Diagnosing transmission problems can be a daunting task for many repair shops new to gearbox repair. However, with some basic knowledge of the transmission system and the right tools, it’s possible to identify the cause of the problem quite intuitively.
If you have a specific error code, please check our transmission error code lists to see if we have a reference for your transmission.
In this article, we will guide you through the diagnosis process of a transmission problem based on the condition of the transmission fluid & oil pan and error codes.
Firstly, let’s talk about the transmission oil. The color and smell of the transmission oil can give you a good indication of the health of your transmission. If the oil is dark and smells burnt, it indicates that your transmission was overheating so friction plates (clutches) may need inspection. If you find metal debris in the bottom of the oil pan, that means there were excessive wear and tear happening in the gearbox; although metal build up is expected in most of the transmission, too much of it can mean some metal part has damaged or deformed, aways check & clean mechanical parts first (and don’t forget valve body and its solenoids). In contrast, a milky color indicates water or external fluid contamination, which could be due to a damaged transmission cooler or a broken pipe. Please keep in mind that contaminated fluid almost always has a severe impact on clutches. Next, let’s move on to the transmission fluid in general. The transmission fluid’s level and condition are also critical. Low fluid levels can cause the transmission to overheat and damage the internal components. Depending on the transmission, if the transmission fluid is also used for hydraulic pressure accumulation, having a low transmission fluid will also impact the hydraulic performance.
Lastly, let’s talk about error codes. When your car’s check engine light comes on, it will generate an error code that can be read using an OBD scanner. These codes can give you a good indication of the problem with your transmission. For example, if the error code indicates a problem with the transmission solenoid, it’s likely that the solenoid needs to be replaced. Bear in mind that a lot of different failures can lead to the same symptom. For example, uncomfortable shifting may be the cause of clutch, valve body blockage, solenoids, or even shift fork delay (when in dual clutch transmission). So, it is necessary to understand the power flow and the general mechanism of a transmission, this will help us better isolate the possible cause. Let’s take an example, if an error code calls a speed sensor’s value implausible, what should be considered? Well, sensor itself is definitely one thing to consider; but we also need to realize that transmission sensor are hall effect sensor, they rely on a magnet sensing wheel to determine the RPM, so we need to check if the wheel is intact; Some RPM signals are also compared with the inputs or calculation of other signals, for example, transmission output RPM may be compared to ABS wheel RPM signal.
In conclusion, diagnosing transmission problems can be challenging, but it’s not impossible. By paying attention to the condition of the transmission oil, transmission clutch, and error codes, you can identify the cause of the problem and take the necessary steps to fix it.