Would someone kindly explain to me all the sensors that feed info to my ECM specific to my 85. Please use the name of the sensor as opposed to an acronym. The more specific the better. I know this is a large task and will appreciate your effort. Also, my SES light comes on when I turn the key on but goes out after the engine starts. Is this normal.
Thanks a lot,
JW

 

I just recently re-pinned my ECM, so I will take a stab at filling you in on an 85's sensors. I'll try to include a brief 101 on what each sensor does. I am assuming you have an automatic, so I'll include the different switches/sensors for the autos.
Before I start, yes, the SES light will come on when the key is in 'run' position but the car isn't started.

As far as input sensors go, the 85 has:

Oxygen Sensor: (narrow band, single sensor, on the left exhaust downpipe). The O2 sensor, aka Ox sensor, feeds the ECM data about how rich the car is running once the car is warmed up. This 'warmed up' state is known as closed loop. You can read more about closed loop in other threads, but as far as a layman is concerned, closed loop is just how the computer reacts when the car is warmed up.

Manifold Air Temperature Sensor: (A temperature sensor, located underneath the plenum close to the distributor), aka MAT sensor has little use on the 85. It is used to turn the EGR system on or off, and it might do the same for the Air Injection Reaction system, but I am not certain on that.

Throttle Position Sensor: (A 5 volt sensor, essentially a potentiometer - it is connected to the throttle body, and opens when the blades on the throttle open - thus, it lets the computer know where the throttle is). Aka TPS, this sensor is a 'heavy-lifter' - the car is quite dependent on the TPS working right.

Coolant Temperature Sensor: (A temperature sensor, located close to the water pump, hidden underneath where the intake duct meets the throttle body). Aka CTS, This sensor is rather important, as it triggers several events regarding engine operation (fan kick-on, for example), and it also changes fueling some.

Park/Neutral Switch: (a multi-position switch, located under the center console, right next to the shifter). This switch is what keeps the car from cranking when it is in gear, and the ECM has a pin for signal from the switch. Not a major input for the ECM, but worth mentioning.

Knock Sensor: (essentially an electronic stethoscope, this sensor 'listens' for cases of engine knock - aka spark knock, or detonation. Knock can harm an engine, and occurs when combustion temps are too high, too little fuel is added, or spark advance is too high.) When the knock sensor picks up a noise coming from the engine, it is passed through a filter, and the ECM then determines if that filtered noise was knock or not. I think the knock sensor on my 85 works great, but some people have cases where the sensor picks up knock where there isn't any. When the ECM decides that a noise was knock, it will retard timing by a few degrees. If knock still occurs, it can pull more timing - I think the most it can pull is around 10 degrees on the stock tune. The knock sensor isn't a primary input to the ECM, but if it malfunctions, it can cause a loss in power, and rough running.

Mass Air-Flow Sensor: (in simple terms, it is a hot wire that air coming in to the engine cools off. More air coming by, the cooler the wire. Less air, hotter wire. The ECM can tell the difference in the wire's temperature, and as such, it knows how much air is entering the engine. When combined with input from the Throttle position sensor, the ECM can know how much load the engine is under. The mass airflow sensor is located behind the air filter, but before the throttle body- it is a black tube-shaped guy.) Aka the MAF, the mass air-flow sensor is a primary input to the ECM. Without it, the ECM has to guess how much air is coming into the engine, which makes the air-fuel ratio based on a guess - which is bad news. A bad MAF can manifest itself as a number of issues, normally lean/rich running, hard starting, no starting, or dying after cranking.

4th Gear Switch: (this switch tells the ECM when the car is in 4th gear (overdrive, automatic transmission only). Aka High gear switch, this isn't a primary input - it essentially just differentiates 4th gear from the others. This allows the torque converter clutch to come on at a different point while in 4th vs in 3rd, as well as a few other things, but they aren't important to a layman.

Vehicle Speed Sensor: (this sensor feeds data to the ECM about how fast the car is going. The sensor is located on the tailshaft of the transmission, on the right hand side.) Aka VSS, this sensor lets the ECM know when to engage the torque converter clutch. This input is also relayed to the digi-dash.

