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Diagnosing a noise can be a real challenge, to say the least. One reason is that what creates a noise does not always look damaged.
Many types of noise are transmission related. Sometimes it's nothing more than a normal noise that sounds louder because of bad mounts or because part of the transmission is touching the frame or underbody of the car. Then there are actual components like pumps, planets, final drives or even something like a valve buzz that can cause you to want to tear your hair out. The trick is to find a way to make the noise change, or stop, and then examine what this change did to affect the noise.
There are several rules that will help isolate the component that is causing the problem. First of all, a component cannot generate a noise if it is not moving. This sounds easy enough but is often overlooked. Next, if the noise is pressure related, it will change when the pressure changes.
The following is a list of components that commonly make noise, and simple methods of diagnosing them:
Pump noises are simple. They change when pressure changes. Take, for example, a 4T60-E that has a noise whether it is in or out of gear. Because of this the torque converter can be ruled out. Putting the transmission in gear stops the entire gear train from turning. This rules out the entire gear train. At this point, the only suspect is the pump (or some other hydraulic component such as the pressure-regulator valve).
The test here is to vary line pressure by changing the vacuum at the modulator. If the noise varies with a change in pressure, its time to look at the pump (including the pump drive shaft). If this were an AXOD, pulling the TV cable would do the trick. With a 4L60-E, disconnecting the solenoid wire harness will work. A restricted filter can also cause pump noises. The best way to check this is with a pressure gauge. If the noise is caused by a clogged filter, the needle will vibrate when the demand for pressure increases.
Torque converter noises are easy to isolate. Since the entire converter (pump, turbine and stator) turns while the transmission is in park and neutral, a converter noise will go away in those ranges. When the transmission is in gear and the drive wheels are stationary, the turbine in the torque converter doesn't spin, because the turbine shaft is stationary. This is when the bearing between the turbine and main housing of the converter is working. If the noise is there only in gear with the drive wheels stationary and goes away in neutral, suspect the torque converter. There is an exception with the AOD transmission. The direct drum of an AOD is always turning while the engine is running. For this reason, a torque converter noise can be easily confused with the noise made by one of the direct drum bearings.
Tip: A torque converter noise will get quieter as the drive wheels begin rolling slowly from a stop.
These make the most difficult noises to diagnose. This is where it is important to remember the first rule: A component cannot make a noise if it is not moving. The trick here is to remember that when two components are rotating in the same direction at the same speed, they are, in effect, not moving relative to each other.
For example, a 4T60-E has a noise in neutral while stationary. The noise goes away when the transmission is in gear. Right away, you can rule out the torque converter. Since the pump is producing the same pressure in neutral as it is in gear, you can also rule it out. The final drive can't be the problem, because a final drive makes noise only when the vehicle is moving. Here is where it gets tricky.
Remember, a component cannot make a noise if it is not moving. Also, two components that are rotating at the same speed in the same direction are considered not moving relative to each other. Since third gear has a 1:1 ratio, The planetary are not rotating relative to each other while the transmission is in third gear. This means that if the noise is caused by either planet, it will go away when the transmission shifts to third gear.
Unfortunately, although the noise is quieter in third gear, it is still there. Further diagnosis requires an intimate knowledge of the unit being tested. Now, the noise did not go away in third gear, but it did change in pitch as the unit shifted to the next gear. If the noise is not coming from a planet and changes pitch as the transmission shifts, it must be from a component that is connected to the turbine shaft. This leaves the two sprockets and their related bearings; all other components have been eliminated.
These noises are perhaps the easiest of all to diagnose. The noise will increase in pitch as vehicle speed increases. The important thing to remember here is that the noise will not change with engine speed or when the transmission shifts, only with vehicle speed. However, its easy to mistake the noise from a bad power train mount for a final drive noise. Here again, check the mounts carefully first.
As with other diagnoses, finding the cause of a noise is a process of elimination. Knowing which component is not causing the malfunction is as important as knowing which component is causing the malfunction. Rule out as many sources as possible by using clear testing methods, and see which components are left.
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