Noise diagnosis is more challenging than it might seem. That is, until you figure out a few key tips. Understanding these tips is a lot better than the alternative: replacing parts until the noise goes away. I’ve done both, and figuring out the noise source beats replacing good parts with other good parts until you find the culprit. Noise diagnosis is a great topic; I’ve written about it twice. The first was almost 30 years ago, in 1996, and again in 2002. The first article was published in the original tabloid format. The second article was in the magazine format. None of these articles are available digitally, so you’re out of luck unless you have old copies. Fortunately, ATRA and Gears magazine now have nearly 20 years of digital archives of previous articles, so this article will be around for a long time.
So let’s get with it. Not much has changed regarding noise diagnosis, even though the transmissions have changed greatly. Before we get into the diagnosis, recognize that transmissions make noise. That’s right, all transmissions make noise. If you’ve ever held a stethoscope to a transmission case, you’d hear it whirring away while it shifts through the gears and does its thing. What keeps these noises isolated are the rubber mounts, securing the transmission and engine to the frame. We’ll cover this aspect later.
There are three components in an automatic transmission that make noise.
- The Torque Converter
- The Pump
- The Geartrain, i.e., the planetary gears sets
TORQUE CONVERTER
Torque converter noise is the easiest to diagnose. Figure 1 shows the components of a typical torque converter. It has four main components.
- Pump: The main housing, which includes the pump. The converter housing is bolted to the engine’s flexplate and turns with the engine. Its turning force flows oil to the next component.
- Turbine: Oil flowing from the converter pump flows to the turbine. The turbine is connected to the transmission input shaft and delivers engine torque.
The turbine turns under two conditions: when the transmission is in neutral or when it’s in gear, and the car is moving. When the transmission is in gear and stationary, the turbine is stationary. It’s a simple concept but too often overlooked. After oil flows through the turbine, it heads to the next component. - Stator: Oil flowing from the turbine heads to the stator. Before we get to the function of the stator, know that this part is the stator, not the part of the pump with a splined shaft. Over time, being mischaracterized, the stator support (the splined shaft from the main pump housing) is commonly referred to as the stator. The only part of an automatic transmission called “stator” is illustrated in Figure 1 and is part of the torque converter. The stator’s job is to redirect the oil flow from the turbine back to the pump. I’ve never heard of a noise complaint from the stator, and if there were problems with it, the problem would be much more severe than an annoying whining sound.
- Converter Clutch: For our purpose, I’ll just recognize that the converter clutch exists. I’ve not seen a converter noise caused by the converter clutch.
THE TAKEAWAY
For all intents and purposes, the only things you’ll find as a noise source in a converter are the bearings between the stator and pump and the stator and turbine. If the converter has a bearing between the converter clutch and the main housing, you can add that to the list, too. These are trivial differences because, in any case, you’re replacing the converter to fix the complaint. The best test is to listen for the noise in neutral. If it’s quiet, then move the shifter into gear (any gear). If the noise appears, then give it a little gas. If the noise increases, you’ve identified the noise source as the torque converter. Simple, right?
PUMP
The next item on our list is the pump. It’s not as easy as a converter diagnosis, but it’s close. There are three types of pumps you’ll find in an automatic transmission (Figure 2):
- Gear and crescent
- Gerotor
- Vane (with or without a slide).
Each of these has its own quirks, but they all share one thing when it comes to noise. Pump noise increases with rpm and pressure demand. With that, if you suspect a noise is pump-related, force it to work harder. Test the vehicle while stationary and in neutral. Right there, you’ve eliminated the torque converter and geartrain.1 Now increase the engine rpm. If the noise pitch rises, then you likely have a pump-related problem.2 While the noise is pump-related, it may not be the pump. A restricted filter or leaking filter seal will behave the same way. When you increase engine rpm, more oil draws through the filter. If your filter is clogged or it’s getting air in the intake, you’ll get a pump whine, and it’ll rise in pitch with increased engine rpm.
Now, go back to an idle and put the shifter into reverse. Most transmissions increase line pressure in reverse. If the noise increases in reverse, then you definitely have a pump-related problem. Gerotor and vane pumps generally aren’t affected by high line pressure as far as noise is concerned. The most vulnerable pump for this is the gear and crescent type. We don’t see these as often anymore, but at one time, they were the most common pump type in production. The problem is that the converter hub keeps the inner gear centered (Figure 3).
If the hub or inside of the gear wears out, then the gear wants to move toward the crescent. That’s because there’s suction on one side and pressure on the other. If you have one of these pumps and the noise gets louder when you shift it into reverse, check the hub and inner gear for excessive wear.
THE TAKEAWAY
Pumps make more noise when they’re worked, either by rpm or pressure. The car doesn’t have to move, and it’ll make noise whether in or out of gear.
GEARTRAIN
Here’s the tricky one. Figure 4 shows an illustration of a simple planet. Complex planetary systems like Raveneux or Lepeltier are just a series of gears or combinations of planets working as a complete gear set. Planetary geartrain noise varies in pitch with speed and volume with load. If you’ve ruled out the converter and pump as the culprit of a noise complaint, then you’re into the geartrain. The challenge is which planet is the culprit. For this example, we’ll use a 4L60E. The chart in Figure 5 shows which elements are working for each gear. In first gear, the low roller clutch holds the rear planet, which is splined to the front ring gear. That’s all the rear planet does in first gear. 
Therefore, the front planet does all the work in first gear. If the noise is only in first gear, then suspect the front planet and related bearings. In second gear, the 2-4 band applies and holds the sun gear for the rear planet. Now, both planets are working. If the noise begins in second gear, suspect the rear planet. It’s a 1:1 ratio in third gear, and none of the planets are working. Everything is spinning as a unit. Here’s a tip. If you suspect a planetary noise and the transmission has a 1:1 ratio, the noise must go away in the gear, or else you’re looking at something else. To finish up, the rear planet is the only one working in fourth gear.
Let’s consider something more modern, say, a 6L80 (Figure 6). The 6L80 doesn’t have a 1:1 ratio, so you’ll have to carefully examine the application chart to point to a specific planet as the culprit. On the other hand, the 8L90 does have a 1:1 ratio for sixth gear. Use that as a clue.
THE TAKEAWAY
Planetary gear noises are the most challenging transmission noises to diagnose. Begin by eliminating the converter and pump, then see if the noise goes away in a 1:1 gear ratio (if it has one). Other than that, it’s a unit-by-unit discovery to see which planets are working the hardest for each gear.
One last thing, and we touched on it in the beginning. Transmissions make noise. It’s normal. The rubber mount’s job is to isolate their normal noises from the vehicle. Always inspect the transmission and engine mounts when faced with a transmission noise. I learned this the hard way in my early years as a rebuilder. That’s it! Noise diagnosis is easy once you know how and when components scream to most. And for your customer’s sake, enjoy the sounds of silence.
1. In rare instances, like the old Powerglide, the geartrain rotates in neutral but then stops when you shift into gear, while stationary. There are no mainstream transmissions that operate this way but who knows what the future holds. In this case you’d first listen to the noise in neutral and then compare the noise to when you put the transmission into gear.
2. Keep in mind that other devices, like alternators and power steering pumps operate from engine rpm and if they’re the cause of a noise their pitch will increase too.
