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The heartbeat of the transmission

How solenoid, hydraulic interaction achieves a specific gear and diagnostic tips
Tuesday, May 24, 2016 - 06:00
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A previous article in Powertrain Pro — “The heart of the matter,” (April 2016) — delved into the operation of the pump in GM’s 8L90 (M5U) transmission. It covered some of the benefits of an off-axis pump, which this transmission uses and its operation. A comment was made by reader named Larry Bloodworth, who pointed out yet another benefit an off-axis pump has — that it can be positioned closer to the sump, lessening the distance of the drawing length through the filter into the pump. This is certainly a beneficial addition to using an off-set pump.

With the pump being the heartbeat of the transmission and pumping fluid into its veins, it brings to life eight forward speeds, converter clutch apply and reverse. This article will provide a glimpse as to how the solenoids interact with the hydraulics to achieve a specific gear. It will also provide an example of diagnostics should a problem develop.

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Beginning with the solenoids, there are a total of nine on the valve body, as seen in Figure 8. There is an assigned letter in the casting for each of the solenoids, as seen in Figures 2 and 3. Here is where some confusion may occur, as the solenoids are primarily referred to by their function and by a number, not their letter. To clear up any possible confusion, the identification of the solenoids is listed as follows: VB casting letter is listed first, followed by its function. In parenthesis is the solenoid number associated with diagnostic trouble codes.

  • A is the TCC Control Solenoid (Solenoid 7)
  • B is the 2-3-4-6-8 Control Solenoid (Solenoid 4)
  • C is the 1-3-5-6-7 Control Solenoid (Solenoid 3)
  • D is the 4-5-6-7-8 Reverse Control Solenoid (Solenoid 5)
  • E is the 1-2-7-8 Reverse Control Solenoid (Solenoid 1)
  • F is the 1-2-3-4-5 Reverse Control Solenoid (Solenoid 2)
  • G is the Default Control Solenoid (Solenoid 8)
  • H is the 1-2-3-4-5 Reverse Boost Solenoid (Solenoid 9)
  • J is the Line Pressure Control Solenoid (Solenoid 6)
    Figure 1 Figure 2
    Figure 3 Figure 4

Five of these solenoids are supplied with a fixed reduced pressure feed called the Actuator Feed oil. They are solenoids D, E, G, H and J. The remaining four solenoids — A, B, C and F — are supplied with Line Pressure.

The Actuator Feed valve in previous GM transmissions is known to wear its bore, causing a loss of solenoid supply pressure. Should this happen with the 8L90, it will have quite an undesirable effect in both forward and reverse ranges.

The solenoids that receive line pressure (A, B, C and F) will regulate that pressure to a lower pressure with which to regulate their respective control valves. These control valves sit forward of the solenoid inside the valve body and are also supplied with line pressure.

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