Fun With Transmissions - September - 2021

It’s a Backyard Party, Special Guests Include the new 10-Speed Solenoids

The call has gone out by the government to increase fuel mileage, and the automotive industry designers have answered. First, it was lock-up converters. Then it was 4-speed overdriven transmissions. Next came the 5, 6, 7, 8, and even 9 speeds. Now, in search of better fuel economy, we have a strong set of 10 speeds in the market.

Here we are in the middle of summer, so welcome back to another backyard party featuring Fun With Transmissions! At this party, we will be checking out the electromechanical solenoids in today’s 8, 9, and 10-speed transmissions (figure 1). Unlike all the solenoids we’re used to seeing since way back in the day (think THM 125) that control fluid flow, these solenoids have a pintle that pushes directly on the valve that controls the fluid flow. Each solenoid\valve assembly controls the apply and release of the clutch it’s assigned to. For example, The A clutch in a Ford or a 1-2-3-4-5-6-R clutch in a GM vehicle.

One of the interesting things about these solenoids is how very little the plunger moves. The pintle, when fully extended, measures a small 0.091″ (figure 2). We can see what the valve looks like in the bore going from the closed position to the open position (Figure 3). When the valve opens, it allows line pressure to go to the clutch.


Now that we know what the solenoid brings to our backyard party let’s check out the hydraulics that the solenoid and valve control. We can see that this solenoid/ valve controls the A/1-2-3-4-5- 6-R clutch (Figure 4). When the solenoid is “ON,” the pintle is out, and the valve moves to the right. This opens line pressure to apply the A clutch. Keep in mind that all the clutch apply solenoids are pulse-width-modulated solenoids, so the pintle does not open all at once. The pintle moves as needed to control the position of the valve. Something interesting to point out, there are no springs behind the valves. They aren’t needed because the exhaust backfill pressure is always there anytime the pump is making pressure.


Exhaust backfill pressure is line pressure that has been lowered by a blow-off valve and spring elsewhere in the circuit. This pressure keeps the apply circuit charged with oil so that when the apply valve moves to the right, the line pressure doesn’t need to fill the entire circuit. The circuit is already charged, and when the valve moves, it raises the pressure in the circuit via the PWM movement of the valve.

One thing that is very critical to remember, this type of solenoid function applies to all of the transmissions described at the beginning of this article, whether it’s Ford or GM. All clutch-apply solenoids and shift valves have the same hydraulic passages and perform the same functions.

All of the units listed have a failsafe gear. That is, the unit will stay in the gear it was in when the failure occurred until the engine is shut off. Once the engine is off and there is no pressure in the unit, the transmission will failsafe to neutral the next time engine is started. The major difference between the two manufacturers is how they keep the transmission in gear while the vehicle is running. Keep an eye out for this in a future article.


These 10-speed solenoids are very precise, hardworking solenoids. So let’s take a break from our backyard party and checkout these workhorses. As mechanics, when we see a part like a solenoid, we can’t help but wonder, “What’s inside there”? So naturally, we cut the solenoid open and disassembled it (Figure 5).

We can see the usual solenoid components, coil windings, sleeve/actuator tube, a plug/armature that rides inside the sleeve, the pintle that moves the valve in the valve body, and the end caps on both ends.

The surprising piece we found was a carbon-fiber sleeve/ membrane. This provides the necessary non-magnetic radial air gap between the armature and actuator tube to minimize magnetic side load when the coil is energized. It also provides a buffer between the armature and the actuator tube. Basically, there is no metal-to-metal contact inside the solenoid (figure 6).

The sleeve/membrane is layered with carbon fiber strands between the layers (figure 7). Keep in mind the membrane is only .005″ thick.


The issues with this solenoid and valve arrangement will be just about the same as a hydraulic solenoid, which controls fluid flow to a valve. The solenoid may get contaminated with metal suspended in the fluid, and the Teflon-coated valves will be exposed to the same contamination. When these valves start sticking, we may see clutch codes and ratios codes as well.

Our backyard party is winding down, and folks are starting to find their way home. We are going to leave the party with a better understanding of how the 10-speed solenoids operate and how the hydraulics work to apply the clutches. We all know that when we have a better understanding of how things work, the easier it is to have Fun With Transmissions!