This article is a prelude and supportive document to a new column, “Thinking It Through.” Over my 40+ years in the transmission industry as a rebuilder, Hotline technician, and trainer, the topic/problem of shift feel comes up a lot. “I’m working on a (you name the vehicle and transmission), and it has a harsh 2-3 shift (or any other shift-feel complaint). One approach is to address the specific shift-feel problem on that particular transmission. That works, but it ignores the bigger picture. And that is to forget the unit for just a moment and consider the factors that influence shift feel, both harsh and soft.
There are seven variables related to shift feel. Stop for a moment and let that settle in. There aren’t just five factors; there aren’t ten; there are seven. This means that whether you’re working on a 1964 Powerglide or 2018 8L90, one or more of these seven factors is causing your shift-feel complaint. At the end of each factor listed, I’ll include questions you might ask to validate that factor. The questions will be in blue.
Let’s get started with the seven factors.
They are:
- Line Pressure — The higher the line pressure, the firmer the shift. This is almost obvious, but it’s worth exploring. When examining pressure related to shift feel, we’re talking about the pressure to the shifting element. But since line pressure affects that, let’s start with that. Is the line pressure normal for the unit you’re working on? If you’re dealing with only one problematic shift, then it’s likely that line pressure is normal.But it’s easy to put a pressure gauge on most units and measure it. Of course, some transmissions don’t have pressure taps or are impossible to get to. Also, computer-controlled transmissions can have a computer malfunction that affects only one shift. Tip. Put a gauge on transmissions that work so you know what normal pressure behavior looks like.
- Flow rate — The faster the oil moves, the firmer the shift. The most common f low rate control is an orifice in a separator plate. We’re all familiar with enlarging an orifice in a separator plate while installing a valve body kit. Another approach is a regulator valve for the flow rate to a shifting element. Regardless of the approach, the faster the oil flows during a shift, the firmer the shift becomes. Here’s another instance where you must know the flow-rate controls for a shift-feel complaint. Do you know the flow-rate mechanisms for the shift you’re testing? Find out what it is and then ask: Has anyone worked on this vehicle, or is this a problem that has just started? If someone just worked on it, perhaps they left a check ball out, for example.
- Accumulator rate — The slower the accumulator strokes, the firmer the shift. This is a simple concept but confusing at the same time. An accumulator works as a controlled leak. Oil going to a shift element gets diverted to an accumulator. The oil flowing to an accumulator lowers the pressure available to the shift element. Once the accumulator bottoms out, the shift element receives the full available pressure. The confusion comes with what slows down or speeds up the accumulator during a shift. Valve body kits often include a heavier accumulator spring to firm up a shift. Does that mean a heavier accumulator spring offers a firmer shift? Not necessarily. If the accumulator compresses the spring during a shift, then yes. But if the spring is on the other side of an accumulator where it relaxes during the shift, then a weaker spring will give you a firmer shift. Does the unit you’re working on use an accumulator for the shift you’re diagnosing? If not, then skip this part. If it does, ask: Does the spring compress or relax during the shift you’re diagnosing? If it relaxes, then it could have a broken accumulator spring. If the spring compresses during the shift, a broken spring will cause a slide bump. Be careful; this can be tricky. Let’s explore this further.
An accumulator goes through three distinct phases (I’ll use the 1-2 shift on a 4L60E for this):
- Pre-Shift (Figure 1): Most accumulator systems have an opposing oil that varies with throttle pressure or engine load. In the 4L60E, it’s called “accumulator pressure.” This pressure will vary based on the load and acceleration rate. In this phase, servo pressure is zero.
- Shifting Mode (Figure 2): As oil flows from the 1-2 shift valve, it goes to the servo and accumulator. If you could sample the servo pressure, you’d see it rise to some level, say 25 psi, and then slowly increase based on accumulator pressure (the green oil) and spring tension. Line pressure has an influence on it, too. If the accumulator was working correctly, the shift would finish while the accumulator was still moving.
- Accumulation Complete (Figure 3): The accumulator has limited travel. Once it reaches the bottom of the bore, the shifting pressure spikes to the level of line oil. Since the shift is complete, you never know when this took place. However, the pressure spike will result in an aggressive shift if it is not complete by the time the accumulator reaches the end of its travel. We call it a slide bump. A slide bump is often misdiagnosed as a harsh shift. The problem with this is that the measures you might take to soften the shift make the problem worse.
If the transmission you’re working on has a pressure port for the shifting element you’re diagnosing, check its behavior with a pressure gauge.
Note: Many later-model transmissions have eliminated accumulators and use direct-apply solenoids for clutch apply and release. In these cases, the solenoid will mimic an accumulator’s influence of pressure on the shifting element. That is, the pressure will rise during the shift, as in Figure 2, and then spike to full pressure once the shift completes, as in Figure 3.
Figure 4 shows a version of the 4L60E where the spring relaxes during the 1-2 shift. A broken spring here will cause a firmer 1-2 shift. These models first have the spring(s) in the 1-2 housing, followed by the accumulator piston.
The following four factors usually come into play after a rebuild.
- Cushion springs and wave plates — The more time that is consumed by a cushion before the clutch or band applies, the softer the shift. This is straightforward. A wave plate or cushion spring allows the piston to move further before it applies its full force to the shifting element. We see these in clutch packs and servos. Some of these are subtle. Ford and Borg-Warner used coned steels in their clutch packs. To the untrained eye, they looked like any other steel. But if you placed them on a flat surface, you’d see that they weren’t flat; one edge (the inner or outer) would have a gap between the plate and the flat surface. Replacing them with flat steels produces a firmer (or more aggressive) shift.Other manufacturers, like Toyota, have used waved friction plates. Replacing them with flat frictions will produce a firmer shift (and usually a shift complaint by the customer). Was the unit in question recently rebuilt? If so, perhaps there’s a missing wave plate (verify if it uses one). If the unit was not recently rebuilt, it may have a broken wave plate or cushion spring. Note: Only consider this after verifying items 1-3.
- Steel plate or drum surface — The smoother the surface, the firmer the shift. This is counterintuitive. Initially, you might think a smoother steel surface (steel clutch plate or band surface) will offer a smoother shift. It’s the exact opposite. It comes back to time. A rough surface causes the shift to begin sooner rather than later. This will only be a factor after a rebuild, not on a unit that’s been working correctly and then develops a shift-feel complaint. Was this unit recently rebuilt? Were any of the steel plates or drum surfaces sanded? If so, you could be chasing a slide bump.
- Friction plate composition — This won’t be defined; however, some friction compositions offer a more aggressive shift than others. We’ve all seen this with Hondas, but many transmissions have specialized plates. You can have a shift-feel complaint by replacing them with other materials. Was this unit recently rebuilt? Were the replacement friction plates from the same manufacturer as the old ones? If the factors mentioned above are okay, suspect a friction incompatibility issue.
- Fluid (ATF) type — This won’t be defined for our purposes. However, some fluid types will offer a more aggressive shift than others. You can also develop a chatter, along with other shift disorders, by using the wrong fluid. Was this unit recently rebuilt or serviced? If so, it likely has the wrong fluid (provided the above factors are okay). If not, a service with the correct fluid may resolve the problem. This is particularly true for chatter/shudder complaints.
There you have it. These are the variables related to shift feel and the considerations I used as a rebuilder and Hotline technician. The technology may change over the years, but these factors remain consistent.
