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Is it the transmission?

Transmission complaints aren’t always cured by a transmission repair
Wednesday, October 29, 2014 - 07:00
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I always get a kick watching the workers at a fast food burger joint put their food together. I don’t think the words “cooking or preparing” are terms that should be used with this process. I refer to it as “food by the numbers” where the people working can take a few different pieces of food, put them together in a certain order which will come up with something that looks and tastes like a sandwich, or something we have been led to believe is a sandwich.  At times, I feel like the automotive technician of today is expected to work and perform in this same manner. Diagnostics by the numbers will work at times, but when the diagnostics by the numbers doesn’t work, it is a must that a logical diagnostic process be followed.

How often does a car come to your bay with an automatic transmission shifting complaint that is not a transmission problem at all? Thinking back over the years, the analysis for automatic transmission shifting problems has changed a lot since the advent of the computer controlled automatic transmission. Terms like powertrain control module, transmission control module or engine control module come to mind when I start to analyze an automatic transmission problem. Whether the vehicle is using a powertrain control module to control both the engine and transmission or an engine control module that is networked with a transmission control module, the outcome is about the same — a computer system capable of operating the engine and automatic transmission in harmony with each other. In today’s world of powerful fuel efficient vehicles, this can be quite a difficult task to have the best of all worlds all rolled up in one tidy bundle.

Without an engine operating as it was designed to operate, the transmission will not operate and shift smooth at the correct time and give the powertrain that seamless transition of power from start through cruise speed. Just recently I was called to a shop to analyze an automatic shifting problem on a diesel powered vehicle. I was told the transmission wouldn’t shift out of second gear. There were even some diesel fuel injection pump codes stored, along with some codes stored in the automatic transmission module. This one ended up being a fuel delivery problem, all the while, the shop was working on the transmission in an attempt to fix the shifting problem. So, how does a technician go about telling the difference between a transmission shifting problem, or an engine power supply problem?

Many times these kinds of transmission problems can be analyzed by using a scan tool and checking for any information that has been stored in the modules. At the start of any diagnostic process, the technician should be on an information gathering quest. I always like to start with the engine module to see if there are any diagnostic trouble codes stored. If there are, I will make a note of them and move on. The next stop will be to take a peek into the transmission control module. If there is any information stored here, take some notes and start putting a plan together. Problem analysis always starts with gathering information from many sources, then that information is put together in a logical progression and a diagnostic direction is started.

The starting point of the problem analysis is to determine whether the transmission shifting problem lies with the engine or the transmission. Many years ago, I was told, “the art of problem analysis is to get the problem to come to you.” To accomplish this task, I will start by using a scan tool and watching engine data. This data will include watching the loop status, engine coolant temperature, Long Term Fuel Trim (LTFT) and Short Term Fuel Trim (STFT), and oxygen sensor activity from both the front and rear oxygen sensors. The vehicle will be test driven on a route where the vehicle can be accelerated from a slow speed to 60 mph at full throttle, then driven at a steady cruise for about a quarter mile. By driving the vehicle in these conditions the technician can get a feel for and capture data for engine operation in most any driving condition. The technician can also get a feel for the transmission shifting points and get a good seat of the pants feel for the powertrain operation and with the captured engine data a determination can be made for the next step in the analysis procedure. Keep in mind, if the engine does not operate properly, the automatic transmission will not perform properly, so the engine operation is always the starting point.

To help explain this analysis process a little easier, there is a 1999 Chevrolet K1500 Suburban sitting out in the parking lot. The vehicle is powered with the trusty 5.7 Vin R small block engine, has the power running through a 4L60 automatic transmission and is showing 188,000 miles on the odometer. The transmission had been rebuilt about 12,000 miles ago. The vehicle owner had taken the vehicle back to the transmission shop to have a transmission shifting complaint analyzed. The transmission shop did all they could do and determined the transmission did not shift properly into fourth gear and the torque converter clutch would not lock.

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