FREE ASE L1 (Advance Engine Performance ) PREP: Scan Tool Actuators
BAR Diagnostic and Repair Class
Let’s discuss some of the Scan Tool-Actuators from the Smog License Course:
A good way to remember what an actuator is, think of it as the hands of the vehicles computer. The actuator actually carries out the command from the computer, in other words it acts out the command.
So anything that the computer controls to come on or do something is called an actuator. Example would be fuel injectors, fan relays on some cars, idle motor, and there are plenty more.
Actuators are used to control various engine functions and switch various electrical circuits.
EGR Duty Cycle
EGR Duty Cycle
The Power Train Control Module (PCM) modulates the EGR flow. This is read in a percentage, 0% the EGR is closed no signal to the valve at all.
While a 100% the EGR is commanded fully open.
The is the amount of percentage the valve is open, 0 % the valve is fully close and 100% means the valve is fully open. This could be very helpful in your diagnosing.
Here is an example; A 1990 Chevy Astro Van come in your shop with a check engine light on, the code is for a rich condition or MAP sensor in some cases maybe both.
As you are looking at your automotive scan tool you notice the EGR (%) percentage is 55% at idle and the EGR Command is off.
This is the problem: The vehicle is at idle and the EGR valve percentage says the valve is open while the command is off. EGR valves don’t work at idle, only cruise speeds.
Reason the (EGR) Exhaust Gas Recirculation valve is stuck open, properly there is a piece of carbon holding the pintle open. This will cause a vacuum leak, keep in mind the MAP sensor works off vacuum.
When there is a vacuum leak and it effects the MAP Sensor, the sensor will the give a bad signal to the PCM then the on board computer (PCM) will deliver more fuel than necessary.
A lot of people would replace the Map sensor, (I made that mistake) instead they should clean the carbon from the EGR Valve port.
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Short Term Fuel Trim (STFT)
Short Term Fuel Trim
Short Term Fuel Trim (STFT) is the computers response to the oxygen sensor.
Here is an example: if the oxygen sensor is showing a lean condition; the STFT will correct rich, which is a positive number. If the oxygen sensor is showing a rich condition the STFT will correct lean, which is a negative number.
The normal numbers for STFT and LTFT is “0” plus or minus 10%.
When you turn the key off the STFT is erased, also the STFT only works in close loop.
Long Term Fuel Trim (LTFT)
Long Term Fuel Trim
Long Term Fuel Trim is an indication of the PCM long term adjustments to the air/fuel mixture. Long Term Fuel Trim is a learn value and it doesn’t erase when the key is turned off. Long term fuel trim has the ability to watch how rich or lean the computer needs to be at, in order to be correct. The PCM, (On board Computer) watches the STFT, if it stays lean or rich to long it moves the whole computer parameters over to compensate for an imbalance air/fuel mixture. The goal is to get the short term fuel trim back to the middle (“0”) plus or minus 10% when this happens the o2 sensor will start to switch and this is what the computer what’s to see, an average of 450mv.
If you what to diagnose, a vehicle for emissions problem or drive ability problem pay close attention to the LTFT. This reading will reliably tell you if the system is compensating for a rich or lean condition. This way you will know what kind of failure to look for.
A fuel trim within + or – 10% is usually considered normal, numbers can go as high or low as 25% or 35%. Keep in mind some computers have different numbers, and on earlier cars they don’t use Long Term Fuel Trim, (LTFT) or Short Term Fuel Trim, (STFT).
They use other fuel adaptive strategies; an example would be Block Learn Multiplier or Integrator, when 128 would be the middle point or the correct adjust or goal (+ or – 10%).
Just remember the concept is the same; just apply it for that manufacture. Understanding how the fuel control system works is critical in using an Scan Tool-Actuators parameter display this will increase your trouble shooting skills.
When the computer is correcting and adjusting for the air/fuel mixture this is called adaptive strategy.
What is Adaptive Stragedy, the ability of the computer to compensate for minor vacuum leaks and drippy injectors, in other words correct for minor rich or lean conditions.
Understanding adaptive stragedy gives us an ideal of what is going on with the fuel system, at idle or a particular RPM and Load.
Remember when using an automotive scan tool and watching long term fuel trim it moves when short term fuel trim moves.
Torque Converter Clutch (TCC)
Torque Converter Clutch (TCC)
When the car speed is approximately 25 MPH recorded by the (VSS) or vehicle speed sensor, the on board computer will energize the TCC solenoid mounted in the transmission. This will allow the torque converter to directly connect the engine to the transmission.
On the Automotive Scan Tool, if the PID (parameter identification data) for the TCC says on, it means the PCM is requesting the TCC to be energize. The TCC de-energizes the TCC when the brakes are press.
A surging problem can be due to a TCC engaging and disengaging due to a defective TPS, defective solenoid or wiring problem with circuit or PCM.
Also a defective brake switch or Park Neutral Switch and a defective VSS Sensor can cause a surging problem cause by the TCC Solenoid.
If you have an automatic transmission, and when the vehicle gets hot it shuts off at a stop light, like you have a stick shift,(manual transmission) in the wrong gear.
Then the vehicle will not start until it cools down, it may be a defective TCC solenoid that is staying in engage
These are the codes in the PCM memory that may or may not be causing a concern. Prior to 1994 -1996 if the code was in memory with no light this was call a soft code. If the code was in memory with the light on this was called a hard code. A quick way to tell if the vehicle has a soft code or a hard code is to follow this procedure to tell the difference.
Extract the code and right it down, then clear the codes according to that manufacturer procedure. Now Drive the car for a few miles then return to the shop, are your house and pull codes again. If the light came back with a code, this is a hard code; if the code did not come back it was a soft code.
Just because the vehicle has a check engine light with a code it does not mean that part is bad. The most common problem with this is a code for an O2 sensor, most people will just replace the sensor and they will be wrong, and the light will return.
Idle Air Control
Idle Air Control Valve (IAC)
The IAC is mounted on the throttle body and it controls how much air bypasses the throttle plate, almost like a controlled vacuum leak. If the idle goes to low the PCM will contract the IAC to let more air bypass the plate to raise the idle or RPM. If the IAC is too high the IAC will extend the IAC to reduce the air around the throttle plate to lower the idle or RPM.
On the automotive scan tool you can see the IAC counts, ranging from 0 fully seated (closed) to 255 fully open (retracted). Some Automotive Scan tools may say 0 – 100%.
When the car is at idle the proper IAC count depends on a few factors; Battery voltage, engine load, and coolant temperature. If the IAC is disconnected or reconnected with the running the IAC may need to be reset.
There are different procedures for different manufacturers consult All Data or Mitchells manuals.
Idle counts might be 15 to 60, idle percentage approximately 20% – 50%, high numbers can be due to sludge in the throttle plate, dirty IAC pintle, or there is a load on the engine.
A very common problem is a vehicle with a vacuum leak on a throttle body fuel injection system, you usually can tell it’s a vacuum leak because the idle will be high and the IAC counts are 0.
This combination of high idle and low IAC counts on your scan tool will be an indication the vehicle has a vacuum leak on a TBI system, and it’s usually the throttle body gasket.
You may have a low count 0% and a high idle, “1500RPM’s” look for a vacuum leak or a bent throttle plate.