, lol.

Manifold Temperature Sensor

Oxygen Sensor

Throttle Position Sensor

MAP Sensor

MAT Sensor

Knock Sensor

Vehicle Speed Sensor(that was already replaced due to it leaking some tranny fluid)

Coolant Temperature Sensor

They're listing these as sensors and I kinda have a rough idea of what they do, but...

AC Request Signal, Neutral Switch and Reference Signal

Anything I missed?

My car is a 1991 Trans Am, 305. vin F, TPI, automatic, AC, etc, pretty much all factory unless a part went bad.

 

Manifold temp and MAT are the same sensor.
The A/C has a pressure switch on one of the lines. I believe when pressure is high it breaks contact.
Automatics have switches built into the shifter in the center console.
Some cars came with an oil temp sensor.

 

Manifold Temperature Sensor

Oxygen Sensor This sensor obviously monitors oxygen content in the exhaust system. Too much Oxygen indicates to the ECM that a lean air/fuel mixture exists and too little Oxygen indicates to the ECM a rich air/fuel mixture exists. The ECM uses inputs from the Oxygen sensor to "fine tune" its fuel delivery to ensure maximum power while obtaining maximum fuel economy which has been determined to be about 14.7:1 Air:Fuel ratio.
If failed, it is usually ignored. If the ECM does not see the sensor respond correctly, it will ignore it and set a code 13. Fuel mileage may suffer as a result, since no "fine tuning" is being done. In some instances, it can fail and the ECM not catch it, resulting in mixtures that are way out of whack.

Throttle Position Sensor This sensor sends and analog signal to the ECM to indicate throttle position. 0.50~ Volts DC usually indicates idle, wheras 4.5~ Volts DC usually indicates Wide Open Throttle (WOT). Should the sensor fail, it could create a hesitation on acceleration, since the ECM would not see the increase in throttle position. It may also sometimes cause idle issues if minimum voltage is not near the 0.5 Volts +/-

MAP Sensor The MAP sensor is an acronym for Manifold Absolute Pressure. Its a fancy name for a vacuum sensor, but it goes off the principle that Atmosphere (what we usually consider 0 PSI) is actually 14.7 PSI (or 101 kPa) at sea level, which is where the term "absolute" comes from. With that being said, even 20" of vacuum is still pressure. 29.92"/Hg is a pure vacuum, and therefore has a kPa reading of 0. 20" converts to 35 kPa, so we can see that 20"/Hg is really 66kPa less pressure then atmospheric pressure. Since we know vacuum is an excellent indicator of load, we can use the MAP sensor to help the ECM decide how much fuel to spray through the injectors. We call this speed density.
Most of the bad MAP sensors I have seen cause an overly rich air/fuel mixture. If the sensor falsely indicates excessive load, the ECM will add extra fuel, because its only going off of what the sensor reported. But it can also cause a lean condition as well. If the vacuum line collapses, trapping high vacuum (low pressure) in the sensor, it will not correctly indicate the increased load to the ECM.

MAT Sensor The MAT (manifold air temp) sensor is the name given to temp sensors mounted in the intake manifold. If the sensor is mounted in the air cleaner housing or intake tubing (usually before the throttle body), its called an IAT (intake air temp). The ECM uses this sensor to compensate for air density. Since cooler air is more dense and therefore has more air molecules, we need to add more fuel to keep our ideal 14.7:1 fuel mixture. Because of this, a failed sensor may cause the car to run lean or rich.

Knock Sensor The knock sensor is a piezoelectric device. This device is used because of its ability to modify a reference signal based on pressure/vibration. When abnormal combustion takes place (pre-ignition/detonation) aka spark knock, this sensor picks up on that and sends a signal to the ECM to reduce the amount of spark timing. If the sensor fails and the ECM cannot detect it, you may hear some audible spark knock. If the ECM senses the sensor has failed, it will automatically reduce spark timing as a default, therefore a noticable loss of performance will be noted. MAF, carbed and TBI cars used an ESC module to interpert the knock sensor signals and relay that info to the ECM, 1990+ Port Fuel cars (V8 TPI and V6 MPFI) had that ESC module located inside the ECM.


Coolant Temperature Sensor Used by the ECM for Cold Start Enrichment (since we no longer have a choke), used to help determine needed fuel demands and to trigger the cooling fans.
An open in the sensor (very high resistance) could cause the car to run rich, since the higher the resistance, the lower the temperature indicates. Cooler temps require more fuel. On the flip side, a short (very low resistance) would indicate very high temperature and the ECM will shut the engine off at about 260° +/-.

They're listing these as sensors and I kinda have a rough idea of what they do, but...

AC Request Signal, It lets the ECM know the A/C clutch may be engaging and therefore will raise the idle speed to compensate for the additional load.

Neutral Switch and Lets the ECM know wether or not your in a drive gear (foward or reverse) or in Park/Nuetral, and it will make changes as needed (idle speed, spark advance, etc...)
It will also prevent you from starting the car unless your in Park or Nuetral as a safety measure.

Reference Signal. This is an output of the ignition module. The pickup coil creates an AC voltage as the distributer shaft rotates. The ECM can't use an AC voltage, it must be converted to a digital signal so it can be processed. So AC voltage from the pickup coil goes into the ignition module, the ignition module converts that into a digital signal, and sends it out to the PCM as RPM and Fuel Control, which we call the Reference Signal.

 

Thank you VERY much, both of you, for the sensors I missed and for the very informative rundown of what they do! I'm going to print this out and use it as a starting guide for any future problems.

 

No kidding, this should be posted as a "useful/important" sticky up top of the page.

Only thing I'd add regarding the knocksensor is it's signal is sent to the ESC (on SD cars, it's in the ECM, MAF is an external module on the firewall) and then used to pull timing out if needed.

 

Your right; MAF, carbed and TBI cars used an ESC module to interpert the knock sensor signals and relay that info to the ECM, 1990+ Port Fuel cars (V8 TPI and V6 MPFI) had that ESC module located inside the ECM.



Edit: I updated my post to show those changes. If there are any other sensors you want added, or any existing sensors you want further explanation on, let me know and I'll add more.