The load PID is displayed on our scan tool and can easily be overlooked. How important is the load PID to engine drivability and transmission operation? The technician’s most powerful tool in their arsenal is their scan tool. The scan tool reveals sensor values and control module commands, and it also offers information from feedback sensors. If we just skim over the PIDS, we may miss a clue that is staring us in the face. Each PID should be analyzed to extract the most information, and each PID should be compared to other PIDs to determine if the PID is accurate. One PID that can offer a ton of information is the load PID. There are two load PIDs in scan tool data. One load PID is found on the OE side of the tool and is listed as calculated load. The other load PID is found on the global side of the scan tool and is labeled absolute load.
What is the difference between the two load PIDs? SAE defines calculated load as the current airflow divided by the peak airflow at wide-open throttle under standard temperature and pressure conditions. Lower values than 100% indicate that the engine is operating at part throttle or in reduced torque mode, provided the load PID is accurate. The calculated load is corrected for altitude and does not represent volumetric efficiency, but it can reflect airflow problems. Here is another way to look at calculated load: it is the percentage of available torque being used at a given moment. If the calculated load is below 100% at wide open throttle, it means the engine is in a derate strategy or total airflow is lower than expected. You should see the calculated load at 100% every time you perform a wide-open throttle action. Engine torque has a direct effect on line pressure inside the transmission. Shift strategies are also affected by engine torque. If engine torque is not calculated correctly, it could result in transmission operation complaints. The transmission could shift too soft or too hard and the shifting strategy could be off. We will display the calculated load on a scan tool that reflects 100% at wide open throttle (Figure 1).
Absolute load is found on the global side of the scan tool. Absolute load is defined as a normalized value of air mass per intake stroke displayed as a percentage. Another way to look at absolute load is to say that the highest value at wide-open throttle correlates with volumetric efficiency. Volumetric efficiency (VE) has a direct correlation with engine torque, which has a huge effect on transmission operation. Low VE could be caused by a restricted air filter, a plugged catalytic converter, an out-of-time camshaft, or a throttle plate that is not opening all the way. One thing a tech must do is determine if the absolute load PID percentage on the scan tool is correct for their engine design and their altitude. Altitude, air temperature, humidity, and engine RPM all influence volumetric efficiency. Let’s look at an absolute PID and analyze the information from a scan tool capture.
This data was taken from a Ford V10 at wide-open throttle on the global side of the scan tool (Figure 2). The absolute load reports 72% at first glance, which may seem a little low. As we stated earlier, altitude has a large effect on VE which is absolute load. This scan data capture was taken at an altitude of 6,000 feet. Altitude reduces VE by approximately 3% per 1000 feet. Let’s do some math: 6 X 3=18%. If the engine were designed to be 100% efficient, then we would only be 82% efficient at this altitude. Our absolute load PID reported 72% so do we have low VE? We have not considered air temperature, humidity, or engine design into this equation. 
The industry offers a calculator to help with the factors we just discussed. The calculator requires the technician to enter engine RPM, air flow in grams per second, air temperature, elevation, and humidity. Let’s enter the information into the calculator and see the results. (Figure 3, a DECS calculator can be found on AESwave.com)
Once all the pertinent information was entered into the calculator, the result was 96% VE. Remember, the absolute load reported a 72% VE, but when we considered all the factors, the actual VE was 96%, which is acceptable. Under what circumstances could the absolute load PID displayed on the global side of the scan tool be incorrect and misleading? The absolute load PID is based on airflow. Air ow is measured by the mass air ow sensor (MAF). 
If the MAF sensor is under-calculating, the airflow absolute load PID will be too low. If the MAF sensor is overestimating the absolute airflow load, the PID will be too high. Here is an example of the MAF underestimating the airflow (Figure 4). We simply reduced the MAF grams per second that was entered into the calculator. Figure 4 indicates VE is only 69% which would be too low at our altitude. In Figure 5, we overestimated the airflow by entering a higher MAF grams per second into the calculator. Figure 5 shows VE at 118%, which would be impossible without a turbocharger, which this engine does not have.
The VE calculator can be helpful in determining the accuracy of the MAF sensor. Is there a way I can test the MAF sensor over the entire load range? Yes, there is a generic scan tool offered by Automotive Test Solutions (https://automotivetestsolutions.com/), called Escan, that will graph VE over the entire load range. The scan tool has two ways of graphing VE over the load range. 
One method used is that the engine you’re testing has been tested by ATS and modeled inside your scan tool for better accuracy. The other method used is a generic VE formula based on the displacement of the engine. Both methods utilize RPM, altitude, and air temperature.
Figure 6 shows a VE graph from the scan tool on a vehicle with low VE, using the generic VE-based formula. The red trace in the graph indicates theoretical air flow. Theoretical airflow is what the engine should be ingesting. The yellow trace represents the actual airflow measured by the MAF sensor. 
In the graph, we can see that the actual air flow is much lower than the theoretical air flow. Does the VE graph indicate that we have an airflow problem? Should we check the air filter? Should we perform a back-pressure test on the exhaust? Not so fast. Let’s first verify the accuracy of the MAF sensor. What is a quick method of verifying the accuracy of the MAF? Let’s use logic for a minute. If the MAF was accurate, the fuel trims would be in their normal range. If the MAF underestimated the air flow the fuel trims would be adding a lot of fuel.
Let’s turn to Figure 7 and see how the fuel trims are reacting. The scan tool has a unique feature that allows the user to combine short-term and long-term fuel trims together and put them in a block diagram over the entire load range. When the blocks turn red, it means the fuel trims are out of range. Our graph indicates that we are adding a lot of fuel, which indicates the MAF sensor under-calculated the air. That means the load PID would be indicating a low load; however, the actual load is not low because we do not have low airflow, but rather a miscalculating sensor.
How will this affect line pressure and shift strategies in the transmission? Line pressure is based on engine load or engine torque. The line pressure command will be too low. That could cause clutch slippage in the applied clutch packs. Low line pressure can also cause long shift times. A low load calculation will cause the shift pattern to change, resulting in shifts that occur earlier than they should. Low line pressure can damage the transmission. If the MAF sensor overestimates the air, the absolute load calculation will be higher than it should be, so the control module will command an increase in line pressure. The increased line pressure will result in harsh, abrupt shifts, and it may delay the shifts to a higher speed. It is essential when dealing with transmission-related complaints that we verify both the calculated load and absolute PIDs are within range and that the MAF sensor is reading accurately.
About the Author: Jerry Stewart develops technical training at AVI and teaches automotive technology at Highlands College in Montana. He bridges the gap between industry and academia, equipping students with real-world skills for careers in the transportation field.
