As we gain experience over the years of building transmissions, we learn the idiosyncrasies of various transmissions—the common failures with specific clutches or bands and the normal causes. We learn about certain valves prone to sticking, valve bore wear, leaks in a circuit from the valve body to the piston, and problems in the release side of servos, to name a few.
Low pressure or poor throttle response is a common issue amongst all transmissions that can lead to repeat failures. While some transmissions rely on a cable to control pressure response, most use engine inputs and control outputs to an electronic pressure control solenoid to get the job done. Over the next few articles, I would like to discuss pressure control from the engine sensors to the pressure tap in various transmissions and how to diagnose inside and outside the box.
Whenever a new job comes in, it’s good practice to check line pressure command and engine load as part of the normal procedures before removing a transmission. Line pressure command should start around 1 amp at idle and drop to 0 – .2 amps at wide open throttle (right before the shift to second). Engine load data should be around 10%-15% at idle and up to 85%-100% at WOT under maximum load (Figure 1). Engine load data is not always available in scan tool data. The alternative is to check the engine’s volumetric efficiency. Load data and volumetric efficiency will be very close on most vehicles, and when available, it saves a little bit of time versus checking VE.
Volumetric efficiency is the engine’s ability to breathe from intake to the exhaust. To check this, we connect a scan tool to the vehicle, put it in graph mode, and collect some data. Mass airflow (in grams per second), intake air temperature, manifold absolute pressure, long-term fuel trims, and O2 sensors. After the vehicle reaches operating temperature, we’ll take it for a test drive. Accelerating from a stop, we need to reach wide open throttle before the one-two shift. We want to see the engine’s capacity for airflow at maximum RPM and load. It’s recommended to do this test a few times and use the highest RPM during the 1-2 shift.
Next, we want to put this data into a VE calculator. A couple of websites that provide these are AGT Volumetric Efficiency Calculator (Figure 2) and OCTOOLS. VE should be 75-95% for normally aspirated engines. Turbocharged engines can exceed 100%. If we see it below 75%, we have an engine problem and need to dig deeper. Various mechanical problems in the engine can cause poor efficiencies, such as restricted intake, restricted exhaust, valve timing, or problems with the mass air flow sensor.
We want to take a closer look at the fuel trims, O2 sensors, and MAP readings. The 02 sensors will range from .2 volts, full lean, to .8 volts, full rich, and fluctuate rapidly (Figure 3). Under normal operating conditions, short-term fuel trims will range between -7 to +7, and the long-term will range between -10 to +10. The Manifold Absolute Pressure sensor reads in voltage on most scan tools (Figure 4). Proper operation can be verified with a vacuum gauge attached to manifold vacuum. It is best to pay attention to the voltage on the scan tool. More than once, I have found the reading is off and showing more than 300 in HG, while the voltages are within normal parameters.
If the fuel trims stay within specs (below 10%) and the 02 sensors are reading rich (full rich is .8 volts) at WOT, we have a system restriction or a valve timing issue. We’ll need to look at the MAP and Baro readings at WOT during acceleration to determine if the restriction is in the intake or exhaust. If the vehicle is not equipped with a barometric pressure sensor, we can check our local weather station for barometric pressure in our area. If the MAP and Baro are nearly the same, this would indicate a restricted exhaust, which we can confirm with an exhaust back pressure gauge. If the MAP and Baro are moving apart during acceleration, this would indicate a restriction in the intake, such as a dirty air filter (or maybe a squirrel hiding its treasure in the breather box). If no restrictions are found, this could be a valve timing issue, and a compression test would be required at this point to check for internal engine mechanical issues.
If the VE is below 75%, with long-term fuel trim exceeding normal parameters (above 10%), and the O2 sensors are reading lean, this would indicate a bad reading from the mass air flow sensor. Leaks in the ductwork can cause this due to loose clamps or damage or a faulty mass air flow sensor. You’ll want to replace the MAF if you don’t find any leaks.
If the volumetric efficiency is above 75%, with engine drivability problems, it will most likely be an ignition or fuel-related issue. If the 02 sensors are rich at WOT and the fuel trims are within spec with drivability problems, we’ll need to check for secondary ignition problems such as wires, coils, and plugs. If the O2 goes lean at WOT and long-term fuel trim exceeds 10%, there is a fuel delivery problem, such as a restricted fuel filter, weak fuel pump, damaged fuel lines, or injector issues.
With a little practice and quick links to these websites on your cell phone, checking the engine for drivability issues will take less than ten minutes. And in the long run, it will keep those trannys rolling and avoid having them back on the bench. I will discuss pressure-related issues inside the box in the upcoming articles.
