Technically Speaking
- Subject: Examination of hydraulic components
- Unit: GM 2ML70 hybrid
- Essential Reading: Rebuilder, Diagnostician
- Author: Wayne Colonna, ATSG, Transmission Digest Technical Editor
Part 3
Having looked at the electrical and mechanical aspects of this rear-wheel-drive two-mode hybrid transmission in the past two issues, a brief look at the hydraulic components will finalize our comprehensive overview of the 2ML70.
There are only two pressure taps available, one to check main line pressure when the gas engine is running while the other checks line pressure via the auxiliary pump when the gas engine is not running (Figure 1).
The valve body (Figure 2) contains six individual valve-body-to-case seals and one valve-body-to-pump manifold seal (Figure 3).
By comparing the case-passage identification given in Figure 4 with Figure 1, you can see how critical these seals are. Of the individual valve-body-to-case seals, there is one for each of the four internal clutch assemblies, and the remaining two are for cooler pressure to each of the hybrid-drive motor/generator assemblies.
You may recall last month’s article taking note of the expansion of the O-rings that go between the stator and case. It is in this area that these two valve-body-to-case seals provides cooler pressure to the generators. The pump manifold seals provide sealed porting for the auxiliary-pump inlet (filter), auxiliary-pump pressure to the valve body, auxiliary-pump pressure regulation and main-line pump regulation via the pressure-control solenoid.
Inside this valve body are one ball that measures 0.5 inch and two encapsulated balls (figures 5 and 6). The half-inch-diameter ball is called the fluid-pump ball checkvalve and shuttles between the use of the transmission’s pump pressure when the gas engine is running and the auxiliary-pump pressure when the engine is off. The other two encapsulated balls have a job to do the likes of which I have never seen.
What we are familiar with is an actuator-feed limit (AFL) valve that supplies regulated pressure to solenoids. In this transmission it not only supplies regulated pressure to all eight solenoids but also to the open side of each of the encapsulated-ball assemblies and through four orifices in the spacer plate to become what is called “backfill pressure” (Figure 7). This backfill pressure can be understood as clutch pre-fill pressure and is supplied to all four clutch assemblies. This allows quicker pressurization of a clutch assembly when full line fills the circuit to apply the clutch, as the circuit is already primed by backfill pressure.
The backfill-pressure circuits that are routed to the 1-3 clutch and the hybrid 2-3-4 clutch are tied into the circuits going to the orifice side of these encapsulated balls. The No. 1 encapsulated ball is called the “Actuator Feed Limit/HD 234 Clutch Feedback ball”; No. 2 is called the “Actuator Feed Limit/13 Clutch Feedback ball.”
The way the backfill-pressure circuit is plumbed to these balls, when their respective clutch is applied, this backpressure becomes clutch pressure. This makes for quite a balancing act. Should actuator-feed pressure working on a larger surface area of the ball become lower than clutch pressure working on the smaller surface area of the ball, the ball will move off its seat and orificed line will keep the AFL circuit charged. It is almost like a backup for the AFL valve should bore wear develop and cause AFL pressure to drop. Between both encapsulated-ball assemblies all four speeds are covered, so any time AFL pressure decides to drop, one of these balls will move off its seat and keep the circuit charged.
Between the valve body and pump, there are only 12 valves and two pressure-relief ball-and-spring lineups (figures 8, 9 and 10). Of course each valve plays an important role in the proper operation of the transmission, but I’ll point out a half dozen of them.
There are two shift-solenoid mode valves in this valve body, and it is good to know that each one routes pressure to two specific clutch assemblies. Shift-solenoid-valve mode A routes fluid pressure to the 1-3 and 4th clutch assemblies; shift-solenoid-valve mode B routes pressure to the hybrid low, 1-2 and hybrid direct 2-3-4 clutch assemblies. Knowing this could help diagnose a stuck-valve scenario.
To finish our comprehensive view of this transmission I extracted information from GM’s 2ML70 Technician Guide for six specific valves and what the associated failures would be should the valve get stuck in either a stroked or non-stroked position.
1-3 clutch regulator valve: Being stuck in the applied position could cause no reverse, no second and no fourth. If stuck in the release position it could cause no first or third gear.
Hybrid low 1-2 clutch regulator valve: Being stuck in the applied position could cause no reverse, no EVT mode high and no third gear. If stuck in the release position it could cause no reverse, no EVT mode low and no first, second or fourth gear.
Hybrid direct 2-3-4 clutch regulator valve: If stuck in the applied position it could cause no reverse, no EVT mode low and no first gear. If stuck in the release position it could cause no EVT mode high and no second, third or fourth gear.
Shift-solenoid-valve mode A: Being stuck in the applied position could cause no reverse. If it is stuck in the release position it could cause no first, second, third or fourth gear.
Shift-solenoid-valve mode B: Being stuck in the applied position could cause no reverse, no EVT mode low, and no first or second gear. If stuck in the release position it could cause no third or fourth gear.
Actuator-feed limit valve: If stuck in the applied position could cause low actuator-feed limit pressure, resulting in no-shift or slipping-shift conditions. If stuck in the release position it could cause high actuator-feed limit pressure, resulting in harsh-shift conditions and damaged components.
