In the previous two articles, we discussed how engine inputs directly affect transmission operation and how GM manages pressure in different applications. In this article, we’ll look at how Chrysler manages line pressure and shift quality and how they relate to engine inputs.
The early three speeds and the RE/ RH overdrive series use mechanical controls much like the GM 700R4 to control line rise, shift quality, and shift timing. All the principles are the same: an increase in throttle pressure increases line pressure. 
However, unlike the 700R4, which uses a boost valve on the spring side, Chrysler uses a pressure-reducing valve (Figure 1) to work against the balance pressure at the other end of the valve. The pressure regulator valve and spring operate at high line pressure and use mainline pressure to balance pressure at the other end of the pressure regulator valve train (against the spring) to maintain a base pressure. Throttle pressure works against that balance pressure to increase line pressure.
Engine inputs like load percentage do not affect line pressure operation of the RH and RE series. Even the later RE series using the motor to control throttle angle is only affected by throttle position sensors or accelerator pedal position sensors. Though engine drivability problems do not directly affect transmission operation in the three speeds and the RE/RH units. Poor engine drivability can create problems with the engine, bringing the vehicle up to speeds to match throttle angle, which will feel like late firm shifts.
The RFE series is similar to the RE/RH in its setup, using the pressure regulator and spring set to high line pressure. However, in the RFE series, a solenoid controls pressure on the end of the valve away from the spring to reduce pressure (Figure 2). This pressure control solenoid is a duty cycle solenoid and operates at 5 percent for maximum line pressure and 62 percent for minimum line pressure. The RFE uses a sensor to monitor pressure, and it is displayed on a scan tool as the actual line pressure. The computer uses this information to regulate the pressure, and this is displayed as desired pressure on the scan tool. The desired line pressure, also known as “torque-based line pressure”, is affected by engine inputs such as load percentage, which is very important to check before any rebuild. Low engine load percentage will result in a lower desired line pressure. While checking actual and desired line pressure, they will appear to match but will be lower than what we need to hold the clutches under heavy acceleration. Which commonly leads to failed overdrive clutches with no noticeable cause of failure inside the transmission.
Also, with this pressure sensor and solenoid setup, the computer can compensate for leaks in the circuit, such as pressure regulator bore wear. As leaks occur, the duty cycle will reduce to maintain the desired line pressure. Monitoring actual and desired line pressure, along with the duty cycle (Figure 3), can help lead to finding and repairing pressure control-related issues inside the transmission. The closer the duty cycle reaches 5% to maintain lower pressures, the greater the leaks inside the transmission. This information is well worth noting on every initial inspection.
The early 41TE and 42RLE series ran higher line pressure, around 120- 140 psi all the time, and boosted pressure to 175-235 psi for reverse. This boost pressure comes straight from the manual valve (Figure 4). They used pulse-width modulated shift solenoids to control shift timing, engagement, and shift quality. Engine load percent only affected how the computer commanded the shift solenoids to operate during the shifts. It had no effect on line pressure. Engine drivability issues normally only affected shift timing and quality.
The later 41TE, 42RLE, and 62TE incorporated a pressure control solenoid and sensor that operated like the RFE units. Engine inputs can affect desired line pressure, low engine load percentage and cause low desired line pressure, which can result in torque converter slippage codes and premature transmission failure.
While the end result is how the transmission operates for the vehicle owner, the shift quality, and timing, not all transmissions manage this same way. Shift quality is sometimes maintained by controlling the solenoids for each friction element in the transmission. Most control the pressure to do the same by mechanical or electronic control. It’s important to know how each system operates so we have a starting point and direction for discovering what has led to a problem in the transmission that has brought the vehicle owner to our door. So we can address that problem and keep these transmissions from appearing back on the bench.
