Tell students that by connecting a pressure gauge to specific access points located on the transmission case, the fluid pressure can be observed to see if it’s at the proper level based on range selection and rpm. Inform them that recording line pressures enables you to spot potential problems in the oil pump, throttle valve, governor and clutch circuits.
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Let students know that to pressure test a transmission, pressure gauges that register up to 300 psi will be needed, along with a tachometer and vacuum gauge, since the tests will be performed at specific engine speeds and vacuum levels. Tell them that transmission testers that incorporate all of these instruments are commonly used.
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Instruct students to operate the vehicle to bring the transmission to normal operating temperature. Tell them to make sure the transmission fluid is at the proper level. Have them raise and safely support the vehicle, then connect the gauge(s) to the pressure port(s). If the transmission uses a vacuum modulator, Tell them to tee the vacuum gauge into the vacuum supply line between the engine and modulator, and have them connect the tachometer according to the gauge manufacturer’s instructions if necessary.
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Tell them to apply the brakes and start the engine. Explain that the engine should idle at the manufacturer’s specified idle speed and the vacuum reading should be approx. 15-20 in. Hg. With the engine idling, have them move the gear selector through each gear range and record the pressure readings. Let them know that some transmissions will not read line pressure when in the Park position. Inform them that if the transmission is equipped with a vacuum modulator, the vacuum reading should be at least 15 in. Hg. Making sure the brakes are firmly applied, tell them to increase the engine speed to 1000 rpm and again move the gear selector through each gear range and record the pressure readings.
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Instruct students that if the transmission is equipped with mechanical TV linkage, they should disconnect the linkage from the throttle and place it in the WOT position. Have them move the gear selector through each range at idle speed and again at 1000 rpm and record the pressure readings. If the transmission is equipped with a vacuum modulator, have them disconnect and plug the line at the modulator. They should then move the gear selector through each range at idle speed and again at 1000 rpm and record the pressure readings.
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Instruct students to compare the recorded pressure readings with manufacturer’s specifications. Let them know that low pressure in Park or Neutral could be caused by the oil pump, pressure regulator valve, valve body or case leaks. Since the clutch packs restrict fluid flow, which raises hydraulic pressure, let them know that low pressure in the forward ranges would indicate a leak in the forward clutch.
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Tell them that high pressure could be caused by a sticking pressure regulator or throttle valve. If line pressure builds up and then drops off, they will know a leak exists. To pinpoint the problem circuit, tell them that they can check the oil pressure difference between line pressure and second, third and fourth (if applicable) clutch oil pressure. Inform them that if the difference between line pressure and any of the clutch circuits is greater than 10 psi, then there is a sealing problem in that circuit.
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Explain that when evaluating pressure test results, they will need to know how the fluid flows through the transmission hydraulic circuits. Let them know that vehicle manufacturers supply diagrams that show fluid flow in each gear range.
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Instruct students that Electronically controlled transmissions have many similarities regardless of manufacturer. Let them know that Transmissions equipped with electronic main line pressure control will generate maximum pressure when electricity is removed from the controls. This feature is meant to be a fail-safe design to protect the transmission.
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Explain that most control devices of electronic transmissions are solenoids that control hydraulic pressure or volume to the transmission components. Let them know that by varying the hydraulic main line pressure, the control module can control shift and converter clutch timing and feel. Tell them a solenoid operating at zero percent duty cycle is not activated at all. In this case, the transmission pressure would be at its highest point. If the solenoid is commanded at 100 percent duty cycle, it is fully open.
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Instruct students that a pressure control solenoid operating at 100 percent duty cycle would cause low transmission pressure. The solenoid would be allowing a large volume of fluid to bleed out of the main line pressure circuit. Explain that the more volume that is allowed to bleed out, the lower main line pressure will be. Let them know that solenoids can work in the opposite way too (closing, as opposed to opening, when applied) so tell them to keep in mind zero percent is always when the solenoid is de-energized and 100 percent is filly energized. Explain that most electrical circuits are designed to operate the solenoid at 60 percent duty cycle or less most of the time.
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Tell students that a common means of testing the control system and the ability of the transmission to generate pressure is to take a pressure reading when the throttle is at minimum opening (low throttle position sensor voltage) during idle. Let them know that this should be the lowest pressure reading obtained. Tell them that disconnecting the pressure control solenoid or using a bidirectional scanner to command the solenoid off should generate the highest pressure.
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Instruct students to refer to the manufacturer’s service information for transmission-specific pressure tables to compare the actual readings to the specs. Tell them by comparing the actual readings to the manufacturer’s specifications; the problem can be isolated as an electronic control issue or a mechanical transmission problem. Inform them to keep in mind that the transmission pump generates volume, the pressure regulator builds pressure, and the electronic pressure control (EPC) solenoid modulates pressure. Let them know that the EPC solenoid will be largely responsible for pressure changes during operation.
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