Tachometer Input: (not technically a sensor, but the distributor does feed RPM data to the ECM. I believe this data comes from the Ignition Control Module, but I could be wrong there.)

That covers it for ECM inputs. Since I'm on a roll, I'll go ahead and do the outputs too - since they are just as important to engine operation.

Assorted Outputs: (I am going to lump in all the non-essential outputs here. This includes the data sent to the digi-dash, data sent to the ALDL connector, the Service engine light, etc.)

Electronic Spark Timing/Electronic Spark Control: These two systems are related, so I'll lump them together. Electronic Spark Timing, aka EST, is the system the ECM commands that advances ignition based on a programmed map. On the most basic level, higher RPM, more advance, higher load, less advance. Electronic Spark Control, aka ESC, is the system the ECM uses to subtract timing - in the event the knock sensor picks up what the ECM determines is knock, ESC is used to remove advance to prevent the knock condition.

Idle Air Control Valve: (this is a stepper motor that the ECM commands to allow air into the engine without having the throttle opened. Essentially, a motor that opens a passageway to let the car get enough air to idle. It is located under the throttle body, with its connector coming in from the right side of the car.) Aka IAC, this motor is important to how the car runs. While a choppy idle isn't always a bad IAC's fault (bad data from an input sensor is usually the case), the IAC itself is what often times will physically cause the bad idle.

Fuel Pump Drive: (the ECM commands the fuel pump to stay on once the car is not cranking. Not much to say here, just that the ECM will continue to power the pump even if oil pressure is lost - a common debate about these cars.)

Fuel Injector Drive: (the ECM commands how long the fuel injectors fire, which directly modulates how much fuel in mixed with air. For what it is worth, these cars are batch fire - the left side fuel injectors all fire together, and the right side does the same.)

Burn-Off Control: (when you turn the car off, the wire in the Mass Air-flow sensor is heated to burn any debris off of it. The ECM commands this action.)

Emissions Systems Control: (The ECM controls both Exhaust Gas Recirculation aka EGR, and Charcoal Canister Purge aka CCP systems. The ECM triggers the solenoids for each system when certain engine conditions are met.) In addition to these systems, the Air Injection Reaction system (aka AIR system) is controlled by the ECM with regards to where the Air pump's flow is directed. The re-direction occurs in the black boxes next to the AC compressor.

Torque Converter Clutch: Aka TCC, or Torque Converter Lockup. The ECM commands this system when certain engine conditions are met.

Fan/AC Control: Not much to say here. The ECM commands these systems on when certain engine conditions are met.

Phew, I think that should give you a good start. Let me know if anything is unclear, or if I missed something, or am wrong somewhere.

 

Wow Blaster. You went far and beyond my expectations. Thank you so much. My days of working on engines were before computers and fuel injection. As you may recall, you responded to a thread I started last week after I took my car through a car wash and shortly after started back firing and would hardly run. When I finally got it home I noticed the temp was 235 and the main fan was not on. Normally comes on at 208. Thus, my thoughts are moister was introduced into the engine compartment and somehow shorted out a sensor. However, after two days the car is running fine and I have driven it twice at about 10 miles each and no backfire. As I need to continue to use this car wash, which I have used many times with no problem, I want to try to determine what caused this malfunction so it doesn't happen again. No codes showed up on the dash screen.
Once again many thanks,
JW

 

Thanks! I know what you mean about computers in cars - I may be young, but I learned cars with carbs. EFI was a mystery to me when I bought the Vette.

It is possible that the coolant temp sensor is acting up. On both my 85 and girlfriend's 86, the CTS connector cracked and fell off on hers.
I didn't mention this in my first post, but the temp you see on the dash Is not the CTS sensor input. The ECM does not see the temp you see on the dash, it sees the one at the water pump. The dash temp comes from the passenger side head. The moisture makes me suspect mass airflow sensor, or the EST/ESC system.

A bad CTS can cause misfires/backfires, as it is a primary input for acceleration enrichment - if you backfire was right when you opened the throttle, I would suspect CTS some. If you had a way to datalog, it would be easy to see what temperature the CTS is reporting to the ECM.

 

 

Yes thank you. That was really well done. If the Mods are paying attention.... this should be a sticky

 

That means alot to me! You're very welcome.