Automatic Transmission Diagnosis

Basic automatic transmission diagnosis should start with some investigative groundwork. Often driver complaints about transmission problems can actually be traced back to malfunctions in other parts of the car.

You also should take the time before beginning any diagnosis to be sure you know exactly which unit is installed in the specific vehicle. Often technical service bulletins about certain model transmissions can provide a lot of insight on common problems associated with that model.

Knowing the year and model of the vehicle may not be good enough for positive identification of the installed unit. Some manufacturers will perform up grades or even change the models during the manufacturing year. The only positive way to identify the exact one installed in your vehicle is to get the identification numbers off the transmission itself.

Usually transmission identification numbers are found on stickers or stamped into the case of the unit. Most often though I find a metal tag that is held by a bolt head or rivet near the shift linkage that controls the valve body. If you decide to remove this metal tag make sure it is reinstalled for future reference.

Many times automatic transmission diagnosis can be traced back to problems elsewhere on the vehicle. To quote a recent problem that I ran into I had a driver complain about banging when he put his vehicle into reverse and applied power to back out of his driveway.

Upon further inspection his transmission problem was actually a broken transmission mount. Replacing the rubber mount solved the problem and there was actually nothing wrong with the transmission itself.

As other examples problems can be caused by poor engine performance, problems in the hydraulic system, or even abuse resulting in overheating of the fluid. Mechanical malfunctions and electrical failures or even improper adjustments can also cause transmission problems.

Automatic transmission diagnosis should start with a few basic steps. First I like to check the condition of the transmissions fluid as well as the level. Low fluid or even overfilled fluid can cause problems. When the fluid is low line pressure can be reduced and caused many different complaints. If the fluid is overfull it can begin to foam up. The air bubbles will also lower the line pressure and cause internal problems.

I also like to take one of my auto scanners and make sure that there are no set trouble codes for the transmission. Diagnostic trouble codes if they are set can help reduce transmission diagnostic time. Often these set codes will direct you to a specific component that needs to be tested or inspected.

Learning as much as you can about not only your specific model transmission but an overview of how they work can also help you diagnose minor problems at home. In the example I mentioned above replacing a transmission mount on some models is not hard but yet on the other models it can be very difficult. If you locate a specific problem check your service manual to see what’s involved with the repair or replacement.

This next link will take you back to the blog for online auto repair from this page about transmission diagnosis. If you would like to learn more about transmissions I have built a repair model on my you fix cars website that talks about specific automatic transmission problems.