In our last discussion, I went over engine load data and how the computer uses this information to determine line pressure command (GEARS, July 2023). As we take our trip down the rabbit hole, from the intake to the pressure tap, we will discuss GM operations, and how to check pressure related issues.
With the exception of the early Allisons, General Motors vehicles use engine data to determine desired line pressure. The computer will raise and lower pressure using an electronic pressure control solenoid to control shift quality and applied force to clutches and bands to prevent slip under heavy acceleration. Early Allisons ran full time high line pressure and controlled shift quality with the use of trim solenoids and valves to change the flow rate from line pressure to clutch pistons apply and release.
General motors will usually display engine load data in percentage and pressure control solenoid command in amperage on the scan tool. At idle we should see around 10% engine load and 0.9 – 1.1 amps for the pressure control solenoid. During a wide open throttle test just before the 1-2 shift, engine load should come up to 90-100% and solenoid command should drop to 0.2 – 0 amps. As engine load comes up, solenoid amperage drops and line pressure should rise. An amp clamp can be placed on the pressure control solenoid wire to verify we have the proper signal from the computer to the transmission. And a pressure gauge on the tap to verify pressure is responding as it should.
On most GM transmissions, the pressure control solenoid is fed by an actuator feed limit valve and uses that oil to act upon the spring side of the pressure regulator valve to raise line pressure as required. The actuator feed limit valve is a pressure regulator for all the solenoids and maintains a consistent pressure while the transmission line pressure varies. Problems with the AFL valve will affect solenoid operation by either starving or flooding the solenoids and usually result in various shift problems as well as line pressure issues.
The pressure regulator valve and spring maintain a base line pressure in early and late applications. Early GM transmissions like the four speeds used a boost valve and sleeve to raise the pressure. The boost valve is connected to the back of the pressure regulator valve with an isolator spring (figure 1), which is usually a stiff thick coiled spring. Wear in the sleeve can leak of boost pressure, or even cause the valve to stick and create line rise issues.
Some of the later six speed (figure 2) and up transmissions still use the actuator feed limit valve to feed the pressure control solenoid, however the fluid from the solenoid is directed to the spring area behind the valve. An end plug was used to seal the fluid from the solenoid in this area so the fluid would act directly on the back of the solenoid to increase pressure as required. Wear on the end plugs can affect line rise, but not base pressure.
The early Allison transmissions (figure 3) used a pressure regulator and spring to keep a high line pressure with no real means of raising or lowering line pressure and was dependent on the trim solenoids and valves to control shift quality. From 2004 to 2009 Allison incorporated a solenoid G to act on the top side of the pressure regulator valve’s bottom spool against spring pressure to reduce pressure as required. Around 2010 the solenoid G was deleted, and a main modulating solenoid was added. This solenoid directed fluid to spring area to act directly on the back side of the valve to raise pressure as required.
On a recent job, a 4L60E came to my bench with no diagnostics before removal and typical 3-4 clutch completely burnt up metal to metal. I did the normal procedures, rebuild with the standard overhaul kit, new pistons, checkball capsule in case, 3-2 plug in valve body, new EPC solenoid, bushings and so on. Vacuum tests all passed except the pressure regulator valve, about 12 Hg vacuum, it was replaced by an oversized valve. The truck didn’t make it past the road test, it slipped in 3rd and 4th so bad it burnt the clutches before they got back to the shop.
After repair, our first move was to attach a gauge to it and see what our pressure at idle and wide-open throttle was. At idle it hovered around 60 psi, as we did a stall test on the lift, the pressure did not rise smoothly, and during full stall we found the pressure was erratic. Maxing out at 150, dropping to 90, coming back up to 130, and various other pressures. It would stop at these pressures for a split second and randomly jump. Our next move, amp clamp on the ground wire to the EPC solenoid.
We found that the amperage on the meter was erratic imitating the pressure on the gauge. Looking at the scan tool information, the amperage appeared to be steady. This was a definite indication of a computer related issue, and not just a loose nut behind the bench. We checked all other systems for any abnormalities, codes in any other systems, and checked powers and grounds to the computer. Everything passed, so we replaced the computer and fixed the problem.
Understanding the importance of line pressure rise and how it is controlled can make diagnostics of most transmission problems much easier, and keep them from landing back on the bench.
